Introducing the TRMM and Asfezari precipitation database: A comparative study

In the present study, the TRMM 3B43 precipitation database was validated as one of the TRMM products. This precipitation database estimates rainfall by applying both satellite observations and rain gauge data. This database utilizes two groups of data: microwave and infrared data. GPCC data are used in the database as supplementary data. The launch of the Tropical Rainfall Measuring Mission (TRMM) satellite in November 1997 by National Aeronautics and Space Administration (NASA) of the US and the Japanese Aerospace Exploration Agency (JAXA) provided more than fifteen years of quality rainfall data for tropical and subtropical rainfall studies. The TRMM3B43 precipitation output comprises 0.25×0.25 grid cells for every month with a spatial extent covering a global belt (180W to 180E) extending from 50S to 50 N latitude. In the present study, Asfezari and TRMM3B43 gridded precipitation databases were compared to each other from 1998 to 2004 that is the joint period of time between the two databases. Asfezari national database has been made using more than 1400 weather stations in Iran. In the Asfezari database the data of synoptic, climatology, and rain stations have been applied to construct it. It covers the period of 1961 to 2000. The correlation coefficient was 0.97 which showed a significant correspondence between Asfezari and TRMM 3B43. The bias of TRMM 3B43 was calculated on 2491 grid cells at a resolution of 0.25 * 0.25 geographical degrees of latitude and longitude for all the seasons. Results showed that there was not a considerable difference between the two databases in a large extent of Iran. Most of the bias has been calculated to be along the Alborze and Zagrous mountains. It seems that most faults of the satellite data are on highlands where the estimation of the rainfall is not as accurate as in other parts of the country. Overall findings showed that the satellite data underestimated the amount of rainfall on the central parts of Zagrous Mountains. The satellite data underestimate the rainfall between 5 to 2 mm in an annual timescale in this part of the country. In some parts of the Alborze and Zagrous mountains, there was an overestimation of precipitation by TRMM 3B43. The amount of this overestimation was between 9 to 25 mm in an annual timescale. In the other parts of the country, the bias of precipitation was -4 to +9 mm. It seems that this database has a fault in the estimation of precipitation that originates from the factor of mountains and the difficulty that the aforementioned satellite data has over highlands. It seems that it underestimates the precipitation over some highlands and overestimates it on some other highlands as well.