Investigation of vertical profile of rain microstructure at Ahmedabad in Indian tropical region
Introduction
Rain is the most dominant impairment for the propagation of millimeter waves. Rain attenuation models are based on the properties of rain drops and interaction between rain drops and electromagnetic waves (Crane, 1996). The Drop Size Distribution (DSD) is an important parameter for calculation of rain attenuation. Rain attenuation is found to be different for different rain types of same rain intensity due to the characteristics change of rain DSD (Maitra and Chakravarty, 2005). It also depends on the vertical extent of rain up to rain height. The ITUR, 2005a, ITUR, 2005b is based on the simplified assumption of constant rain height derived from the zero degree isotherm height and a uniform vertical rain structure (Ajayi and Barbaliscia, 1990). These assumptions may not be valid for tropical regions and lead to unsatisfactory results.
It is well known that the rain characteristics are largely different in tropics from the temperate counterparts (Green, 2004). ITU-R recommendations are based on extensive study in temperate regions. The vertical structure of rain thus can give useful insight in the process peculiar to the tropical region. Vertical profiles of rain for different temperate regions have been reported using Doppler radar, Micro Rain Radar (MRR), etc. by different researchers (Clemens et al., 2006, Peters et al., 2006, Peter et al., 2002). But, there is a dearth of observations in the tropical region, especially for India (Kunhikrishnan et al., 2006, Cha et al., 2007). It is thus required to identify the actual vertical rain structure and test the assumptions for different rain types for this region.
Indian Space Research Organization (ISRO) is currently planning to conduct a “Ka Band Propagation Experiment” to estimate the rain attenuation at Ka band over India. For this purpose, a Ka band beacon transmitter will be sent with GSAT-4 satellite. Different instruments like Micro Rain Radar (MRR), Disdrometer and Raingauge are deployed at different locations over India to measure different meteorological parameters associated with rain. For the present study, vertical profile of rain has been observed with a vertically pointing MRR at Ahmedabad (23.06°N, 72.62°E). A Disdrometer is also used to measure the drop size at the ground level.
In this paper, we present some preliminary results based on a few case studies during monsoon periods of the year 2006. The possibility of rain classification using MRR is investigated and used in this study.
Vertical profiles of rain microstructures, such as, mean drop size, rain rate, liquid water content and average fall speed of the drops have been analysed here for different rain types from propagation point of view. This will provide better understanding and insight into the physical processes of rain attenuation. The DSD characteristics of MRR are also compared with that of ground based Disdrometer. The current study is an attempt to demonstrate the usefulness of such analysis for rain attenuation study in the tropical region.
Section snippets
Data collection
At Ahmedabad (23.06°N, 72.62°E), located in the western part of India, rain mainly occurs in the monsoon period (i.e. July–September). The vertical profiles of rain parameters are observed using a MRR. It has a temporal resolution of 30 s and vertical resolution of 200 m. The 200 m resolution is taken to accommodate the nearly complete profile of the rain up to 6 km over the Indian region. A Disdrometer, located adjacent to MRR, is used to collect DSD information at ground level. It also has an
Rain classification scheme
Classification of rain is an important research topic in radar meteorology. It is very useful for a large number of applications, from improvement of radar estimation of rainfall in remote sensing to rain attenuation estimation at higher frequencies. There are various methodology developed to discriminate the rain type. Houze (1993) proposed use of vertical air velocity and hydrometeor fall velocity for classification of rain since vertical air motion is small compared to the fall velocity of
Experimental observations
The time series of vertical reflectivity of MRR is studied for different rain events. Once the bright band is observed in the profile, a more in depth study in terms of different rain parameters is performed. The drop size distribution observed by MRR at 200 m level is then compared with the measurement taken by Disdrometer, providing a better understanding of the results as well as validity of MRR measurement.
The MRR data of the monsoon of 2006 reveal quite a number of events with the prominent
Discussions and conclusions
Information on rain microstructures of different types of rain is of practical importance for satellite communications at frequencies above 10 GHz. Using a MRR and a ground based Disdrometer observations some case studies of tropical rain over Ahmedabad are presented. From the MRR observation, rain is classified into three different types. These three types of rain are characterized by different microphysical parameters like DSD, fall velocity, LWC, rain rate and radar reflectivity. The
Acknowledgements
Authors are grateful to scientists of Space Applications Centre, ISRO to assist in maintaining the instrument and data collection. Authors are also sincerely thankful to Mr. Deval Mehta and Dr. K.S. Dasgupta, senior scientists, ISRO, for reviewing the work.
References (24)
- et al.
Prediction of attenuation due to rain: characteristics of the 0 °C isotherm in temperate and tropical climates
Int. J. Satell. Commun.
(1990) - Awaka, J, Iguchi, T., Okomoto, K. Early results on rain type classification by the tropical rainfall measuring mission...
- et al.
Estimation of the melting layer from a Micro Rain Radar (MRR) data at the Cloud Physics Observation System (CPOS) site at Daegwallyeong Weather Station
APJAS
(2007) - Clemens Marco, Peters, G., Seltmann, J., Winkler, P. Time–height evolution of measured raindrop size distributions, in:...
Electromagnetic Wave Propagation through Rain
(1996)- et al.
Long-term radar observations of the melting layer of precipitation and their interpretation
J. Atmos. Sci.
(1995) - et al.
Mesoscale air motions associated with a tropical squall line
Mon. Weather Rev.
(1982) Propagation impairment on Ka-band SATCOM links in tropical and equatorial regions
IEEE Antennas Propag. Mag.
(2004)- et al.
The terminal velocity of fall for water droplets in stagnant air
J. Meteorol.
(1949) Cloud Dynamics
(1993)
Cited by (52)
Study of melting layer features related to atmospheric parameters over a tropical location
2023, Advances in Space ResearchA study of rain drop size distributions and associated rain microphysical processes over a subtropical station in the Northeast India
2023, Journal of Atmospheric and Solar-Terrestrial PhysicsThe interseasonal features of precipitation microphysics over Thiruvananthapuram and Kolkata - the two tropical stations of Indian sub-continent
2021, Journal of Atmospheric and Solar-Terrestrial PhysicsInvestigation of vertical microphysical characteristics of precipitation under the action of low-frequency acoustic waves
2021, Atmospheric ResearchCitation Excerpt :Although understanding the vertical precipitation structure of precipitation particles is important, there has been little relevant research due to limitations of observation equipment. To address this issue, Ka-band vertical directional micro rain radar was recently designed to investigate the evolution of precipitation from near ground to high altitude using detected echo power (Das et al., 2010; Peters et al., 2005). It has attracted wide attention due to its easy portability, low cost, and high sensitivity to precipitation.