عنوان مقاله [English]
Precipitation is one of the most important component of climate type specification which has been attracted specialists in different fields. It also plays an important role in hydrological cycle and world climate and has application in various sectors such as; weather forecasting, environment, agriculture, water basin management, flood probability occurrence and climate change. Traditional methods which have been applied for precipitation measurement are based on the synoptic, climatological, and raingauge meteorological stations and have difficulties such as high expense, shortage of number of stations, and lack of raingauge over impassable regions. Since the prominent climate of Iran is dry and semi-arid, knowledge of amount and temporal variation of precipitation in each region could be essential for planning and management of surface water resource.
In this regard, precipitation estimation using TRMM satellite is one of the modern precipitation product approaches which have been considerably applied in meteorological studies now. The Tropical Rainfall Measuring Mission’s (TRMM) Microwave Imager (TMI) is a nine channel passive microwave sensor designed to provide quantitative rainfall information over a wide swath under the TRMM satellite. By carefully measuring the minute amounts of microwave energy emitted by the Earth and its atmosphere, TMI is able to quantify the water vapor, the cloud water, and the rainfall intensifying the atmosphere. The data used in this study are TRMM-TMI monthly products (3A-12 V6) which are global belt (40°S - 40°N) monthly average of surface rain rate (mmh-1), convective surface rain rate (mmh-1), stratiform surface rain rate (mmh-1) and 14 vertical layers (surface until 18 km above surface) hydrometeor contents (cloud liquid water (gm-3) precipitating water (gm-3), cloud ice (gm-3), precipitating ice (gm-3) and latent heat (degh-1) for 0.5 x 0.5 degree grids. In this research TRMM-TMI monthly data have been downloaded from the website
This paper is aimed at investigating and recognizing of monthly mean of convective and stratiform precipitation using TMI sensor of TRMM satellite data based on rate latent heat release during 13 years (1998-2010). Recognition of two precipitation regime associated with convective and stratiform clouds have not ever been carried out in the country due to their conflict mechanisms and considering of restricted existing observation stations. On the other hand, the best recognition of the two precipitation regimes has been presented by TRMM satellite and the other thing has not been replaced up to now. It should also be noted that in view point of cloud seeding feasibility study, convective clouds have higher potential for cloud seeding compared with stratiform clouds due to the more precipitable water of it. Therefore, detection and distinction of convective and stratiform precipitation type is very important and necessary. As a result, considering of the importance of precipitation regime which includes cloud type indirectly is one of essential indices in cloud seeding feasibility studies.
For this purpose, in this paper, monthly mean spatial distribution of two types of convective and stratiform precipitation have been drawn up for 12 months, from January through December using TRMM_TMI data processed by GrADS software under Linux Operating System. The results showed that high concentration of convective rainfall have been occurred over the high elevations of Northwest, middle of Zagros and Central Alborz respectively. The pattern of stratiform and convective rainfall are similar but the amount of stratiform rainfall is far less than convective rainfall and considering the seasonal condition, the difference of the maximum and minimum of stratiform and convective rainfalls have been observed about 100 to 200 mm per month respectively. Considering the separation of two precipitation types algorithm which is based on the evaporation latent heat mechanism, the precipitation which have had convective mechanism (including frontal, cyclonic, mountainous precipitation) all have been assumed convective. Therefore, more portion of precipitation belongs to the convective precipitation Maximum convective precipitation amount have been occurred in spring (March, April, and May) over west, northwest, and western coast of Caspian Sea. Since, the winter synoptic systems type is in transition to summer type in spring, accordingly, the air near the surface warms and atmospheric condition would be suitable for convection. The maximum of convective rainfall have been observed over northwest, and Kordestan’s elevation and central Zagros elevation for 8 months of year. The maximum of stratiform rainfall have also been occurred over high elevations of Zagros mountains. Due to the shortage of observation stations over high elevations and also their unsuitable dispersion, the satellite data could be able to complete the lack of data. However, they would be associated with some errors in some cases. For example, it could not be observed suitable estimation of precipitation over southwest region of Iran based on the TRMM data.