C. Suresh Raju

Microwave Radiometric Observation of a Waterspout Over Coastal Arabian Sea

This letter discusses the background thermodynamic conditions of a convective cloud during the occurrence of a waterspout. This study is conducted using a very unique experimental observation of a ground-based multifrequency microwave radiometer which was set to scan the atmosphere in seven elevation angles. The spatio-temporal variations of the cloud microphysical parameters during the evolution of a multicell convective cumulus system are studied. Humidity and temperature anomalies deduced from the radiometric observation could clearly explain the convective processes like the formation of an intense updraft of moist air, convective heating due to large latent heat energy release, and cooling of the lower atmosphere below 2-km altitude by the downdrafting dry air. The measurements from collocated IR radiometer, surface met sensors, and calculated CAPE showed the formation of an intense convection in a humid warm atmosphere over a shallow warm ocean (conducive to formation of a waterspout). Studies on the evolution of cloud parameters during the life cycle of convective precipitation are of great interest in weather forecasting.

Microwave radiometer observations of interannual water vapor variability and vertical structure over a tropical station

The intraseasonal and interannual characteristics and the vertical distribution of atmospheric water vapor from the tropical coastal station Thiruvananthapuram (TVM) located in the southwestern region of the Indian Peninsula are examined from continuous multiyear, multifrequency microwave radiometer profiler (MRP) measurements. The accuracy of MRP for precipitable water vapor (PWV) estimation, particularly during a prolonged monsoon period, has been demonstrated by comparing with the PWV derived from collocated GPS measurements based on regression model between PWV and GPS wet delay component which has been developed for TVM station. Large diurnal and intraseasonal variations of PWV are observed during winter and premonsoon seasons. There is large interannual PWV variability during premonsoon, owing to frequent local convection and summer thunderstorms. During monsoon period, low interannual PWV variability is attributed to the persistent wind from the ocean which brings moisture to this coastal station. However, significant interannual humidity variability is seen at 2 to 6 km altitude, which is linked to the monsoon strength over the station. Prior to monsoon onset over the station, the specific humidity increases up to 5–10 g/kg in the altitude region above 5 km and remains consistently so throughout the active spells.

A Detailed Study of Land Surface Microwave Emissivity Over the Indian Subcontinent

Microwave emissivities of land surfaces on global basis have been derived using Special Sensor Microwave/Imager brightness temperature data. These derived emissivities are compared with other reported emissivity values to demonstrate the accuracy of the retrievals. Following these results, detailed analyses on the microwave emissivities of the Indian subcontinent are carried out using the monthly mean emissivity estimate for two years. The Indian subcontinent has a wide variety of geographic and biospheric classes with distinctly different emissivity characteristics. The spectral and monthly variations of microwave emissivity for different tropical land surface classes are examined. This study is significant for microwave radiance assimilation in weather forecast models and also for the utilization of the data from passive microwave sensors onboard the Indo-French satellite “Megha-Tropiques,” which is dedicated to tropical atmospheric studies.

Atmospheric Boundary Layer Characterization Using Multiyear Ground-Based Microwave Radiometric Observations Over a Tropical Coastal Station

The continuous ground-based microwave radiometer profiler (MRP) observations of lower atmospheric temperature and humidity profiles are used to investigate the diurnal evolution of atmospheric boundary layer height (BLH) over a tropical coastal station. The BLH estimated from the MRP observations is compared with concurrent and collocated measurements of mixing layer height using a Micropulse Lidar and the BLH derived from radiosonde ascends. The monthly mean diurnal variation of the BLH derived from the multiyear (2010–2013) MRP observations exhibits strong diurnal variation with the highest around the local afternoon (~12:00–15:00 IST) and the lowest during the nighttime (~100–200 m). The daytime convective BLH is maximum during the premonsoon season (March–May) with the peak value (~1300 m) occurring in April and minimum in the month of July (~600 m). This paper presents the potential of MRP observations to investigate the continuous diurnal evolution of the BLH over a tropical coastal region manifested by a thermal internal boundary layer (TIBL) at much better time resolution, which is essential for understanding the rapid growth of the boundary layer and the TIBL during the forenoon period.

Microwave emissivity of arid regions at 10 GHz – potential for subsurface studies

ABSTRACT The land surface microwave emissivity at 10 GHz over Australia is estimated using brightness temperature measurements from the Tropical Rainfall Measuring Mission Microwave Imager. The emissivity characteristics of dominant surface classes and its seasonality are analysed and reported, which is a vital information to study soil moisture and effect of precipitation. However, the deeper penetration of microwaves into dry soil media leads to the departure of effective radiating temperature from the skin (surface) temperature due to diurnal heating of the dry arid surfaces. The emissivity retrieved from satellite observations thus exhibits diurnal variability associated with the disparity between effective radiating temperature and skin temperature. The manuscript projects the utility of this diurnal variability to understand the thermo-physical properties of the subsurface.

Flood Extent Analysis Over the Major River Basins in the Indian Subcontinent Using Satellite Microwave Radiometric Data

Every year South Asia suffers from widespread floods along its major river basins, especially during the southwest monsoon season calling for planning, mitigation, and hazard management strategies. This study demonstrates the application of land surface microwave emissivity data in identifying and quantifying flooded areas. It employs an indigenously developed scheme based on microwave radiative transfer to retrieve emissivities at 19 GHz from satellite microwave radiometers and to estimate emissivity polarization index (EPI) from it. By assigning thresholds to the EPI for delineating inundated areas, this study examines the inter-annual variability of floods over the IndoGangetic plains for the period 2007-2010 and the cataclysmic flood of 2010 in Pakistan.

Tropical Convective Cloud Characterization Using Ground-Based Microwave Radiometric Observations

Characterization of the microphysical and thermodynamical properties of convective events over the tropical coastal station Thiruvananthapuram (TVM) has been carried out based on multiyear microwave radiometer profiler observations. The analyses have been extended to develop a methodology to identify convective events, which is based on the radiometric brightness temperature (Tb) difference threshold, at 30 and 22.23 GHz channels, and the results are compared with reflectivity and rainfall intensity deduced from concurrent and collocated disdrometer measurements. Eighty-four of such convections were identified using the aforementioned methodology over the station during 2010-2013, i.e., both for pre- and post-Indian summer monsoon months, and further evaluated by computing their stability indexes. The occurrence of convective systems peaks in the afternoon and early-morning hours with genesis, respectively, over the land and the sea.