Elsevier

Advances in Space Research

Volume 45, Issue 10, 17 May 2010, Pages 1235-1243
Advances in Space Research

Investigation of vertical profile of rain microstructure at Ahmedabad in Indian tropical region

https://doi.org/10.1016/j.asr.2010.01.001Get rights and content

Abstract

The microstructure of rain has been studied with observations using a vertical looking Micro Rain Radar (MRR) at Ahmedabad (23.06°N, 72.62°E), a tropical location in the Indian region. The rain height, derived from the bright band signature of melting layer of radar reflectivity profile, is found to be variable between the heights 4600 m and 5200 m. The change in the nature of rain, classified on the basis of radar reflectivity, is also observed through the MRR. It has been found that there are three types of rain, namely, convective, mixed and stratiform rain, prevailing with different vertical rain microstructures, such as, Drop Size Distribution (DSD), mean drop size, rain rate, liquid water content and average fall speed of the drops at different heights. It is observed that the vertical DSD profile is more inhomogeneous for mixed and stratiform type rain than for convective type rain. It is also found that the large number of drops of size <0.5 mm is present in convective rain whereas in stratiform rain, drops concentration is appreciable up to 1 mm. A comparison of measurements taken by ground based Disdrometer and that from the 200 m level obtained from MRR shows good agreement for rain rate and DSD at smaller rain rate values. The results may be useful for understanding rain structures over this 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)

  • G.O. Ajayi 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...
  • Joo-Wan Cha 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:...
  • R.K. Crane

    Electromagnetic Wave Propagation through Rain

    (1996)
  • F. Fabry et al.

    Long-term radar observations of the melting layer of precipitation and their interpretation

    J. Atmos. Sci.

    (1995)
  • J.F. Gamache et al.

    Mesoscale air motions associated with a tropical squall line

    Mon. Weather Rev.

    (1982)
  • H.E. Green

    Propagation impairment on Ka-band SATCOM links in tropical and equatorial regions

    IEEE Antennas Propag. Mag.

    (2004)
  • R. Gunn et al.

    The terminal velocity of fall for water droplets in stagnant air

    J. Meteorol.

    (1949)
  • R.A. Houze

    Cloud Dynamics

    (1993)
  • International Telecommunication Union Recommendation P 838-3. Specific attenuation model for rain for use in prediction...
  • International Telecommunication Union Recommendation P 893-3. Rain height model for prediction methods, Propagation in...
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