Sachin M. Deshpande

Microphysics of clouds and rain over the Western Ghat

In an attempt to unravel the interactions between cloud microphysics and dynamics that make shallow clouds precipitate heavily in this region, some unique observations of rain and cloud microphysical parameters are presented here from two stations, Pune and Mahabaleshwar, one each on the lee and windward sides, respectively, of the Western Ghat (WG) mountains in peninsular India. To elucidate rain microphysics, we used the raindrop size distribution (DSD) by fitting three parameter Gamma functions to the observed raindrop spectra. Over Pune, during stratiform rain with bright band (BB) at 0°C isotherm; concave upward DSD shapes are observed below the BB which becomes concave downward at lower altitudes. It is due to breakup process of large raindrops which increases drop concentration at midsizes suggesting coalescence, collision, and breakup processes. Both slope and intercept parameters of Gamma DSD decrease during no BB condition as altitudes decrease, signifying collision and coalescence processes. Over Mahabaleshwar, bimodal and monomodal DSD are observed during light and heavy rainfall, respectively. With shallow storm heights, small raindrops mainly contribute to both types of rainfall. The DSDs are parameterized, and their radar reflectivity factor-rainfall intensity relationships are evaluated suggesting the dominance of collision-coalescence processes. Aircraft measurements of orographic clouds over the WG suggest interaction of cloud mass with the ambient updraft speed. The orographically forced updrafts foster rapid condensational growth of cloud droplets triggering coalescence process within few hundred meters of cloud depth. Hence, these clouds are dynamically forced to produce precipitation over the WG.

Time evolution of monsoon low-level jet observed over an Indian tropical station during the peak monsoon period from high-resolution Doppler wind lidar measurements

Doppler wind lidar measurements of horizontal winds at an Indian tropical station, Mahbubnagar (16.73°N, 77.98°E, 445 m above mean sea level), were used to investigate the time evolution of the monsoon low-level jet (MLLJ) during the southwest monsoon season. Vertical profiles of zonal wind in the altitude range of 100 to 3000 m above surface (at every 50 m height interval and 5 min time averaged) obtained during the period 25 July to 23 August 2011 are considered for the analysis. The zonal winds in the altitude up to 3000 m above ground are predominantly westerly throughout the period and on almost all the days there is a westerly wind speed maximum around 500 m above ground during nighttime. Soon after local sunrise, the core of this wind speed maximum (jet) gets lifted up and by afternoon, the westerly wind maximum is shifted to a higher altitude of 2000 m–2500 m without much change in its magnitude. Analysis of the high-resolution lidar data strongly indicates that the same nocturnal LLJ seems to be moving up and evolving into a daytime westerly MLLJ reported in several previous studies. Wind speed and direction derived from the wind lidar agree reasonably well with simultaneously observed GPS upper air sounding wind measurements. Further analysis shows that the time-height evolution of the jet core is closely associated with daytime convection and boundary layer growth. The presence of clouds over the region seems to inhibit this type of time evolution.

Inter-comparison of wind profiles in the tropical boundary layer remotely sensed from GPS radiosonde and Doppler wind lidar

Winds play a very important role in the dynamics of the lower atmosphere, and there is a need to obtain vertical distribution of winds at high spatio-temporal resolution for various observational and modelling applications. Profiles of wind speed and direction obtained at two tropical Indian stations using a Doppler wind lidar during the Indian southwest monsoon season were inter-compared with those obtained simultaneously from GPS upper-air sounding (radiosonde). Mean wind speeds at Mahbubnagar (16.73° N, 77.98° E, 445 m above mean sea level) compare well in magnitude for the entire height range from 100 m to 2000 m. The mean difference in wind speed between the two techniques ranged from −0.81 m s−1 to +0.41 m s−1, and the standard deviation of wind speed differences ranged between 1.03 m s−1 and 1.95 m s−1. Wind direction by both techniques compared well up to about 1200 m height and then deviated slightly from each other at heights above, with a standard deviation in difference of 19°–48°. At Pune (18○32′ N, 73○51′ E, 559 m above mean sea level), wind speed by both techniques matched well throughout the altitude range, but with a constant difference of about 1 m s−1. The root mean square deviation in wind speed ranged from 1.0 to 1.6 m s−1 and that in wind direction from 20° to 45°. The bias and spread in both wind speed and direction for the two stations were computed and are discussed. The study shows that the inter-comparison of wind profiles obtained by the two independent techniques is very good under conditions of low wind speeds, and they show larger deviation when wind speeds are large, probably due the drift of the radiosonde balloon away from the location.

Statistical Characteristics of Convective Clouds over the Western Ghats Derived from Weather Radar Observations

X-band radar observations at Mandhardev (18.04°N, 73.85°E) are used to investigate statistics of convective clouds over the Western Ghats during monsoon season (June-September, 2014). Convective storms (cells) are identified using an objective-tracking method to examine their spatio-temporal variability, thus quantifying the time continuous aspects of convective cloud population over the region for the first time. An increased frequency of storm location and initiation along the windward mountains compared to coastal and lee side highlights orographic response to southwesterly flow, with superimposed diurnal cycle. An eastward progression of convective activity from upstream the barrier through windward slopes of mountains over to the lee side is observed. Storm area, height and duration follow lognormal distributions; wherein, small-sized storms contribute more to total population and unimodal distribution of 35-dBZ top heights (peaking at 5.5 km) depicts the dominance of shallow convection. Storms exhibit a pronounced diurnal cycle with a peak in afternoon hours, while the convective area maximum is delayed by several hours to that of precipitation flux. Cell lifetime and propagation show that cells move with slow speeds and have mean duration of 46 minutes. They align east-west nearly parallel to mountain ridges and their direction of movement is steered mostly by large-scale winds at lower levels. Based on top heights, convective cells are further classified into cumulus, congestus and deep clouds. In general, congestus (deep) cells are most abundant in the windward (leeward) side. A lead-lag relationship between congestus and deep cells indicates midtroposphere-moistening by congestus cells prior to deep convection.

The Assessment of Global Precipitation Measurement estimates over the Indian Subcontinent

Accurate and real-time precipitation estimation is a challenging task for current and future space-borne measurements, which is essential to understand the global hydrological cycle. Recently, the Global Precipitation Measurement (GPM) satellites were launched as a next-generation rainfall mission for observing the global precipitation characteristics. The purpose of the GPM is to enhance the spatio-temporal resolution of global precipitation. The main objective of the present study is to assess the rainfall products from the GPM, especially the Integrated Multi-satellitE Retrievals for the GPM (IMERG) data by comparing with the ground-based observations. The multi temporal scale evaluations of rainfall involving sub-daily, diurnal, monthly, and seasonal scales were performed over the Indian subcontinent. The comparison shows that the IMERG performed better than the TRMM-3B42, although both rainfall products underestimated the observed rainfall compared to the ground-based measurements. The analyses also reveal that the TRMM-3B42 and IMERG data sets are able to represent the large-scale monsoon rainfall spatial features, but are having region-specific biases. The IMERG shows significant improvement in low rainfall estimates compared to the TRMM-3B42 for selected regions. In the spatial distribution, the IMERG shows higher rain rates compared to the TRMM-3B42, due to its enhanced spatial and temporal resolution. Apart from this, the characteristics of raindrop size distribution (DSD) obtained from the GPM mission dual-frequency precipitation radar (DPR) is assessed over the complex mountain terrain site in the Western Ghats, India using the DSD measured by a Joss-Waldvogel disdrometer.

UHF wind profiler observations of monsoon low-level jet (MLLJ) and its association with rainfall over a tropical Indian station

High resolution data of horizontal winds profiles (zonal and meridional) in the lower troposphere, derived from a UHF wind profiler at a tropical Indian station, Pune (18 o 32’ N, 73 o 51’ E, 559 masl) during a 3-yr period (June 2003-May 2006) has been utilized to study seasonal and intra-seasonal variability of winds. Winds display a systematic seasonal evolution with behavior opposite in phase in the two altitude regimes below and above a height of 4-5 km. In the lower region, during the southwest monsoon months (June to September) winds are predominantly westerly with a peak in the 1.5-3.0 km range indicating the occurrence of the monsoon low-level jet (MLLJ). Soon after September, winds in this height region change from westerly to easterly and these easterlies continue in winter months (December to February). Above a height of 4 km, westerlies are observed during post-monsoon (October to November) and winter periods. The MLLJ is observed to be strong during normal/good monsoon years. On a day-to-day scale during southwest monsoon months, winds exhibit considerable intra-seasonal variability and periods of strong MLLJ seem to be associated with occurrence of spells of rainfall over the region.

Characteristics of precipitation over Western Ghat as observed by Doppler weather Radar and disdrometer

The structure of clouds during the convective and stratiform precipitation over the high-altitude tropical station, Mahabaleshwar, has been studied in the present paper by using Doppler weather Radar and several other in-situ measurements. The station, receiving an average annual rainfall of 5000 mm represents one of the important points of heavy precipitation belt of Western Ghats orographic region of India. It has been found that the region experiences a shallow convection with the cloud top height merely exceeding 6 km. The drop size of diameter 1–2 mm contributes more in the span of rain rate range of 2–4 mm/hr compared to other rain rate classes. Correspondingly, drops of larger diameter dominate the convective rain with respect to the stratiform one.