| Of the total rainfall received on earth's surface, a large amount is stratiform, which has a long lifespan, light to moderate intensity, and is horizontally widespread. Steady state solutions have been sought using both one- and two-dimensional numerical models to simulate rain evaporation in the lower levels of stratiform regions.;Two 1-D models are developed. The first one does not consider the vertical air velocity and uses prescribed steady temperature and moisture profiles. An analytical approximation of rain evaporation is derived for this case. Another more realistic 1-D model solves for the steady state vertical profiles of temperature, water vapor, and raindrop size distribution (RDSD) explicitly for a prescribed, steady, vertical air velocity profile.;Both of the 1-D models show that the RDSD changes systematically due to evaporation, especially at the small size end. The variations of RDSDs due to evaporation are applied to radar rainfall measurement. It is shown that the detection of the rain evaporation using the vertical profile of radar reflectivity (Z) is possible. However, when the environment is moist and/or rainfall rate (R) is high, vertical variations of radar reflectivity may fall below the detection threshold of the radar. Furthermore, the empirical Z-R relation used to estimate rainfall rate changes systematically with height because of the variation of RDSD. Significant errors may result if a fixed Z-R relation is used to estimate rainfall rate or rain evaporation amount.;The dynamical interactions between rain evaporation and mesoscale downdraft are studied using a 2-D model. The simulated magnitudes of downdraft and cooling rate due to rain evaporation agree well with observations. The 2-D simulations also reveal the details of the 2-D structures of the buoyancy force and pressure gradient force, which are not easily observable.;Sensitivity tests with the 2-D model show that rainfall rate is the most important parameter which determines the cooling and downdraft strength in stratiform region. The cooling and downward mass transport in stratiform regions over the tropical areas may be estimated fairly accurately from the knowledge of the duration, coverage, and rainfall rate of the stratiform rain. These can be easily observed by, for example, a space-borne radar. |
