Revision of convection, radiation and cloud schemes in the ECMWF IFS (2000)

Q. J. R. Meteorol. Soc. (2000), 126, pp. 1685-1710

Revision of convection, radiation and cloud schemes in the ECMWF Integrated Forecasting System

By D. GREGORY*, J.-J. MORCRE’ITE, C. JAKOB, A. C. M. BELJAARS and T. STOCKDALE European Centre for Medium-Range Weather Forecasts, UK

(Received 7 December 1998; revised 13 January 2000)

SUMMARY Revisions to the convection, radiation and cloud schemes recently introduced into the European Centre for

Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) are described, together with discussion of their impact upon model performance. Seasonal simulations with observed sea surface temperatures (SSTs) for JundJuly1August 1987 and DecemberIJanuaryFebruary 1987188 with a low-resolution (T63) version of the model are used to assess the impact of the revised schemes, concentrating upon tropical climate and variability. While the revisions improve the physical basis of the schemes, and each improves aspects of the seasonal climate, overall improvement does not result until the changes are combined. Biases in simulated temperature and top-of-atmosphere and surface energy budgets are reduced, leading to a substantial decrease of an equatorial SST cold bias in coupled ocean-atmosphere simulations used in seasonal forecasting. At higher resolution (T213) changes to temperature and wind fields are similar to those found in seasonal simulations, with little impact upon medium-range forecast performance in mid latitudes, although these forecasts proved a more critical test of the impact of the schemes upon mid-latitude flows. The paper demonstrates the methodology used in the development of parametrizations of physical processes at ECMWF, and points to the need to balance parametrization improvements especially between schemes which are highly interactive.

KEYWORDS: ECMWF IFS Numerical weather prediction Parametrization Physical processes

1. INTRODUCTION

Model development at the European Centre for Medium-Range Weather Forecasts (ECMWF) is an ongoing process in which scientific developments and knowledge of model deficiencies in different applications are combined. The atmospheric model at ECMWF is part of the Integrated Forecasting System (IFS, developed in co-operation with MCtCo-France) and is used for medium-range forecasting, data assimilation, proba- bilistic forecasting, wave forecasting coupled to a wave model, and seasonal forecasting coupled to an Ocean model. Various applications put emphasis on different aspects of the model. Apart from good performance for medium-range forecasting and data assimilation, it is also necessary to have good quality heat fluxes at the ocean surface for coupling with the Ocean model in seasonal forecasting (Stockdale 1997) and good surface winds for coupling with the Ocean wave model (Gunther et al. 1992).

In recent years, changes to the data assimilation system and to the dynamical component of the model, together with changes to the parametrizations of the ECMWF model have brought about improved forecast performance and an improved model climate (Brankovic and Molteni 1995). Examples are the introduction of a new land surface scheme (Viterbo and Beljaars 1995), a revised subgrid-scale orography scheme (Lott and Miller 1997) and a prognostic cloud scheme (Tiedtke 1993). This paper describes further revision to the convection, radiation and cloud parametrizations introduced into the operational version of the IFS during December 1997.

The revisions were undertaken for several reasons. Firstly, to improve the physical basis of the parametrizations and their performance as measured against observations and detailed models (such as line-by-line radiation codes or fine-scale cloud-resolving models for convection). Secondly, the changes aimed to correct errors in the top of atmosphere (ToA) and surface energy budgets, important for coupled ocean-atmosphere

* Corresponding author, present address: The Hadley Centre for Climate Prediction and Research, The Met. Oflice. London Road, Bracknell, Berkshire RG12 2SY. UK.

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1686 D. GREGORY el al.

simulations of the model used in seasonal forecasting. Some aspects of the medium- range forecast performance of the model are also improved.

A brief description of and justification for revisions is given. Reference is made to off-line case-studies using single-column models compared with observational data or detailed modelling studies of the processes concerned, to illustrate the improved physical basis of the revised schemes. The impact of each individual change is considered using 120-day seasonal forecasts with observed sea surface temperatures (SSTs). This provides insight into how each component contributes to the overall improvement of model climate brought about by the combined package. The effect of the changes upon coupled ocean-atmosphere simulations used in seasonal forecasting at ECMWF is considered, as is the impact upon medium-range forecasts using a 4-dimensional variational (4D-Var) data assimilatiodforecast system.

2. MODEL DESCRIPTION

(a) The ECMWF Integrated Forecasting System The study focuses on the atmospheric model component of the ECMWF IFS.

This is a global model employing semi-Lagrangian dynamics, with a hybrid vertical coordinate (Ritchie et al. 1995) being pure terrain-following in the lower troposphere and pure pressure coordinates near the top of the model. Here the 3 1-level configuration of the ECMWF model is used, with horizontal resolutions of T63 and T213 for seasonal simulations and 10-day forecasts, respectively. The model time steps for these resolutions are 60 and 20 minutes respectively. In the control physics package used in this study, the radiation parametrization (Morcrette 1990) uses a two-stream formulation, with a photon-path distribution method with two spectral intervals for the short wave (SW). For the long wave (LW) an emissivity model with six spectral intervals is used. Moist convection is represented by a bulk mass flux scheme (Tiedtke 1989). The prognostic cloud scheme of Tiedtke (1993) is used, with water detrained from convection being a source of cloud mass and cloud water. Several modifications have been included since its implementation into the ECMWF operational model in 1995. The original formulation of cloud-top entrainment has been replaced by one related to the radiative heating in the top layer of a cloud. Clouds erode by evaporation, the rate coefficient being increased by a factor of five in the presence of shallow convection in order to bring about an improved radiation budget in the subtropics. The vertical diffusion parametrization is based on K-theory with: a K-profile closure (Beljaars and Viterbo 1998) for the unstable boundary layer; a Richardson number dependent diffusion coefficient for stable situations above the surface layer (Viterbo et al. 1999); and a Monin-Obukhov type closure in the surface layer (Beljaars and Holtslag 1991). The soiVsurface scheme consists of a skin layer and four soil levels (Viterbo and Beljaars 1995) while the subgrid-scale orographic scheme is described in Lott and Miller (1997).

(b) Revised convection scheme

(i) Diagnosis of deep or shaElow convection (‘switching ’). The original version of the Tiedtke convection scheme used the presence of moisture convergence into a column of the atmosphere due to atmospheric motion to determine whether a point is likely to contain deep or shallow convection. If moisture convergence is positive then the parameters of the scheme are set to be those appropriate to deep convection. If the converse is true parameters appropriate for shallow convection are used. In the revised scheme this ‘switching’ has been replaced by one based upon the depth of convection. If

REVISED SCHEMES IN THE ECMWF IFS 1687

the convective cloud depth exceeds 200 hPa the convection is deemed to be deep, while a cloud with a depth lower than this threshold is treated as shallow convection.

(ii) Deep convective closure. The previous operational version of the scheme retains a closure similar to that described in Tiedtke (1989). Convective mass flux at cloud base is estimated from the assumption that the integrated moisture (for deep convection) and moist static energy (for shallow convection) of the sub-cloud layer remains constant when convection is active. This has been termed 'boundary-layer quasi-equilibrium' by Raymond (1995)*. In the revised version of the scheme the estimation of cloud base mass flux for shallow convection remains unchanged, while the closure for deep convection is changed to one based upon the concept that convection acts to reduce convective available potential energy (CAPE) towards zero over a certain time-scale. This is similar to that introduced by Fritsch and Chappell(l980) in a mass flux scheme for use in mesoscale models. Its use in global models has been described previously by Nordeng ( 1994) using an earlier version of the ECMWF model, and Gregory (1 995) in The Met. Office Unified Model.

CAPE is defined as:

where g is the acceleration due to gravity, T the temperature and z height. Superscript c refers to the in-cloud values, and the virtual liquid-water temperature is given by TV1 = T(l + 0.608q - l ) , with 1 being the liquid-water content of the parcel and q the specific humidity. An overbar denotes an average value.

The rate of change of CAPE with time due to convection is:

aCAPE

assuming that the in-cloud properties are constant in time. Mass-flux theory predicts that the effect of convection on large-scale temperature

and moisture structures is dominated by compensating subsidence (see for example the study by Gregory and Miller (1989), among others). Neglecting the effect of convection on the large-scale water content of the atmosphere and ignoring the contribution of the grid-box mean water content to virtual temperature, (2) can be expressed as:

1 aT aql dz, (3) (F)coW v' - 1 '7 (. (at :) + (1 + 0.608@% where c p is specific heat at constant pressure, and Mc, the cloud mass flux, accounts for both updraught and downdraught mass fluxes. Expressing this as a combination of the mass flux at the base of the updraught (Mf) and top of the downdraught (MFD) , together with functions, q, which describe the variation of the mass fluxes with height (derived from the entraining/detraining plume model of convection), then

aCAPE

* Although the closure of the deep convection scheme used in the control simulations here is similar in philosophy to the original Tiedtke scheme, it differs in that only updraught fluxes are considered to balance the moisture supply to the sub-cloud layer. In the original scheme both updraughts and downdraughts were considered. The difference appears to have come about through a coding error in the IFS when it was developed from an early version of the ECMWF model.

1688 D. GREGORY et al.

The initial mass flux at the top of the downdraught is taken to be proportional to the mass flux at the base of the updraught,

where a is 0.3 (as in Tiedtke 1989). Equations (4), (5) and (3), together with the assumption that convection acts to reduce CAPE towards zero over a time-scale t (which needs to be specified) give, after rearrangement, an expression for the mass flux at the base of the updraught:

('";") MBUD =

1 aT 0.608 aq) dz' (6) /g(qUD-aqDD) (= T (-+:) az + (1 + 0.608a

The impact of these changes has been assessed in several case-studies using the ECMWF single-column model, results shown here being from simulations using a time step of 15 minutes and a convective adjustment time-scale of two hours. Fig- ure l(a) shows the net convective mass flux profile predicted by the control and revised convection schemes for an idealized cold-air outbreak case (Kershaw and Gregory (1997Nases 232 with surface sensible- and latent-heat fluxes of 123 and 492 W m-2). Modification of the switching algorithm is beneficial in this case. With surface forcing alone, the control scheme diagnoses shallow convection with entrainment rates set to 1.2 x m-l 150 hPa above this. These values dilute the parcel during ascent, leading to an underestimation of the mass flux in the upper part of the cloud layer. The shallow convection closure of the control scheme also overestimates the cloud-base mass flux. With the revised scheme, deep convection is diagnosed, entrainment rates being one quarter of those for shallow convection, resulting in deeper convection and a mass flux profile in better agreement with that diagnosed from cloud-resolving model simulations.

For deep convection which exists in association with ascent on the large-scale, the shape of the net convective mass flux is little changed, both schemes using the entrainment formulation described in Tiedtke (1989). However the intensity of the convection is increased, as shown by the net convective mass flux from single-column simulations of convection over the Inner Flux Array (IFA) of TOGA-COAREi (Tropical OceadGlobal Atmosphere Coupled Ocean-Atmosphere Response Experiment) between 20 and 26 December (Kruger 1997)* (Fig. l(b)). Using the CAPE adjustment closure increases the mass flux at most heights, leading to a larger net heating in better balance with the imposed large-scale forcing. However the entrainment formulation causes the peak in the mass flux to be placed just above 5 km rather than 3 km as indicated by cloud- resolving model simulations of the case (Kruger, personal communication). The proportion of precipitation originating from the convection scheme is increased with the CAPE closure, from 53 to 60% of the 13 mm day-' total precipitation rate over the whole simulation. This is consistent with the increase in the mass flux predicted by the convection scheme, although it is difficult to say which is more accurate as estimates of the fraction of total rainfall in the tropics produced by convection vary between 50 and 70%.

* Intercomparison case 2 of the Working Group 4 (Deep Precipitating Convection) of the Global Energy and Water Budget Experiment Cloud Systems Study.

m-l at cloud base, decreasing linearly to 3 x


Figure 1. Single-column model simulations showing the impact of the revised convection scheme upon mass flux (Kg m-*s-') profiles. (a) Idealized cold-air outbreak (10-hour average); results from The Met. Office Cloud Resolving Model (dotted) with control (solid) and revised (dashed) convection schemes. (b) Convection in the IFA of TOGA COARE between 20 and 26 December 1992 (6-day average); results from control (solid) and revised

(dashed) convection schemes. See text for details.

(c) Revised radiation scheme

(i) Water-vapour continuum. Following work by Zhong and Haigh (1995; hereafter ZH), a revised continuum has been incorporated into the LW radiation scheme. LW cooling rate profiles from off-line tests using the climatological tropical atmosphere of McClatchey et al. (1972) from the control and revised schemes, together with those from a line-by-line code (Mlawer et al. 1997-RRTM3A) and the ZH parametrization are compared in Fig. 2. Cooling-rate profiles for mid-latitude summer and winter and sub- arctic winter atmospheres of McClatchey et a/. (1972) show similar sensitivity. The new parametrization corrects the overestimation of the clear-sky cooling in lower layers and produces increased cooling higher up, in much better agreement with the more recent and sophisticated line-by-line code. Table 1 compares the outgoing LW radiation (OLR) at the ToA and the surface downward LW flux (DLF) for the McClatchey et al. (1972) profiles. For both these quantities the revised LW code is closer to the ZH radiation code than the control version.

1690

0.0

0.2

f 0.4

2 n

fn fn

m 0.6

0.8

1 .o

D. GREGORY et al.

-4 -3 -2 -1 0 Longwave Heating Rate Wday

Figure 2. Comparison of clear-sky long-wave radiative cooling rates for the tropical profile of McClatchey er al. (1972) for the operational (OPE long dash), the revised (NEW, solid), the rapid radiative transfer model (RRTM; short dash) and Zhong and Haigh (ZH; chain dash) radiation codes. The ordinate is pressure in eta coordinates.

TABLE 1. OLR A N D DLF (W m-*) COMPUTED BY CONTROL, ZH A N D REVISED Lw SCHEMES FOR THE CLIMATOLOGICAL ATMOSPHERES OF MCCLATCHEY et d.

( 1 972)

OLR OLR OLR DLF DLF DLF Atmos. Control ZH Revised Control ZH Revised

Tropical 296.8 288.3 292.2 391.9 392.5 392.1 Mid-latitude summer 286.5 278.4 282.0 344.8 346.3 345.0 Mid-latitude winter 233.2 228.2 230.1 218.8 221.2 220.2 Sub-Arctic summer 269.0 262.8 265.6 296.1 297.6 296.6 Sub-Arctic winter 200.7 197.0 198.5 167.4 171.2 170.9

(ii) Cloud optical properties. The control radiation scheme used the Smith and Shi (1992) scheme to specify the LW optical properties of cloud. This is replaced by the Ebert and Curry (1992) scheme, also used in the SW part of the revised scheme. A crucial parameter in defining the optical properties of clouds is the effective radius of cloud water and ice particles. For ice the constant value of 40 microns used in the control scheme (Morcrette 1994) is replaced by the observationally derived formulation of Ou and Liou (1 995). In this the effective radius of ice particles increases with temperature, from 40 microns at 240 K to 120 microns at the freezing point, the variation usually being attributed to accretion on falling crystals.

As discussed by Edwards and Slingo (1996), spectral overlap between the gaseous components of the atmosphere (mainly H 2 0 and C02) and the liquid-waterhce bands leads to an overestimation of SW absorption by ice clouds when only 2 bands are used to represent the SW spectrum. Although these errors are known to decrease with increasing spectral resolution, for computational reasons the revised scheme retains 2 bands in the SW but the cloud optical properties are modified to match those of


TABLE 2. VARIATION OF L w SURFACE EMISSIVITY ACROSS SURFACE TYPES WITHIN (800-1250 cm-')

AND OUTSIDE THE L w WINDOW

Spectral Surface Landcategory Surface window emissivity

Outside LW Land Desert

ocean Sea-Ice

Inside LW Land Desert

Ocean Sea-Ice

window Non-desert

window Non-desert

0.99 0.99 0.99 0.99 0.93 0.96 0.98 0.98

higher spectral resolution (4 bands)* removing the enhanced SW cloud absorption of the control scheme. A further modification to the treatment of clouds in the SW is the introduction of a treatment of cloud inhomogeneity (Tiedtke 1996), in which the cloud- water path used in the SW scheme is reduced by a factor of 0.7.

(iii) Surface Properties. The control radiation scheme uses a spectrally invariant value of 0.996 for LW surface emissivity regardless of surface type. As reported by Kondraty'ev (1972) this corresponds to water, while recent studies have shown a considerable variation across different land types and soil moisture contents (for example, Masuda et al. 1988). Considering work by Taylor (1979) and van de Griend et al. (1989) the revised scheme allows for variations in the LW surface emissivity (see Table 2).

Sea ice albedo has been modified following Morassutti (1991) who linked visible and near-infrared sea ice albedo to temperature. To account for the presence of snow on sea ice a constant value of 0.55 has been replaced by a value which varies in the range of 0.5 to 0.7 for surface temperatures between 277.15 K and 272.15 K, with values being held at the lower and upper values outside of this temperature range.

( d ) Revised treatment of ice fallout The fallout of cloud ice in the model is governed by the equation:

where Di is the terminal fall speed of ice particles and 1 here is the cloud ice content. A spatial discretization of this equation for a layer, k, can be written as:

where for simplicity a constant density ( p ) is assumed. The control scheme has a diagnostic treatment of snow, with ice leaving a model

layer falling to the surface in a single time step, evaporating as it falls through lower layers of the model. This is equivalent to assuming (uil)k-' = 0 in (8), and obviously introduces a strong dependence on vertical resolution. In the revised scheme, ice falling out of one layer contributes to the ice-water content of the layer below, provided that * The revised SW scheme can be used with either 2 or 4 spectral intervals, with either 1 or 2 bands in the near infrared.

1692 D. GREGORY ef al.

the lower layer is cloudy. For ice falling into clear sky a conversion to snow is assumed as in the control scheme. For both the control and revised scheme, (8) is integrated analytically in time as described in Tiedtke (1993).

3. IMPACT OF CHANGES ON SEASONAL SIMULATIONS

It is common for new parametrizations to be introduced into a general circulation model (GCM) as a package rather than a series of individual changes. This comes about for several reasons. Firstly, improvements in the physical basis of one scheme may uncover poor performance in another and so introduction of one alone may not result in overall improvement in model performance. In other words the ‘accuracy’ of various schemes must be balanced, especially when there is a large degree of interaction (although it is difficult to define accuracy in an absolute sense). A second consideration is more practical; in an operational environment such as ECMWF it is more practical to introduce a single package containing a number of changes rather than a series of individual smaller changes. However, it is important to understand how each component of the package affects the overall system and here, before the impact of a combined package of changes is considered, the impact of individual changes is discussed.

Seasonal simulations of December/January/February 1987/88 (DJF87/88) and June/ July/August 1987 (JJA87) using a T63 version of the ECMWF IFS are considered. The 120-day integrations with observed SSTs start from 1 November 1987 and 1 May 1987, respectively. Experience suggests that changes to the mean thermodynamical structure of the tropical atmosphere and mid latitudes can be broadly assessed using single forecasts, the signal being made more robust by the use of zonal-mean diagnostics. Other quantities, such as precipitation or measures of tropical variability, are more variable, and the use of ensemble means is preferable. Here ensembles of three forecasts are used, starting one day apart. For example an ensemble mean for DJF87/88 is made from simulations starting from 30 and 3 1 October and 1 November. The response of the mid- latitude circulation patterns to the physics changes are more difficult to assess using such a limited number of simulations, and comments here will be restricted mainly to the impact upon the tropics and thermal structure in mid latitudes.

(a) Convection The convection change has substantial impact upon precipitation patterns simulated

by the IFS, bringing the model closer to observed precipitation estimates provided by the Global Precipitation Climatology Project (GPCP) climatology (Huffmann et al. 1997). During JJA87 (Fig. 3-an ensemble mean over 3 simulations), the intensity of rainfall along the inter-tropical convergence zone (ITCZ) is reduced, in the Atlantic and also, most noticeably, in the east Pacific. Over the Indian monsoon region underestimation of rainfall over central India is corrected, as are low precipitation amounts to the west of the Philippines. The coverage by rainfall amounts greater than 5 mm day-’ over south-east Asia is more continuous and peak values reduced. The low-level monsoon jet is better captured (not shown), with core jet speeds increasing over the Arabian Sea. Reduction in the intensity of the ITCZ is associated with a reduction of the zonal mean vertical velocity in the ascending branch of the Hadley circulation.

The GPCP climatology places the maximum zonal mean rainfall in the tropics south of the equator in DJF8788, with contributions from the South Pacific convergence zone (SPCZ) and South America dominating. With either convection scheme (Fig. 4(a)), the model overestimates the intensity of the ITCZ north of the equator, although with the revised scheme the peak is broadened towards the equator due to larger precipitation


Figure 3. Total precipitation (nun day-') for June/July/August 1987 from: (a) Global Precipitation Climatology Project climatology; (b) from an ensemble of three 125 day T63L31 simulations starting from 29 and 30 April and 1 May with control convection scheme; and (c) as (b) but with revised convection scheme. Contours at 0.1.1.

2,3,5,8, 16and32mmday-',shadedabove5mmday-'.

1 694 D. GREGORY er al.

8 - (a) 8 - (c)

4 -

Figure 4. Zonal mean precipitation (mm day-') for December/January/February 1987/88 from an ensemble of 125-day T63L31 simulations starting from 30 and 31 October and 1 November 1987. (a) Global Precipitation Climatology Project (solid) compared with simulations using control (dash) and revised (chain dash) convection schemes. (b) Split of total precipitation (solid) into convective (dotted) and stratiform (dashed) components for control convection scheme. (c) as (b) but for revised convection using an adjustment time-scale of 2 hours. (d) as

(b) but with revised convection scheme using an adjustment time-scale of 4 hours.

rates in the central Pacific. In a similar manner to the TOGA COARE single-column model simulation, the proportion of total tropical precipitation of a convective nature is increased with the new scheme (Fig. 4(b) and (c)). The new scheme also increases the temperatures in the upper troposphere of the tropics, by 1 K at 200 hPa (Fig. 5(a)), this being consistent with a more active convection scheme as seen in section 2(b), although part of the signal is a radiative response to an increase in upper-level cloud cover.

The convective adjustment time-scale is theoretically poorly defined, and must be established by experiment. Nordeng (1994) argues that its value should be chosen such that convection comes into equilibrium with large-scale ascent while maintaining realistic thermodynamic structures, essentially allowing the convective mass flux to be of similar magnitude to the grid-scale (resolved) vertical motion. The T63 simulations presented thus far use an adjustment time-scale of 2 hours. It is commonly found that the magnitude of the resolved vertical motion increases with horizontal resolution such that a doubling in resolution leads to a doubling in peak vertical velocity. In the IFS a linear scaling of the convective adjustment time-scale with spectral resolution is used, with the 2-hour time-scale at T63 being used as the reference value. Such considerations are common to other adjustment approaches to convective parametrization; Slingo and Blackburn (1992) and Slingo et al. (1994) discuss them in relation to the Betts-Miller scheme in the UK Universities Global Atmospheric Modelling Programme GCM.

Further ensemble forecasts for DJF87/88 were carried out with adjustment time- scales of 1 and 4 hours. Reducing the adjustment time-scale to 1 hour has little impact upon the seasonal-mean precipitation and temperature fields of the model, while increasing the value to 4 hours leads to an increase in stratiform precipitation (Fig. 4(d)), the convectivehratiform ratio being similar to the control simulation. Upper-tropospheric temperatures are lower, closer to those of the control simulation


A m s Y

W K

W 7 a n 1

LATITUDE

Figure 5. Difference in zonal mean temperature (K) for DecembedJanuaryFebmary 1987188 ensembles of 125- day T63L31 simulations starting from 30 and 31 October and 1 November I987 between the revised and control convection schemes for different adjustment time-scales. (a) Adjustment time-scale of 2 hours, (b) adjustment

time-scale of 4 hours. The contour interval is 0.5 K.

(Fig. 5(b)). Increasing the adjustment time-scale reduces the convective mass flux for a given thermodynamic profile, the signals seen in tropical rainfall and temperature being consistent with a less active convection scheme, and the cloud scheme playing a greater role in balancing the effects of large-scale ascent.

Slingo et al. (1994) showed that simulated tropical variability is affected by the nature of the convection scheme. Introduction of the revised convection scheme affects synoptic activity within the model. Figure 6 shows the standard deviation of 850 hPa relative vorticity averaged over DJF87/88 for the control model and for simulations using the revised convection scheme with adjustment time-scales of 2 and 4 hours (each an ensemble of three simulations). With the control convection scheme (Fig. 6(b)) a band of high variability extends from the Philippines along the ITCZ to east of the date line. Strong variability is also seen along the southern edge of the SPCZ and southern Indian Ocean. The magnitude of tropical variability is reduced with the revised scheme (Fig. 6(c)) in the SPCZ, southern Indian Ocean and along the Pacific ITCZ in the vicinity of the date line. However it is increased east of the Philippines, associated with a spurious band of high precipitation. While reducing the adjustment time-scale to 1 hour has little impact upon tropical variability (not shown), increasing it to 4 hours (Fig. 6(d)) sees greater variability along the Pacific ITCZ. In the west Pacific the pattern is similar to that found in the control simulation, although the magnitude is increased. Figure 6(a) shows the standard deviation of 850 hPa relative vorticity estimated from


(a) 45"E 90"E 135"E 180" 135"W 9OW 45W . .

0"

(b) 45"E 90"E 135"E 180" 1359111 9OW 459111

0"

(c) 45"E 90"E 135"E 180" 135"W 90W 45W

0"

(d) 45"E 9O"E 135"E 180" 135"W 9O"W 45"W - .

0"

Figure 6. Standard deviation of 850 hPa relative vorticity for December/January/February 1987188 between 30"N and 30"s from: (a) ECMWF re-analysis and an ensemble of 125 day T63L31 simulations starting from 30 and 31 October and 1 November 1987, (b) using the control convection scheme, (c) as (b) but using the revised convection scheme with an adjustment time-scale of 2 hours; and (d) as (c) but with an adjustment time-scale of

4 hours. Contour intervals are 0.5 x s-' with shading above 1 x lop6 s-I.

the ECMWF re-analysis (ERA). Generally all of the simulations exhibit larger levels of variability than ERA suggests is realistic for this period; this is perhaps surprising as ERA was carried out at T106 while the simulations here are at T63. However there are several difficulties in drawing firm conclusions from comparisons with ERA data. While ERA captured the frequency of tropical cyclones well (Serrano 1997), there are inconsistencies between consecutive analyses in terms of features in the vorticity field (J. Slingo, personal communication) which may lead to the standard-deviation of relative


n

Y 5

w 7 w K

1

LATITUDE

Figure 7. Difference in zonal mean temperature (K) for JundJuly/August 1987 between T63L31 simulations starting from 1 May 1987 with the revised and control radiation schemes. The contour interval is 0.5 K; dashed

contours are negative.

vorticity underestimating tropical variability. A further difficulty is that T63 simulations appear to underestimate the level of variability within the tropics. Seasonal simulations at the higher resolutions of T ~ 1 5 9 and T ~ 3 1 9 (not shown) with a version of the IFS incorporating the combined revised physics package, show that tropical variability (as estimated here) increases with resolution, suggesting that a T63 model has inadequate resolution to make definitive statements about tropical variability.

Both the control and revised convection scheme run with a 4-hour adjustment time- scale have a larger proportion of the total rainfall from the cloud scheme; this suggests that the convection scheme is less active, with the cloud scheme playing a larger role in providing diabatic forcing for the tropics. Further experimentation (not shown) in which convective activity is reduced by applying a limit on the magnitude of CAPE which must be exceeded before the deep convection scheme can be activated, shows that variability is further increased, with stratiform precipitation becoming dominant within the tropical belt. Other measures of synoptic variability, such as the tracking of individual synoptic systems (Vitard, personal communication), also show an increase in synoptic activity when convective activity is reduced and the dynamics directly interact with the cloud scheme.

(b) Radiation Although the revised radiation scheme has a stronger physical basis and compares

better with line-by-line models and observations, its lone introduction to the model does not produce an overall improvement in temperature biases (Fig. 7-JJA87 from a single simulation from 1 May 1987). The upper troposphere is cooled, increasing model bias as compared to ERA climatology, a consequence of the revised H20 absorption in the LW and the reduced absorption of solar radiation by ice clouds. Biases in the lower troposphere are reduced due to warming brought about by the trapping of thermal radiation. The impact of the scheme upon the ToA radiation budget is also mixed. OLR (Fig. 8(a)) is in worse agreement with Earth Radiation Budget Experiment (EWE) observations in the tropics and mid latitudes, although a small improvement is seen in the subtropics. While stand-alone experiments (Table 1) indicated lower clear-sky OLR with the revised scheme, the higher values seen in the global model are a consequence of changes to the treatment of cloud-radiation interactions, primarily the variation of ice-effective radius with temperature. ToA SW flux (Fig. 8(b)) is closer to E W E in the tropics and sub-tropics indicating improved modelled albedo, due in the subtropics to the introduction of the cloud inhomogeneity, and in the tropics to lower cloud water/ice


-1 60

-1 80

-200 E

5 -240

g -220

-260

-280

-300 9dN 60*N 30% d 30dS 6dS 9C

Latitude (deg) 400 I

n

"E e 150 - 100-

50 - 0 - 90°N 60°N 30% 0" 30's 60% 9C

Latitude (deg)

'S

S

Figure 8. Zonal mean top of atmosphere: (a) outgoing long-wave radiation; and (b) solar flux (W m-*) from T63L31 simulations from 1 May 1987 using the control (dashed) and revised (dotted) radiation schemes,

compared to Earth Radiation Budget Experiment measurements (solid).

paths and larger ice-effective radius. However the increased ice-effective radius leads to worse results in the summer hemisphere due to a lowering of the brightness of ice clouds.

Combining the revised convection scheme and new radiation scheme moderates the impact on temperature and ToA radiation budget, especially for the temperature field above 400 hPa (not shown). However cooling in the upper levels of mid latitudes remained. While this had little impact upon seasonal simulations, at T213 forecast performance was degraded due to an increase in eddy kinetic energy in the mid-latitude storm tracks as baroclinic eddy activity responded to decreased stability. This led to the search for other physically based modifications which would further balance the impact of the revised radiation.

(c) Cloud Previous comparisons to observations (CEPEX*, Jakob and Morcrette 1996) have

shown that the cloud-ice content of the model using the original ice fallout formulation are too small by up to a factor of 2 or more. This is supported by Rizzi and Jakob

* Central Equatorial Pacific Experiment.

REVISED SCHEMES IN THE ECMWF IFS 1 699

150

- Old Ice 1

Figure 9. Zonal mean cloud-ice path (g m-*) for JundJulylAugust 1987 from T63L31 simulations starting from 1 May 1977 with control ice fallout (solid) and revised scheme (dashed).

LATITUDE

Figure 10. Difference in zonal mean temperature (K) for JundJulylAugust 1987 between T6 simulations starting from 1 May 1987 with the revised and control ice fallout treatment. The contour interval is 0.5 K.

(1996) who compared OLR from short-range forecasts with satellite observations. The revised formulation gives substantial increases in ice-water path (Fig. 9-ZOnd mean for JJA87 from a simulation starting 1 May 1987). Such ‘global’ values are difficult to validate due to lack of observational data. Lin and Rossow (1996) have attempted to retrieve ice-water paths for non-precipitating clouds from ISCCP* data combined with SSM/It observations. They found a systematic difference in ice-water path between mid latitudes of the summer and winter hemispheres, with values in the range 50- 100 g m-2 in the summer hemisphere and 80-120 g m-2 in the winter hemisphere. Although the zonal mean includes all ice cloud (precipitating and non-precipitating) the variation between the two hemispheres is captured by the model, the new ice fallout scheme giving ice-water paths within the range quoted. In the tropics values with the new scheme are larger than in Lin and Rossow (50-100 g m-2), although here the model values include contributions from precipitating cirrus associated with deep convection, and so the validity of the comparison is questionable. Klein and Morcrette (1997), using * International Satellite Cloud Climatology Project.

Special Sensor Microwavehnager.


&N 60'N 30'N 0" 30's 6@S 9( -308

Latitude (deg) 400 1

Latitude (deg)

S

S

Figure 1 1. Zonal mean top of atmosphere: (a) outgoing long-wave radiation; and (b) solar flux (W m-') from T63L31 simulations from 1 May 1987 using the control (dashed) and revised (dotted) ice fallout treatments,

compared with Earth Radiation Budget Experiment measurements (solid).

cirrus observations made during the FIRE* I1 to evaluate model performance, also found the larger ice-water contents with the new scheme were more realistic.

Increased cloud ice has a large impact upon the model due to its interaction with radiation. In the tropics upper-tropospheric temperatures increase by 2 K (Fig. 10- JJA87) associated with an increase in upper-level cloud cover and ice-water content. Warming is also found in northern mid latitudes, although in the southern hemisphere in the winter storm track (between 40 and 60"s) cooling is found through much of the troposphere, due to an increase in OLR which dominates increased solar absorption by clouds. Cooling in the lower stratosphere, just above cloud top, is typical of the signal expected for increased cirrus amounts in the upper troposphere as evidenced by idealized experiments by Slingo and Slingo (1988). The temperature change in the tropical troposphere increases model bias as measured against a climatology based on ERA, although as noted above the control radiation scheme gives anomalously high solar absorption in clouds, a consideration in introducing the original ice fallout formulation into the IFS. Increased cloud amounthe water brings better agreement between modelled OLR and ERBE measurements (Fig. I l(a)) in the tropics and mid

* First ISCCP Regional Experiment.


n

p" d a

a n

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3 u)

ti 1

S LATITUDE

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LATITUDE

Figure 12. Difference in zonal mean temperature (K) for June/July/August 1987 between ensembles of 125-day T63L31 simulations starting from 29 and 30 April and 1 May 1987 with: (a) control physics and ECMWF re- analysis; (b) revised physics and ECMWF re-analysis; and (c) revised physics minus control physics. The contour

interval is 0.5 K, dashed contours are negative.

latitudes of both hemispheres. ToA solar flux (Fig. 1 l(b)) is reduced in the tropics and northern hemisphere, and while this improves the realism of the simulation in the latter, in the tropics albedo is overestimated.

( d ) Combined package Each of the parametrization changes has considerable impact upon the seasonal

climate of the IFS, although individually each change does not necessarily lead to reductions in the mean bias of the model, due to errors in other components of the physics. However, a combination of the changes does reduce model errors. Results

1702 D. GREGORY et nl.

f

-308$" 6$N 30% d 30% 64s 9( Latitude (deg)

400 I

3 .c. 2004

9 100 v ) i!

50

0 98" 6$N 30% 0" 30% 66s 9(

Latitude (deg) Latitude (deg)

S

'S

Figure 13. Zonal mean top of atmosphere: (a) outgoing long-wave radiation and (b) solar flux (W m-2) from an ensemble of 125-day T63L3I simulations starting from 29 and 30 April and 1 May 1987 using the control (dashed) and revised (dotted) physics packages compared to Earth Radiation Budget Experiment measurements

(solid).

presented are for ensemble-mean representations of JJA87 carried out using Cy 16r2 of the IFS*. The impact of the combined changes upon the simulated precipitation field is similar to that seen with the convection change alone and is not considered further. It is rather the impacts upon the thermal structure of the model together with the ToA and surface energy budget, which play an important role in determining the accuracy of coupled ocean-atmosphere simulations, that are briefly considered. Although only simulations of JJA87 are considered, similar signals are found for DJF87/88. A more complete description of the impact of the combined package upon both seasons can be found in Gregory et al. (1998).

As discussed above, the revised radiation and cloud schemes have large and op- posite impacts upon the temperature field of the upper troposphere in the tropics. The increased cooling, associated with the revised continuum together with the removal of

* The model used here also incorporates a correction of the treatment of convective momentum transports (Gregory 1997). Experiments with the combined package include a change to the numerical treatment of the vertical diffusion scheme, which is called three times within a model time step, improving the accuracy of the surface drag coefficient. In 7213 forecasts with the IFS coupled to a wave model this gives improved performance but has no impact upon model climatology (Beljaars and Janssen, personal communication).


d. 3d.s 6d.s 8 Latitude (deg)

's

Figure 14. Zonal mean ocean surface net heat flux (W m-*) for JundJuly/August 1987 from an ensemble of 125-day T63L31 simulations starting from 29 and 30 April and 1 May 1987, using control (dashed) and revised

(dotted) physics package, compared to the climatology of da Silva (1994; solid).

the anomalous absorption in the SW scheme, is opposed by warming due to increased cloud and ice-water contents, resulting in reduced tropical tropospheric temperature biases (Fig. 12). A similar balance occurs in the upper troposphere of the northern and southern hemisphere mid latitudes, although in these regions the combined package gives slightly lower temperatures. At low levels the revised radiation scheme removes the cold bias seen in the control simulation in the tropics, subtropics and northern mid latitudes, although between 40 and 60"N a warm bias results. Cooling in the lower stratosphere of the tropics is a direct radiative effect of increased tropical cloud cover and ice-water contents. Overall the modelled temperature structure is improved, while the physical basis of the radiation and cloud schemes is more realistic.

Zonal mean OLR is improved with the combined package in both the tropics and mid latitudes (Fig. 13(a)). While changes to the radiation scheme alone lead to higher OLR in the tropics, the overall signal is dominated by the increased ice-water contents produced by the revised treatment of ice fallout (and also to a lesser extent by the convection scheme). Values along the ITCZ are slightly underestimated, but differences to ERBE are lower than with the control physics. Changes in the ToA incoming solar flux are smaller (Fig. 13(b)), with albedo errors being reduced south of the equator, a consequence of the cloud inhomogeneity factor introduced in the radiation scheme, although the size of the correction is lessened by increased high-cloud amounts caused by the revised ice fallout. In the northern hemisphere the ToA solar flux is larger than in the control physics, in worse agreement with ERBE. Although increased cloud amounts due to the revised ice fallout decrease the error in albedo here, the signal in the combined package is dominated by the temperature-dependent effective radius.

Changes to the ToA radiation budget also have an impact on the net surface heat flux over the oceans (Fig. 14). Between 30"N and 60"s the combined package gives a net ocean surface heat flux which is in closer agreement with the climatology of da Silva


c

Longitude

Figure 15. Drift in sea surface temperature (K), compared with observations, for the last month of a six-month forecast at T63 with the coupled ocean-atmosphere version of the Integrated Forecasting System, averaged over an ensemble of 24 forecasts starting from 1 January, 1 April, 1 July and 1 October for 1991 to 1996 using: (a)

control, and (b) revised physics package. Contour interval is 1 K.

et al. (1994); increases in surface heating come mainly from increased solar radiation at the surface together with a smaller contribution from a reduction in surface evaporation. This corrects a surface cooling bias in the control simulations, leading to increased SSTs in the tropics of coupled ocean-atmosphere simulations as seen in the following section. However, north of 60"N, due to reduced ToA albedo and so increased surface downward SW radiation, the overestimation of the net surface flux already present in the control simulation is increased.

( e ) Coupled ocean-atmosphere simulations Decreased errors in the tropical surface energy budget lead to improved SSTs in

simulations with the coupled ocean-atmosphere version of the IFS at T63. Figure 15 shows the annual mean drift in SST after 6 months for an ensemble of forecasts using the control (CY 151-8) and revised (at CY 16r4) packages. Errors are substantially reduced in the tropical Pacific, a cold bias of over 2 K on the equator is reduced to near 1 K. The latitudinal variation of the error is also flatter. Similar reductions in drift are seen in the Indian Ocean together with the North and South Atlantic.


However, substantial local errors still remain. In coastal regions strong gradients in SST are inadequately resolved by the Ocean model and little should be inferred about the atmospheric model fluxes in these regions. The SST in the coupled model is of course sensitive to ocean components as well as to the atmosphere, uncertainty in the ocean surface mixing contributing to large-scale errors seen in the coupled system. Although some of the error undoubtedly comes from the ocean model, it is thought that even in the case of the revised physics package the errors in the atmospheric fluxes are dominant, with warm biases increasing in high latitudes and low-level stratus regions in the subtropics.

In high latitudes a small part of the warm bias is due to inappropriate treatment of the sea ice boundary regions in the coupled model. However, this is not sufficient to explain the errors, and increased surface solar heating due to lower albedo in the mid latitudes plays a role in increasing model errors with the revised physics. As discussed above, the reduced albedo results from the use of the Ou and Liou (1995) ice-effective- radius treatment (despite increased ice-water paths), again illustrating the uncertainty in our present knowledge of ice clouds.

In low-level stratus regions of the subtropics an existing warm bias in the control model is increased, due to larger surface solar flux caused by accounting for inhomogeneity in estimating the cloud-water path. This has a flat longitudinal signal across the subtropics and, while reducing albedo errors in many areas in stratocumulus regions, where cloud cover is underestimated, albedo is further reduced. The warming of the sea surface in these regions is an undesirable feature which will have to be addressed in the future.

Although the mean bias of the SST in the coupled system is of importance, the goal of coupled modelling at ECMWF is seasonal forecasting. The ability of the coupled model to predict tropical SST anomalies, especially those associated with the El Niiio Southern Oscillation cycle in the equatorial Pacific, is crucial to this. In contrast with the use of the control and revised physics in forecasts of Pacific SST anomalies, many of the individual tropical forecasts are significantly altered, the difference in SST being more than the uncertainty inherent in the coupled system due to atmospheric chaos. Some of the forecasts are better, some are worse. The overall level of skill is fractionally higher with the new physics, but the difference is not significant. Given the small number of cases tested (24 start dates spread over 6 years) this is not surprising; only a dramatic improvement (or deterioration) could be detected reliably. It is hoped that more discerning tests of model changes can be made in the future, although the resources needed for thorough testing of a new model version are very large.

4. IMPACT OF CHANGES ON MEDIUM-RANGE FORECASTS

As well as seasonal forecasts, a major use of the IFS is to provide deterministic forecasts at high resolution out to 10 days. The combined physics package was tested over several periods during recent northern summers and winters as part of a assimilatiodforecast system using a 4D-Var analysis system (Rabier et al. 1998). Overall the impacts upon measures of forecast performance in the mid latitudes are generally neu- tral, although beneficial changes in the temperature structure of the tropics are clearly seen. Figure 16(a) shows the change to the temperature structure brought about by the physics package at T + 120 h for a series of forecasts from 16 January to 1 February 1997; the pattern of changes is similar to those seen in seasonal simulations, although only a third of the magnitude. Temperature biases of the upper and lower troposphere are reduced in the tropics and mid latitudes; the lower stratosphere is cooled, although

1706 D. GREGORY ef al.

LATITUDE

(b)

LATITUDE

Figure 16. Mean over 15 cases of the zonal mean day 5: (a) temperature difference (K), and (b) zonal wind difference (m s-'), between revised and control physics packages, from T213L3 1 four-dimensional variational data assimilatiodforecast experiments for the JanuaryRebruary 1997 period. Contour intervals are (a) 0.25 K,

(b) 0.5 m s-I .

this corrects a warm bias seen with the control physics. Zonal-mean winds (Fig. 16(b)) in the tropics are more westerly by 1 m s-', correcting an easterly bias in the control forecasts; once again the pattern of changes is similar to that seen in seasonal forecasts, where in DJF87/88 an easterly bias of 6 m s-' at 200 hPa between 20"N and the equator was halved (not shown).

5 . FURTHER DISCUSSION

Revised parametrizations and their impact upon the ECMWF IFS have been described. The switch from the original moisture convergence closure of the deep convection scheme to one based upon adjustment of CAPE towards zero on a specified time-scale, improves the distribution of precipitation in seasonal simulations. Although such a closure was first suggested in the context of cumulus parametrization within mesoscale models (Fritsch and Chappell 1980) it appears to work well at much coarser resolution. As the IFS is used across a wide variety of resolutions, even down to what may be considered to be a coarse mesoscale resolution (around 40 km), the switch to an instability-based closure may prove to be a better choice as the use of moisture convergence closures at high resolutions can be problematic. Kuo et al. (1998) report on the performance of several different convection schemes in simulations of a mid-latitude cyclone using the National Center for Atmospheric Research MM5 mesoscale model


at 20 km horizontal resolution; they found that a Kuo scheme allowed deep layers of saturated absolute instability to develop with the possibility of CISK* -like instabilities leading to grid point storms. With a Kain-Fritsch scheme (Kain and Fritsch 1993-a development of the Fritsch-Chappell scheme still using a CAPE adjustment closure) more realistic vertical structures were simulated which were less susceptible to CISK-type instabilities. Study of precipitation amounts from individual T213 forecasts (not shown) show that the new scheme gives smoother rainfall patterns in the tropics and mid latitudes, suggesting that the original moisture convergence closure reduced the horizontal- scale of resolved vertical motion. A similar effect is seen at T63 (Fig. 3) where peak rainfall amounts over India are reduced and the area of precipitation widened.

The radiation and ice-fallout changes both have a firmer physical basis and improve the thermal structure of the model in the upper and lower troposphere. The radiation changes include an improved continuum treatment and remove the anomalous absorption of solar radiation by cloud due to errors in the overlap of spectral bands in the SW part of the spectrum. The ice-fallout scheme is numerically more accurate, removing a dependency upon vertical resolution and giving higher ice-water contents which appear to be more realistic, although available observations are unreliable. The large degree of balance between the radiation and cloud changes, especially changes to the temperature of the upper troposphere, points to the need to maintain a parallel level of sophistication among parametrizations which are highly interactive, and once again brings to focus the continuing need for further observational studies to quantify the nature of cirrus clouds. Together, the new radiation and cloud schemes improve the ToA and surface energy budget, bringing some improvement to tropical SST errors predicted in coupled ocean- atmosphere simulations, although errors remain in subtropical coastal regions and high latitudes. With the limited testing available, no clear improvement is seen in the ability of the coupled version of the IFS to predict seasonal anomalies.

The main focus of this paper has been the impact of the revised physics upon the seasonal climate of the ECMWF model, with only limited emphasis placed upon improvements in medium-range forecast performance. However, the value of such deterministic tests in the development of the overall package should not be lost. As described above, while the radiation and convection changes together act to improve the tropical seasonal climate of the model, T213 forecast performance in mid latitudes was reduced (as measured by root-mean-square height and wind errors). This signal was not so apparent in the T63 simulations, perhaps because the scales involved in the development of baroclinic eddies are less well resolved; also, variability of the mid latitudes found in seasonal forecasts acts to mask any signal. The combined use of low-resolution seasonal forecasts with emphasis upon the tropics, and high- resolution medium-range forecasts to study mid-latitude sensitivity, appears to provide a practical method of evaluating the performance of parametrizations. Comments by Randall (1996) have also emphasized the constraint on parametrization development that medium-range forecasts can provide.

As noted in the introduction, the combined package of changes was introduced into the IFS during December 1997. Together with further changes to the cloud and convection schemes and increased vertical resolution (Jakob et al. 1999) which further correct biases in the surface radiation budget of the tropics, the combined package will form the basis of the atmospheric component of the parametrization package in a 40- year re-analysis using a 3D-Var version of the ECMWF data assimilation system and a revised operational seasonal-forecasting system.

* Conditional Instability of the Second Kind.


ACKNOWLEDGEMENTS

The authors acknowledge useful discussions with Martin Miller during the development and implementation of the revised physics package and the assistance of members of the data assimilation group at ECMWF in carrying out the 4D-Var experimentation. Coupled simulations were carried out by members of the Seasonal Forecasting Project group at ECMWF. Comments by Keith Shine, Julia Slingo and an anonymous reviewer which ‘reshaped’ an original version of the paper are gratefully acknowledged.

Brankovif, c. and Molteni, F.

Beljaars, A. C. M. and

Beljaars, A. C. M. and Viterbo, P. Holtslag, A. A. M.

da Silva, A. M., Young, C. C. and

Ebert, E. E. and Curry, J. A.

Edwards, J. M. and Slingo, A.

Levitus, S.

Fritsch, J. M. and Chappell, D. J.

Gregory, D.

Gregory, D. and Miller, M. J.

Gregory, D., Morcrette, J.-J., Jakob, C. and Beljaars, A.

Gunther, H., Lionello, P. and Janssen, P. A. E. M.

Huffman, G. J., Alder, R. F., Arkin, I?, Chang, A., Ferraro, R., Gruber, A,, Janowaik, J., McNab, A., Rudolf, B. and Schneider, U.

Jacob, C. and Morcrette, J. J.

JAob, C., Gregory, D. and Teixeria, J.

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Revision of convection, radiation and cloud schemes in the ECMWF IFS (2000)

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Transcript of Revision of convection, radiation and cloud schemes in the ECMWF IFS (2000)