3-DIMENSIONAL SINGLE DOPPLER RADAR ANALYSIS OF AN OCCLUDED-FRONT

IL NUOVO CIMENTO VOL. 12 C, N. 2 Marzo-Aprile 1989

Three-Dimensional Single Doppler Radar Analysis of an Occluded Front.

V. LEVIZZANI

Istituto FISBAT-C.N.R . - Via de' Castagnoli 1, 1-40126 Bologna

A. BOSCOLO BOSCOLETTO

Istituto di Geologia Applicata-C.N.R. - Corso Stati Uniti 4, 1-35100 Padova

F. PRODI

Istituto FISBAT-C.N.R . - Via de' Castagnoli 1, 1-40126 Bologna Dipartimento di Fisica deU'Universitd - Ferrara

(ricevuto il 14 Novembre 1988)

S u m m a r y . - - Single Doppler radar analysis has been applied to an occluded front associated with the last stages of cyclonic activity over the NorrkSping area in Central Sweden. Validation was conducted of the analysis methods, such as reflectivity contouring and wind calculation from Doppler radial velocity using velocity azimuth display (VAD). The structure of the cloud system was identified as composed of a stratocumulus cloud layer, horizontally divided into two sectors of high reflectivity (> 20 dBZ), oriented parallel to the surface occluded front, each extending around 80 km W-E and 50 km S-N. Embedded in these two precipitating areas were local regions of very high reflectivity (up to 50 dBZ), indicating the presence of scattered heavy precipitation and showers. The case study represents a test of the capabilities of single Doppler radar analysis techniques to be introduced in the next future into the operative forecasting environment in Italy.

PACS 92.60 - Meteorology.

1 . - I n t r o d u c t i o n .

The use of radars in the last th ree decades has great ly increased since the possibility to quanti tat ively est imate the amount of precipitation from

231

232 V. LEVIZZANI~ A. BOSCOLO BOSCOLETTO and F. PRODI

operational weather radars was first introduced (1.~). Recently weather radars have entered the operational forecasting environment as a first-hand tool for very short-range weather forecasting(36). Together with meteorological satellites, radars constitute the framework of Nowcasting systems, which are radically transforming the modern methodology of weather forecasting, due to their precision coupled with great space and time data coverage (7,8).

At the same time, improved radar capabilities have given new impulse to research concerning formation and evolution of precipitating systems (911). This has led to new insights into the internal structures of convective and frontal systems, especially when microphysical considerations have been included (1~18).

A single Doppler radar only measures the reflectivity field and the radial component of the wind. Although dual Doppler coverage is preferable, there is a great deal of weather features, essential for forecasters, that can be identified and studied with a single radar (17). Radars with Doppler capabilities are not yet operational in Italy for weather forecasts, but their installation is being considered for the near future. It has then been devised to prepare ourselves to the introduction of Doppler weather radars by acquiring and evaluating analysis techniques to be used with single Doppler radar data. In the following a case study of an occluded frontal area is presented, as an example of single Doppler radar analysis capabilities.

(1) L.J. BATTAN: Radar Meteorology (University of Chicago Press, Chicago, Ill., 1959). (2) D. ATLAS: Adv. Geophys., 20, 299 (1964). (8) A. BELLON, S. LOVE JOY and G. L. AUSTIN: Mort. Weather Rev., 108, 1554 (1980). (4) K. A. BROWNING and C. G. COLLIER: An integrated radar-satellite nowcasting system in the UK, in Nowcasting, edited by K. A. BROWNING (Academic Press, London, 1982). (5) S. BODIN: Blueprint for the future swedish weather service system in Nowcasting, edited by K. A. BROWNING (Academic Press, London, 1982). (6) D. W. REYNOLDS: Bull. Am. Meteorol. Soc., 64, 264 (1983). (7) K. A. BROWNING and B. W. GOLDING: Meteorot. Mag., 113, 302 (1984). (8) SMHI Promis Report No. 7: Annual Report 1986-87, edited by B. DAHLSTROM (Swedish Meteorological and Hydrological Institute, 1988). (9) p. V. HOBBS: Rev. Geophys. Space Phys., 16, 741 (1978). (lo) R. A. HOUZE jr. and P. V. HOBBS: Adv. Geophys., 24, 225 (1982). (11) Z. A. BROWNING: Contemp. Phys., 27, 499 (1986). (12) C. L. ZIEGLER: J. Atmos. Sci., 42, 1487 (1985). (13) C. L. ZIEGLER: J. Atmos. Sci., 45, 1072 (1988). (14) V. N. BRINGI, R. M. RASMUSSEN and J. VIVEKANANDAN: J. Atmos. Sci., 43, 2545 (1986). (1~) V. N. BRINGI, J. VIVEKANANDAN and J. D. TUTTLE: J. Atmos. Sci., 43, 2564 (1986). (16) R. M. RASMUSSEN and A. J. HEYMSFIELD: J. Atmos. Sci., 44, 2783 (1987). (17) j . WILSON, R. CARBONE, n. BAYNTON and R. SERAFIN: Bull. Am. Meteorol. Sou., 61, 1154 (1980).

THREE-DIMENSIONAL SINGLE DOPPLER RADAR ANALYSIS ETC. 233

2. - M e t e o r o l o g i c a l s i t u a t i o n .

The meteorological situation examined pertains to the passage of an occluded front over Southern and Central Sweden on October 26, 1986. The radar is located at SMHI (Swedish Meteorological and Hydrological Institute) in NorrkSping (58.36 ° N, 16.11 ° E), about 120 km Southwest of Stockholm.

From the 850 hPa chart of October 26 (fig. 1) at 00 UTC the synoptic situation was characterized by the presence of cyclonic activity centred above the North Sea, between the British Isles and Scandinavia. A low pressure of comparable intensity lies north of Iceland ( -68°N , 15°W) and an extensive high-pressure system drives the circulation above the Atlantic.

The low of the North Sea, in which we are interested, seems to be fading away under the influence of the Atlantic high and of the approaching large low, embedded in the polar circulation, which shows up around 60 ° N, 55 ° W. The next day this impression was confirmed: a large low-pressure system takes over in North Atlantic and the Scandinavian low under consideration is quickly disappearing.

On the other hand, the day before (October 25), the low pressure was centred above the British Isles and the surface chart showed a cold front approaching Scandinavia, already subject to some degree of occlusion. On October 26, the

Fig. 1. - 850hPa chart at 00 UTC on October 26, 1986 (Courtesy of Deutscher Wetterdienst).

234 v . LEVIZZANI, A. BOSCOLO BOSCOLETTO and F. PRODI

front is already completely occluded and at 12 UTC is passing over Stockholm, as is shown in fig. 2. The life cycle of the occlusion is essentially concluded before 12 UTC of October 27, when the surface analysis shows no traces of it.

The local situation in Norrk6ping at 8.35 UTC, the time the radar volume was obtained, was characterized by the presence of an occluded front in the last stages of evolution. As such, the stratiform clouds associated with the warm front were mixed with the cumuliform clouds of the cold front and stratocumulus clouds were observed both in G6teborg and Stockholm (see fig. 2) at 12 UTC. Rain was detected in G6teborg, which at this time was already in the post-frontal area.

• S t o c k h o L m

. g //-"//

225

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GSteborg A 8 O54

57 11

B 8 120

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Fig. 2. - Surface chart at 12 UTC on October 26, 1986 (left): the occluded front is indicated together with the radar site in NorrkSping (58.36 ° N, 16.11 ° E) and the locations of the nearby WMO meteorological stations of G6teborg (A), Stokholm (B) and Gotland Island (C). Wind profiles at 00 UTC for stations A and B are reported (centre). Station observations at 12 UTC are shown on the right.

3. - The instrument.

The radar system, developed by Ericsson Radio Systems, consists of a linear and logarithmic receiver, preprocessor, data processor, display system and a conventional C-band weather radar from EEC, WSR-81. The characteristics of the radar (1s,19) are as follows: 5.6 cm wavelength, 0.9 degree beam width, 480 km

(18) T. ANDERSSON and B. LINDSTR(]M: Preprints XXII Conference on Radar Meteorology, September 10-13, ZUrich (American Meteorological Society, 1984), p. 244. (~9) B. EKENGREN and L. DAHLBERG: Preprints XXII Conference on Radar Meteorology, September 10-13, Zi2rich (American Meteorological Society, 1984), p. 520.

T H R E E - D I M E N S I O N A L S I N G L E DOPPLER RADAR ANALYSIS ETC. 235

maximum range and 2 ~s pulse width in amplitude mode, 120 km maximum range and 0.5 ~s pulse width in Doppler mode, and 250 kW peak power. The pulse repetition frequencies are: 250 Hz in amplitude mode and 900/1200 Hz in Doppler mode. The system is controlled by a 32-bit minicomputer of the VAX series.

The sampling rate is 1.8 MHz corresponding to 83m range bins, with consequent 12 range samples per integrated cell. Prior range integration, ground clutter range bins as well as frequency channels containing background returns are removed. A correction for the attenuation is applied, together with an R 2 correction, where R is the range. The average wind speed and the spectrum width are estimated every I km. The total number of range cells is 120 covering the interval 0 - 120 km. The analysed radar volume in Doppler mode has an amplitude of (240 x 240x 12)kin 3, with a (1x l x 1)kin 3 grid cell resolution.

4. - R e f l e c t i v i t y a n a l y s i s .

The radar volume used for the present study refers to a single radar scan from NorrkSping on October 26, 1986 at 8.35 UTC. The radar volume, as indicated in the previous section, has the dimension of (240 x 240 x 12)km 3, with a cell

60

6

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0 --60 -40 -20 0 20 4.0 60

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Fig. 3. - Three-dimensional perspective of the entire reflectivity field. Values in excess of 20 dBZ are visible. The radar is located in (0, 0) and the distances are relative to it: along X positive means East, negative West and along Y positive means North, negative South.

16 - II Nuovo Cimento C.

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resolution of 1 km 3. The radar was operating in Doppler mode and the data were automatically processed in CAPPI format. The result is a radar volume formed by constant-elevation planes starting at 0.5 km and ending at 6.5 km, the height above which no more meaningful data were found for the present case, with an increment of 1 km. At each elevation, both reflectivity and Doppler radial velocity were measured at every grid point of the superimposed grid of (240 × 240)km 2. The radar is positioned in the centre.

The radar volume data were processed by means of the radar software package CEDRIC (Custom Editing and Display of Reduced Information in

T H R E E - D I M E N S I O N A L S I N G L E D O P P L E R R A D A R A N A L Y S I S ETC. 237

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Cartesian Space) developed at the National Center for Atmospheric Research (NCAR), Boulder, Colorado. This software package is a very flexible and powerful tool for the manipulation, editing, analysis and display of Cartesian- space radar data fields (20.2,). CEDRIC performs both single and multiple Doppler radar interactive analysis. As a brief overview of its capabilities, it is worth mentioning: editing of single data in the volume, algebraic manipulation of data, vertical integration, remapping with different sets of coordinates, derivation of radial velocities from u, v, w components, uniform advection and, very impor- tantly, a whole set of display functions ranging from scatter plots, to contouring, three-dimensional perspectives and vector maps.

A first look at the radar volume as is presented in fig. 3 shows the volume where reflectivity exceeded 20 dBZ and immediately reveals the stratified character of the cloud system. The cloud was well confined below 6 km of altitude and shows two distinct sectors located almost symmetrically North and South of the radar. There seemed to exist then two active areas, or distinct cells where rain was occurring.

(~) C. G. MOHR and L. J. MILLER: Preprints XXI Conference on Radar Meteorology September 19-23, Boston (American Meteorological Society, 1983), p. 569. (21) C. G. MOHR, L. J. MILLER, R. L. VAUGHAN and H. W. FRANK: J. Atmos. Oc. Technol., 3, 143 (1986).

238 V. LEVIZZANI, A. BOSCOLO BOSCOLETTO and F. PRODI

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l Yii . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . , , , , . . . . . . . . . . . . . . • : ' : ' : ' : ' : ' 1 " . - - . : ' : ' : ' : ' : ' : ' r : ' : ' ; ' . : ' : ' : ' : : ' • ' : ' " ( - . ' . ' : ' [ ' 2 " : ' . ' : ' • • " • • "

i. 0 $... ~:.;.:.:.:.:.:(i:r~"r-:¢+~+:~:~:+:U.zx+ i ~ ..':r:: :: ~:")~kil...i':':'~-i:i ........ +'. •. • .,. ~) .......... i r • • .:.:...'e:-:..., .I : .: :,:.:.:.1.:~:: ........ T ....... • .........

: ~ , : : : " i ' 2 : : : : : : " ! : ~ : : F i ' i ' " " i ' I + i ' i i i i ' i i i i i i i i : : * . . . . . . . . . . . . . . . " ~ . . . . . . . . . . . o. 5 I t ~ii!,,, :,, ~ i ~ , , .:~ ......................... + , + , , + , , . ~ i i , + , ::: :: ::,~:::: ::: :: :: ~ fill ~gil 0 , 0 I I I I I I I I ] I I I I I I I I I I I I I I I I I I I I I I I I I j ] I I ] I I I I I I I I I I I I I I I I 11

-60. -40. -20. O. 20. 4 0 . 6 0 . X ( k m )

x x x x x x x x x x xxxxx

: : : : : : : : : : : : : : : : : : : :

~15,0

~ . 0 ,.,....,,,

1 5 . 0

5.0

Fig. 6. - West-to-East vertical sections of the reflectivity volume; categories of reflectivity are as in fig. 5. a) Section at a distance of 30 km South of radar; b) section at a distance of 30 km North of radar.

V. LEVIZZANI, A. BOSCOLO BOSCOLETTO and F. PRODI

Fig. 7. - Horizontal section of the reflectivity volume as in fig. 4, obtained by means of Bologna Image Processing System (BIAS). The dimensions of the image are (240x240) km 2 and the areas of highest reflectivity are shown in orange (20 ~< Ze ~< 30 dBZ) and yellow (Z~/> 30 dBZ).

Fig. 8. - Horizontal section of the Doppler radial velocity field at 0.5 km obtained as in fig. 7. Different colours are directly related to the VAD profile at 0.5 km shown in fig. 9, clockwise from North.


This consideration is confirmed by the horizontal section of the reflectivity field at 0.5 km altitude in fig. 4. Contours at 20, 30 and 40 dBZ are plotted and the corresponding areas, where the reflectivity values lies in the three categories (20 < Ze < 30, 30 < Ze < 40 and Ze > 40) are enhanced by superimposing a grey scale. The two separate sectors of the cloud system, identified by the two high-reflectivity zone where precipitation was occurring, clearly show up, simmetrically centred about 30 km South and North of the radar along a South-North line drawn 30 km west of radar site. The horizontal sections at higher altitude (not shown) indicated that the system was most active below 4.5 kin.

In fig. 5 a vertical cross-section in the South-North direction is given, with more categories of reflectivity than in fig. 4, to better illustrate the cloud top and external outline. The two-cell structure is even more evident. Two successive vertical sections in the West-East direction across the two cells are shown in fig. 6. Figure 6a) is for the cell 30 km South and fig. 6b) for the cell 30 km North of radar. It is evident that the north cell is larger due to a low reflectivity tongue in its east sector having values between 20 and 30 dBZ (see also fig. 4).

Based on the reflectivity analysis, the cloud system was not clearly organized in a rainband structure, as described by Houze and Hobbs (10) for occlusions. Precipitation was concentrated in isolated spots, where local weak convection was occurring, surrounded by areas of drizzle. Apparently the system was rapidly approaching complete dissipation with the rainbands separated into scattered showers.

As a graphical aid for the description of the cloud system, fig. 7 shows the reflectivity field at 0.5 km altitude resulting from the image processing of the whole (240 × 240)km ~ field. The image processing was conducted by means of the Bologna Image Analysis System (BIAS)(22), first tested for meteorological imagery on our VAX Station II/GPX. The great advantages of image processing are to be found in the possibility of superimposing radar imagery onto the corresponding satellite imagery of the same meteorological situation. The cloud systems, as seen by radar, are related to the mesoscale and synoptic situation in which they are embedded, so that a better evaluation of their characteristics is possible when viewed in this superimposed perspective. Additional insight into the structure of precipitating systems can be inferred since information from outside the radar range becomes available for analysis. Moreover, using radar- derived precipitation estimates and comparing them with satellite brightness temperatures at the same location, it is possible to assign precipitation levels to portions of the cloud system outside the radar range, but covered by satellite imagery. These kinds of products demonstrated that usage of combined radar

(~) A. DI IORIO: B.I.A.S.-Bologna Image Analysis System-Version 1.1 (Digital Equipment Co. and Department of Astronomy, University of Bologna, 1987).

240

and satellite capabilities.

image


processing are especially important to Nowcasting

5. - Wind field analysis.

A very important piece of information that can be extracted from the Doppler spectra concerns the wind field. The use of pulsed Doppler radar for measurement of the horizontal component of the wind field was introduced by, among others, Lhermitte and Atlas (~), Browning and Wexler (2,), Locatelli and Hobbs (25) and used in many case studies, such as those of Testud et al. (2~) and Fukao et al. (~7).

The velocity azimuth display method (hereafter referred to as VAD) is founded on the expression of radial velocity measurements along a circle at constant elevation, obtained by slicing the radar cone at that elevation, in terms of a Fourier series. By means of the divergence values obtained from this analysis, the mass continuity equation is then integrated upward for extraction of vertical velocity profiles. This is based on the hypothesis that wind and precipitation fields are horizontally uniform, which is true if the wind field v(u, v, w) is assumed nearly linear within the measurement circle so that u, v and w components are well approximated by the zeroth- and first-order terms of a Taylor series about the radar location (x0, Y0, z0).

The radial component of the wind along the VAD measurement circle may be written as

( 1 ) V r ( ¢ ) = w s i n (0e) -~- V h COS (8 - - ¢ ) =

= U s in (¢) COS (0e) + V COS (¢) COS (0e) + W s in (0e) ,

where D = ~ + = is the direction from which the wind blows; w and vh are the vertical and horizontal component of the wind vector, respectively; 0e, the beam elevation angle; and ¢, the azimuth angle. By expanding (1) in a Taylor series, considering X and Y coordinates, since the measurement circle lies on a horizontal plane, and expressing them in terms of radar coordinates, an expression is obtained that reveals how the linear wind field contributes only to

(23) R. M. LHERMITTE and D. ATLAS: Proceedings Ninth Weather Radar Conference, Boston (Americal Meteorological Society, 1961), p. 218. (~4) K. A. BROWNING and R. WEXLER: J. Appl. Meteorol., 7, 105 (1968). (25) j . D. LOCATELLI and P. V. HOBBS: J. Appl. Meteorol., 17, 1076 (1978). (~) J. TESTUD, G. BREGER, P. AMAYENC, M. CHONG, B. NUTTEN and A. SAUVAGET: J. Atmos. Sci., 37, 78 (1980). (27) S. FUKAO, M. D. YAMANAKA, T. TSUDA and S. KATO: Mon. Weather Rev., 116, 281 (1988).


the fwst three components of a Fourier expansion of Vr onto the fundamental period 0 ~< ¢ ~< 2=. Therefore Vr becomes

(2) 1 Vr(¢) = ~ a0 + al cos (¢) + blsin (¢) + ae cos (2¢) + b2sin (2¢),

where an, b~ (n = 0, 1, 2) are the zeroth, first and second harmonic of the Fourier series. By expanding eq. (1) in Taylor series and comparing it with the Fourier expansion of eq. (2), the following physical quantities of the horizontal wind field are obtained: speed, direction, divergence and deformation.

The vertical component of the wind vector is calculated by means of vertical integration of the mass continuity equation and assuming no variation in time of the mass density (3p/3t = 0). The equation is expressed as

1 apw0 (3) div Vh + = 0,

~(Z) ~Z

where p(z) is the atmospheric density which is given by ~(z) = ~0 exp [ - z/H], with ~0 air density at ground level in (gm-3), z the height in (km) and H the scale height of the atmosphere in (km-1). The divergence of the horizontal wind, obtained as described before, is also given by

(4) div Vh = (ao -- 2W0 sin (Oe))/rcos2(Oe),

where a0 is the zeroth harmonic of Fourier series. We can then write

Ot~Wo (5)

3z = - ~(a0 - 2w0 sin (Oe))/rcos2(G),

which can be solved for the vertical wind component, starting from an estimate of w0 at some height z = Zl.

As has been previously noted, the first approach to wind field analysis starts with the acquisition of Doppler spectra. In fig. 8 a horizontal cross-section of the radial velocity at 0.5km altitude is shown, resulting from the BIAS image processing of the (240 × 240) km ~ field. The scanning radius was 120 km. The lower part represents the wind velocity directed toward the radar, with a maximum value of 16 ms -1, while the upper part represents wind receding from the radar, with a maximum of 13 ms -1.

In fig. 9, some of the VAD analysis results are presented. Radial wind velocity (when measurable) is plotted vs. azimuth angle for the whole set of altitudes between 0.5 and 5.5 km, keeping a constant VAD radius of 30 km. The observed VAD profiles are plotted together with those obtained using Fourier coefficients along the VAD circle (solid curves). Observed data show a nearly sinusoidal variation which does not change appreciably with altitude. The deviations of the experimental data from the theoretical curves are mainly

242 v. LEVIZZANI, A. BOSCOLO BOSCOLETTO and F. PRODI

70 t I [ I' I l [ ~ ! i I i [ i ! I i

t 60

50

E v

b 0 0

~-0

30

20

10

I

-10

-2o t o 100 200 300

azimuth (cLegrees)

Fig. 9. - VAD profiles v s . azimuth angle at a range of 30 km from radar at the different altitudes (0.5 to 5.5 km, with a step of I km). Experimental points (dotted) are superimposed to theoretical VAD profiles (solid). The radial velocity scale refers to the bottom curve (0.5 km), and the other curves have been shifted upward of 10 ms -1 each with respect to preceding-level curve.


0.4

~. 0

-0.4 0

-o.B

• ~ -1.2

,~ -1,6 ~ 180

~_. 17o C: ~ 60

.~ 150 140

~ ~6

14 f

' ~ 61 0

, , ~ 1 , , , , i , , , , i , , , , i , , , , i , , ~ , 1 ,

J

* , , l J , , , l , , , , l , , , ~ l , , , , l i , i i l i l l p l , , , , l J J l i l l i l i l , , 1 , 1

5 10 15 20 25 30 35 40 45 50 55 r ~ s ( k m )

Fig. 10. - Wind speed (lower), wind direction (centre) and divergence (upper) as derived from VAD analysis expressed v s . the radius of the VAD circles at an altitude of 0.5 km. The strong decreasing at the shortest ranges is mainly due to ground clutter.

attributable to nonuniformity of the particle's speed in the azimuth direction, at that distance from the radar.

Along many VAD measurement circles, a certain amount of data is often missing, which produces wrong estimates of the wind velocity. This happens, for example, when there is uncertainty in the radial velocity measurement because of low signal-to-noise ratios, or when precipitation is not present in the scan volume. When this happens it is necessary to extract the VAD profiles using some artifice to ensure a more precise measurement of the wind field. The procedure involves the generation of missing data by means of bilinear interpolation using the nearest neighbours of the missing data. The procedure continues with the comparison of the nearest VAD circles. In the case that it is impossible to generate missing data because there is little data on consecutive horizontal planes in the vicinity of the examined location, the VAD plot is approximated by a series of broken lines on the fundamental period 0 ~< ¢ ~< 2= using consecutive experimental points. Then, as was previously introduced, the pseudo-VAD circle is compared with the nearest real VAD circle and adjusted

244

B6/IO/2B {R$ OF 6/29/BB)

60,

40.

20.

-g

-20.

-40.

a)

B 35 O- 8 35 0 NORRK ORIGIN={ 0.00,

VRD-DERIVED WIND FIELD + REFLECTIVITY

\

\

\o\


Z= 1.50 KM REFLEC O.O0} KM X-RXIS= 90.0 DEG

4'

\

\

.t k~" '0' I

-60. -60. -40. -20. O. 20. 40. 60.

X(km) U-NIND ,V-WIND EVERY 6 PT. 20. m/s "

20. Os

30. O r,

40. Or.

Fig. 11. - VAD-derived horizontal section of the wind field overlayed onto reflectivity contours ((20, 30, 40)dBZ) at 1.5 km altitude, a) Wind vectors plotted every sixth point; b) streamlines obtained from the wind field of fig. lla).

accordingly. This correction regarding missing data along the VAD circle is particularly important for the wind measurement by single Doppler radar.

The curves of the wind speed, wind direction and divergence plotted vs. the radius of the azimuthally scanned circle at an altitude of 0.5 km are depicted in fig. 10. The apparent decrease and change of the wind characteristics at short range is mainly attributable to ground clutter(~). The mean value of the

(~) R. J. DOVIAK and D. S. ZRNI~: Doppler Radar and Weather Observations (Academic Press, Orlando, Fla., 1984).


BB/I0/26 8 35 O- 8 35 0 NORRK Z= 1.50 KM REFLEC {AS OF 7/20/88] ORIGIN={ 0.00, 0.00] KM X-AXl$= 90.0 DEG

VAD-DERIVED STREAMLINES + REFLECTIVITY BO.

40.

20.

O. v

-20.

-40.

-60. -60. -40. -20. O. 20. 40. 60.

X(km) U-WIND .V-WIND EVERY 6 PT.

20. Os 30. Oo 40. O,

Fig . 11. - (Continued).

horizontal velocity is estimated around (9.92 _+ 1.53) ms -1 at 0.5 km, increasing to (14.16+ 1.08)ms -1 at 4.5km, with errors increasing with distance from the radar. The direction from which the wind blows remains almost constant around 170 ° (clockwise from North), changing to 180 ° with increasing altitude. The divergence also shows the effects of the ground clutter at short range with values approaching zero from the negative side, consistent with the cyclonic circulation.

The wind analysis results (speed, direction and vertical profile) are in good agreement with the radio sounding wind data shown in fig. 2, particularly with the G~teborg sounding at 00 UTC. The Stockholm station at 00 UTC was not yet


86110126 (FIS OF 7111188}

S-N CROSS SECTION WIND + REFLEC 6 1 5 _ 1 i i i i i i i i i i i i i i i i i i i i i i i i 1 1 1 1 i i i I i i i i i i i i i i ] 1 1 i l l l I I I I I I I I

8 35 O- 8 35 0 NORRK X= -30.00 KM REFLEC ORIGIN={ 0.00, 0.00) KM X-RXIS= 90.0 DEG

6.0- 5.5-

5 . 0 -

4.5

4.0

a.s ~3.0

2.5

2.0

1.5

1.0

0.5

0.0 -60.

~ ~ ~ o F ~ ~ ~ ~ T ~ ~ ~ ~

_ / . ~ ~ r ~ - ~ - - ~ - - > - ' > - - > - - > . - > - > - > - >

I I I j I .

f, . ' - , II i , i / I o ' l i :: ', . . . . . . i l l

I I I ] I I I I ~ I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I i I I I I I I I I L :

-40. -20. O. 20. 40. 60. Y(km) V-HIND ,H-WIND EVERY 4 PT. 20. m/s

_ 2 0 . O s

~ 3 0 . 0 o

- - ~ O . O L

Fig. 12. - VAD-derived vertical section of the wind field overlayed onto reflectivity contours as in fig. 5. Vectors and reflectivity values are plotted every fourth point; a vector of magnitude 20 ms -1 is reported for reference (lower r ight corner).

reached by the front, so that comparison with radar-derived wind data is not appropriate.

VAD-derived horizontal wind vectors at an altitude of 1.5 km are plotted in fig. l la) , superimposed on the refiectivity contours at (20, 30, 40)dBZ (every sixth vector is plotted). In fig. llb), the same field is shown using streamlines to give an idea of the overall flow direction.

The integration of the mass continuity equation (5) allows the extraction of the vertical component, w, of the wind field. The analysis described above points out quite clearly the absence of strong updrafts or downdrafts within the stratocumulus clouds. Of course, the presence of local, small-scale convective structure (< 1 km) cannot be rejected since it is not detected with 1 km- resolution data. For calculation of the vertical component, a boundary condition was assumed of w0(zl)= 0ms -1, where Zl is the altitude of the radar site above the sea-level, and the equation was integrated upward. Figure 12 shows the same reflectivity plot of fig. 5 with the wind vector overlay plotted every 4th point. As is evident, vertical motions are essentially not detected and the flow is stratified.

A vertical profile of the mean wind velocity, separated into the three components, is given in fig. 13, confirming the stratification of the flow and its northward direction.


5.0

4.5

4.0

3.5

3.0 -/

~ 2.5

~ 2.0'

1.5

1.0

0.5

0 - 1 0

1 1 , 1 , 1 , l l l , l i l i l ' l ' l ' l ' l

tl

I

X

I I

I I

I

Y I I I I I

I I

I !

I

!

I !

I !

I I

I ! !

I I I i i I , I i I i I i I i I i I i I i I J I

- 8 - 6 - 4 --2 0 2 4 6 8 10 12 14 rnecLn w]ncL veLocity(m/s)

Fig. 13. - Vertical distribution of mean values of wind components u , v and w as derived from VAD analysis.

6 . - C o n c l u s i o n s .

A study of an occluded front passing over Norrk6ping, Sweden, on October 26, 1986 has been presented to demonstrate the capabilities of single Doppler radar analysis. Reflectivity analysis revealed a two-sector structure of the cloud system within the radar range. Each lobe showed high-reflectivity values (up to 50dBZ, limited to the first 1.5km), associated with areas where heavy precipitation was occurring. Horizontal and vertical sections of the radar volume clearly identified the cloud system as consisting of stratocumulus layers, in accordance with station observations.

VAD-derived wind fields characterized the system even more quantitatively, evidencing its stratified features. The wind field, calculated in its three components (u, v, w), was essentially uniform at all levels. Mean wind speed varied from 10 ms -1 at 0.5 km to 14 ms -1 at 4.5 km, while wind direction slowly rotated from 170 ° to 180 ° with increasing altitude.

Single Doppler radar analysis proves to be an effective technique for studying cloud systems when used in connection with conventional meteorological data. It


is highly recommended not only for research, but also for operational services, particularly for Nowcasting procedures. It offers the potential for greatly improved short-term forecasts to assist every kind of human activity in the nation (e.g., water reservoir management, agrometeorology, constructions, ...).

The present work has been funded by the Gruppo Nazionale Difesa Catastrofi Idrogeologiche (GNDCI) within the ,,Arno Project~) and by ,,Progetto Strategico Arno)~ of the Consiglio Nazionale delle Ricerche. The authors are greatly indebted to Ericsson Radio Company for having kindly provided the radar data and to Digital Equipment Co. and Dr. A. Di Iorio for the use of B.I.A.S. software package. Drs. R. M. Rasmussen and C. G. Mohr, at that time at NCAR, Boulder, Colorado, deserve all the gratitude for their suggestions in dealing with CEDRIC package. One of the authors (ABB) wishes to express his appreciation to Prof. S. Fattorelli of Dept. of Territory and Agro-Forestal Systems, Section of Water Resources and Soil Conservation, University of Padova and to Prof. C. Friz, Director of the Applied Geology Institute of C.N.R. in Padova (Italy) for their support during the research; moreover he is greatly indebted to Mrs. G. Gatto for the courtesy during his C.N.R. fellowship.

• RIASSUNTO

Tecniche di analisi dei dati da singolo radar Doppler meteorologico vengono applicate allo studio di un fronte occluso associato alla fase terminale di decadimento di un'area ciclonica sulla verticale di NorrkSping nel Sud della Svezia. Scopo principale dello studio ~ la validazione dei metodi di analisi dei dati, in particolare la mappatura dei campi di riflettivit~ e il calcolo dei campi di vento dalla velocit~ radiale Doppler utilizzando il velocity azimuth display (VAD). I1 sistema nuvoloso risulta costituito da stratocumuli ed suddiviso in orizzontale in due settori di elevata riflettivit~ (>20dBZ), orientati parallelamente al fronte occluso e ciascuno con un'estensione intorno a 80 km in direzione W-E e 50 km in direzione S-N. I volumi ad elevata riflettivit~ (fmo a 50 dBZ) rinvenuti all'interno di queste due zone di precipitazione rivelano la presenza di intense precipitazioni isolate e di acquazzoni. Lo studio costituisce un test delle potenzialita delle tecniche di analisi dei dati da singolo radar Doppler, il quale dovra essere quanto prima introdotto nel sistema di previsioni meteorologiche in Italia.

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3-DIMENSIONAL SINGLE DOPPLER RADAR ANALYSIS OF AN OCCLUDED-FRONT

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Transcript of 3-DIMENSIONAL SINGLE DOPPLER RADAR ANALYSIS OF AN OCCLUDED-FRONT