Spatio-temporal characteristics of precipitating clouds and rainfall over the Western Ghats of India

Abstract

One of the heaviest rainfall receiving regions during the Indian summer monsoon season is anchored around the orographic barriers of the Western Ghats (WG) in peninsular India. The high rainfall is attributed to the forced ascent of the moisture-laden air in the low-level jet that impinges the WG mountains almost orthogonally. Most of the region is forest covered and not accessible by road, and therefore, in situ (i.e., rain gauge) observations are very sparse. The spatial distribution of WG rainfall is primarily inferred from satellite observations and model analyses. In the past studies, the position of rainfall maximum with respect to the mountain peak is dependent on data spatial resolution: coarse resolution data show peak nearer to the coastline, whereas, finer resolution (5 to 10 km) data suggest the peak to be closer to or over the mountain. Given the terrain complexity, high-resolution observations and modelling are required to understand the true nature of spatio-temporal characteristics of clouds and rainfall over this region.

This thesis addresses major knowledge gaps over the WG region. These include the absence of high-resolution rainfall maps at <5 km resolution, limited knowledge of temporal characteristics of rainfall microphysics, poorly documented vertical structure of precipitating clouds across the WG, and insufficient understanding of orography-dynamics interactions. Moreover, very few model sensitivity experiments have been conducted to understand these interactions and the impact on orographic rainfall.

The study uses X-band (9.535 GHz) Doppler weather radar installed by the Indian Institute of Tropical Meteorology (IITM) Pune at Mandhardev (18.04° N, 73.87° E, 1.3 km AMSL), a high-altitude location in the WG. PPI (plan position indicator) data from 2018 and RHI (range height indicator) data collected during 2017-2018 have been used in the thesis work. A high-altitude cloud physics laboratory (HACPL) is operated by IITM at Mahabaleshwar (17.92°N, 73.66°E, 1.38 km AMSL), located ~25 km from the radar site. The HACPL has a suite of advanced instruments including a Joss–Waldvogel disdrometer (JWD, model RD-80), a 2-dimensional video disdrometer (2DVD), an automatic weather station (AWS) and a rain gauge. These data have been utilized in the thesis work. In addition, I used Ku-band precipitation radar data (KuPR) of GPM, Shuttle Radar Topography Mission (SRTM) Global 30 arc second (~ 1 km resolution) data for orography, rainfall products from Integrated Multi-satellite Retrievals for GPM (IMERG) L3 daily data (version 6) and Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) 2A25 data (version 7). Surface winds and boundary layer height (BLH) data are taken from hourly ERA5 reanalysis data of the European Centre for Medium-Range Weather Forecasts (ECMWF). An important work in this thesis is the quality control of the radar reflectivity factor (Z) which is used in deriving the spatial distribution of rainfall at 1 km x 1 km horizontal resolution. The original Z was corrected for the bias and the corrected Z is considered in rainfall estimation using appropriate Z-R relations (R is rain rate).

In the first part of thesis, I study microphysical properties of rain events of different durations along with their diurnal variations, primarily focusing on the pre-monsoon and summer monsoon seasons. Main findings are the following. Durations of rain events span from 0.1 to 160 h. Rain rate (RR), mass weighted drop diameter (Dm) and precipitation liquid water content (LWC) decrease with duration up to 1 h, remain steady between 1 and 20 h, and then increase for larger durations. Drop number concentration (NT) shows an increasing trend with duration up to around 10 h and then decreases. Rain events are categorized into short- (< 10 h) and long-duration (> 10 h) events based on rain drop characteristics. The long-duration events account for ~ 15% of the total number of events and contribute ~ 75% to the monsoon rainfall. These events are strongly linked to the large-scale monsoon circulation and dominate during the active phase of monsoon. Radar Z profiles show that shallow clouds dominate all events but frequency of deep clouds is higher in short events. Next, radar-derived rainfall distribution at 1 km × 1 km spatial resolution is presented during the summer monsoon season of 2018. Rainfall characteristics (rainfall amount, rainfall frequency, and rainfall rate) are reported. The analysis reveals the exact position of rainfall maxima across the WG. Spatial distribution is not uniform along the Ghats and pockets of heavy rainfall are observed. A rainfall maximum zone (RMZ) with seasonal rainfall exceeding 7000 mm is observed. Within RMZ, rain rates are moderate but rain frequency is very high. Strong diurnal variation is observed in rainfall which peaks during 12-18 h local time. Diurnal changes in thermodynamic instability, wind speed, wind direction and atmospheric boundary layer height are the main factors governing the afternoon peak in rainfall amount. The position of rainfall maxima is strongly tied to the location of maximum slope rather than peak elevation of the local mountain. Observations suggest mountain geometry, wind orientation and diurnal heating are important in deciding the spatial distribution of rainfall over the Western Ghats. A hypothesis is formulated that heavy rainfall over RMZ is mainly due to steep slope and winds striking the barrier at more perpendicular angles.

The latter half of the thesis investigates the vertical structure of precipitating clouds influenced by orographic features as they move across the WG. Radar observed 18-dBZ echo top height (ETH) shows higher values over the low-lying slopes, reaches minimum near summit, and increases in lee side. The maxima of ETH and rainfall are not coincident and ETH maximum occurs ahead of the maximum rainfall. ETH shows several highs and lows across the WG which may be linked to gravity waves.

The final part of the thesis describes WRF-ARW model simulations. To test the proposed terrain slope-wind angle hypothesis, three numerical experiments have been conducted using the Advanced Weather Research and Forecasting (WRF-ARW) model with 3 levels of nested domains (27 km, 9 km and 3 km) and initialised with ERA5 data. The WRF was run for three cases, (1) CTL run with the original topography, (2) EXP1 run in which the terrain is smoothed to understand the impact of terrain slope, and (3) EXP2 in which the terrain is rotated to modify the angle at which winds strike the mountain barrier. WRF simulations showed rainfall concentration over the elevated slopes and away from the summit of the WG, in agreement with the radar observations. The sensitivity experiments suggest that accumulated rainfall decreases when the terrain is smoothened or when winds strike the terrain at less perpendicular angles. Peak rainfall decreases substantially (up to 50 %) as slopes become gentle, especially in areas of heavy rainfall. In addition, moisture convergence and orographic lift also decreases along the steep slopes with smoothing of slopes. There is also a decrease in height and strength of updrafts along with reduction in moisture lifting over the windward slopes of the WG. On the other hand, EXP2 simulations show northward shift of rainfall maxima and a redistribution of rainfall is seen in the region. Findings suggest that changes in terrain orientation can significantly alter the incident angle of winds, moisture transport pathways, and areas of moisture flux convergence. The key conclusion is that the spatial distribution of rainfall over the Western Ghats results mainly from the combined influence of terrain slope and the angle at which winds strike the mountains, with the latter playing a dominant role in the regions of maximum rainfall.