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Observation of a Small Hadley-Like Circulation Using Wind Profiler Radar

Dadang Subarna & Didi Satiadi

National Institute for Aeronautics and Space Jl. Dr. Junjunan 133, Bandung 40173

e-mail:dangsub@yahoo.com e-mail: satiadi@bdg.lapan.go.id

Abstract

Equatorial and tropical region exhibits unique characteristics due to relatively larger amount of insolation it receives, creating a low pressure of convergence zones, driving global circullation such as Hadley circullation. Near zero Corriolis force makes the equatorial regions governed dominantly by hydrotropic balance between gravity and buoyancy, generating stable equatorial waves and unstable atmospheric convection. The convection is inherently coupled with the circulation, powered by abundant moisture and heat energy, creating about two third of clouds and rain of the globe. These conditions create the unique atmospheric conditions, climate and ecosystem in the tropics.

Development of the Atmospheric Observation Station at Kototabang, West Sumatra, give an opportunity to investigate global atmospheric circulation in the equatorial region which is currently under researched. This station was developed by collaboration between the National Institute of Aeronautics and Space (LAPAN) and Research Institute for Sustainable Humanosphere, Kyoto University. Research has been conducted to investigate three dimensional structure of wind circulation using the Equatorial Atmosphere Radar (EAR), Boundary Layer Radar (BLR) and Sound Detection and Ranging (SODAR) installed at the Kototabang Station and the adjacent Global Atmospheric Watch Station. Five years data have been analyzed to give the statistical characteristic of wind circulation in three dimension. The results showed clearly the presence of a small hadley-like circulation observed by the radar. This circulation can be explained well by analyzing the sun prosecution relative to the equator. The circulation changed direction in April and November, which corresponded with the sun equinoxes. During these months rain peaked due to creation of convergence zones. The rain profile followed equatorial type with two peaks at April and November. The results show that climate at Kototabang is strongly governed by the solar radiation, which create convective system and local circulation coupled to global circulation. This also shows that the radars installed at Kototabang station are able to capture the circulation important in the study of the atmosphere in the equatorial regions. Keywords: Small Hadley Circulation, Equatorial Atmosphere Radar, Kototabang.

1 Introduction

Atmospheric dynamics in the equatorial region exhibit unique characteristics. With little corriolis force, this region is dominantly governed by hydrostropic balance between gravity and buoyancy forces, generating stable atmospheric equatorial waves and unstable atmospheric convections. This region also receives large amount of solar radiation energy, creating a band of low pressure convergence zone, driving the global Hadley Circullation. Atmospheric convections are very active in this region, fueled by abundant heat energy and moisture, producing almost 2/3 of clouds and rain of the world. In this region, the convective processes were highly coupled with the global circullation and waves. These conditions create unique atmospheric conditions, climate and ecosystem in the equatorial region.

The major driving force of atmospheric circulation is solar heating, which on average is largest near the equator and smallest at the poles. Heating at the equator causes air to rise and flow toward the poles in the upper troposphere. As the air cools it sinks and flows back toward the equator it drift to the west as the Tradewinds. The atmospheric circulation transports energy pole-wards, thus reducing the resulting equator-to-pole temperature contrast. The mechanisms by which this is accomplished differ in tropical and extra-tropical latitudes. Large-scale circulations develop in Earth's atmosphere due to uneven heating of its surface by the Sun's rays. Daytime solar heating is greatest near Earth's equator, where incoming sunlight is nearly vertical to the ground, and smallest near both poles, where sunlight arrives nearly horizontal to the ground. Near the poles, heat lost to space by radiation exceeds the heat gained from sunlight, so air near the poles is losing heat. Conversely, heat gained from sunlight near the equator exceeds heat losses, so air near the equator is gaining heat. The heated air near the equator expands and rises, while the cooled air near the poles contracts and sinks. The combination of these two processes sets up a general circulation pattern: air rises near the equator, flows

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north and south away from the equator at high altitudes, sinks near the poles, and flows back along the surface from both poles to the equator. This type of flow is called Hadley circulation after George Hadley, an English meteorologist of the eighteenth century, who first described the process.

The Hadley cell (HC) plays a pivotal role in the earth’s climate by transporting energy and angular momentum pole-ward and by organizing the three dimensional tropical atmospheric circulation. The tropical Hadley cell, by definition is the zonal mean meridional mass circulation in the atmosphere bounded roughly by 30 degree South and 30 degree North. It is Characterized by equator-ward mass transport by the prevailing trade wind in the lower troposphere, and pole-ward mass transport in the upper troposphere. This lateral mass circulation links the mean ascending motion in the equatorial zone with subsidence in the subtropics, and represents a major part of the large-scale meridional overturning between tropics and subtropics. The change of the Hadley cell as an expression of climate change associated with a warming trend in global mean surface temperature during last half-century (IPCC, 2001).The Hadley cells play a central role in climate. Among other things, they contribute to pole-ward energy transport, affect the tropical ocean circulation and control the creation of dry subsidence regions in the subtropics which strongly impact the planetary energy balance (Caballero, 2007). In this paper we will show observation of small Hadley Like Circulation using Wind Profiler Radar. Annual charateristics of horizontal winds have been investigated using the Equatorial Atmosphere Radar (EAR) installed at Kototabang Station. Five years of data have been analyzed to give statistical characteristics of wind circulation in three-dimensions. The results showed clearly the presence of a small Hadley-like circullation observed by the radar. This circullation were explained well by analyzing the sun position relative to the equator. The circullation changed direction in April and November, which corresponded to the sun equinoxes. During these periods, rain peaked due to the creation of a convergence zone. These results explained the equatorial type of rainfall observed in the area. Observation using the Boundary Layer Radar (BLR) and radiometer also confirmed these results.

2 Theory

The mean meridional circulation envisioned by Hadley (1735) and other authors cited in Lorenz’s (1967) historical review, consists of a pair of equator-to-pole, thermally direct cells, one in each hemisphere, symmetric about the equator (Dima, 2002). The ‘‘Hadley cell’’ appellation continues to be used for the tropical, annual mean meridional circulation dominated by the year-round rising motion over the equatorial belt and sinking motion over the subtropics (Glickman 2000), but the same term is also widely used to denote tropical MMC in general. Lindzen and Hou (1988, hereafter LH) drew attention to the cross-equatorial cell, characterizing it as a‘‘solsticial pattern,’’ as opposed to the classical, equatorially symmetric ‘‘equinoctial pattern.’’ Their theoretical analysis suggested that the distribution of mean meridional circulation is highly sensitive to small equatorial asymmetries in the imposed temperature of the underlying surface: so much so that the displacement of the thermal equator from the geodetic equator by even a few degrees of latitude would give rise to a solsticial (or equatorially asymmetric) circulation as strong or stronger than the classical, equatorially symmetric ‘‘Hadley cell.’’ Mindful of this sensitivity, they predicted that the solsticial component would dominate the tropical mean meridional circulation throughout most of the year and that is amplitude would exhibit more of a‘‘square wave’’ than a sinusoidal dependence on Julian day. Figure 1 shows a meridional cross-section of Hadley Circulation. Solar radiation creates a band of low pressure zone, where air rises in thunderstorms, and warmer stratosphere forces the air to move horizontally pole-wards and then sinks creating high pressure region. At the surface wind will flow from high to low pressure region, completing the circulation.

Figure 1 . A meridional cross-section of Hadley Circulation (source:http://geog.arizona.edu/)

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Figure 2 shows a three-dimensional view of the circulation. Coriolis force will turn the wind clock-wise in the northern hemisphere, and anti clock-wise in the southern hemisphere .(source:http://geog.arizona.edu/).

3 Data dan Method

The main instrument used was the Equatorial Atmosphere Radar (EAR), a large array of doppler wind profiler radar installed at Kototabang Station (100.32E, 0.20S, 856MSL). The radar gives three-dimensional structure of wind velocities, spectral width and echo power, with vertical range of 2-20km, vertical resolution of 150m and time resolution of 10 minutes. Other instruments used were an Optical Rain Gauge (ORG), a radiometer, and a Boundary Layer Radar (BLR) located at the nearby Global Atmospheric Watch (GAW) station.

The Kototabang Station is operated under close collaboration between National Institute of Aeronautics and Space (LAPAN) and the Research Institute for Sustainable Humanosphere (RISH) of Kyoto University, Japan. Figure 3, 4 and 5 shows respectively the photograph of the EAR, Optical Rain Gauge (ORG) and the radiometer used in this study.

Figure 3. Photograph of the Equatorial Atmospheric Radar (EAR) located at Kototabang West Sumatera Indonesia.

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Figure 4. The Optical Rain Gauge (ORG) located at Kototabang West Sumatera Indonesia

Figure 5. The radiometer located at Kototabang West Sumatera Indonesia

4 Results and Discussions

Figure 6 shows average zonal wind velocity against height and month observed by the EAR during year 2001—2006. Generally, in the upper region the wind went to the west, and in the lower region the wind went to the east. However, between April and November (when the sun was in the northern hemisphere) the wind speed increased, probably due to enhancement by monsoonal effect.

Figure 6. Average zonal wind velocity against height and month observed by EAR during year 2001-2006

Figure 7 shows average meridional wind velocity against height and month observed by EAR during year 2001—2006. Between April and November (when the sun was in the northern hemisphere), in the upper region the wind went to the south, and in the lower region the wind went to the north. Between November and April (when the sun was in the southern hemisphere), the opposite occured.

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Figure 7. Average meridional wind velocity against height and month observed by EAR during year 2001-2006

Figure 8 shows average rain rate against month recorded by the Optical Rain Gauge (ORG) during year 2002—2006. Rain peaked in April and November, when a convergence zone formed at Kototabang, which was correlated with the change of the circulation direction. This also explained the equatorial type of rain profile at Kototabang, which is mainly governed by the sun position and the corresponding atmospheric circulation generated.

Figure 8. Average rain rate against month recorded by optical rain gauge (ORG) during year 2002-2006 Figure 9 shows average downdrafts against height and month observed by the Boundary Layer Radar (BLR) during year 1998—2003. Strong downdraft wind occured around April and November due to high rainfall that occured from the melting layer.

Figure 9. Average downdraft wind velocity against height and month observed by Boundary Layer radar (BLR) during year 1998-2003

Figure 10 shows average atmospheric density against height and month observed by the radiometer during year 2002—2004. Relatively higher density occurred in April and November.

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Figure 10. Average atmospheric density against height and month observed by Radiometer (RDM) during year 2002-

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The observed phenomena described above can be explained using the circulation diagram in figure 11.

Figure 11. Wind circulation diagram with respect to the sun position and location of Kototabang Stasion

In this diagram, the arrow represents wind circullation, the yellow circle represents the sun position, EQ is the equatorial line, KT is Kototabang location, NH and SH are northern and southern hemisphere respectively. Those results are consistent with the other scientist research as shown figure 12.

Fi gure 1 2. Annual mean streamfunction field based on the NCEP– NCAR reanalysis and (b) the scaled monthly contribution of the mean meridional circulation to the annual mean. The contour interval in (a) is 1010 kg s21 (Dima, at all 2003) The annual-mean streamfunction shown in figure. 12 is dominated by the equatorially symmetric component, with ascent in the tropical belt and subsidence in the subtropics. Upon close inspection it is evident that the axis of symmetry is not precisely on the equator, but a few degrees to the north of it, and that there is a particularly strong meridional gradient of the streamfunction across 78N, which corresponds to the mean latitude of the intertropical convergence zone over the Pacific and Atlantic.

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5 Conclusions

In this study, we observed a small Hadley-like circulation using the Equatorial Atmosphere Radar (EAR) installed at the Kototabang Station. The circulation can be explained clearly by considering the sun position with respect to the station location and the corresponding generated wind circulations. The circulation changed direction in April and November, creating a convergence zone and rainfall peak observed at Kototabang during those months. This explained the equatorial type of rainfall profile known for the region by using wind circulation analysis. These results also indicated that climate in the region is strongly governed by the incoming solar radiation, energy, which created local convective systems , and probably coupled with large-scale circulations. The results also showed that the Equatorial Atmosphere Radar (EAR) installed at Kototabang Station is able to capture the wind circulations, which is important in the study of the atmosphere in the equatorial region.

Acknowledgments. Thanks to the CPEA team, RISH, Kyoto University, for the collaboration, data provision, and support.

6 References

Caballero, R 2007: Geophyiscal Research Abstracts., 9, 10762 Dima, I. M., and Wallace, J.M, 2003: On the Seasonality of the Hadley Cell. Journal of the .Atmospherics. Sciences., 60, 1522–1527 http://geog.arizona.edu/~comrie/geog430/17gencirc/gencirc.htm Lindzen, R. S., and A. Y. Hou, 1988: Hadley circulations for zonally averaged heating centered off the equator. Journal of the .Atmospherics. Sciences., 45, 2416–2427.