Abstract. To evaluate satellite rainfall estimates of Tropical Rain Measurement Mission (TRMM) level 3 output (3B42) (TRMM_3B42) over Iran (20°–45° N, 40°–65° E), we compared these data with high-resolution gridded precipitation datasets (0.25°×0.25° latitude/longitude) based on rain gauges (Iran Synoptic gauges Version 0902 (IS0902)). Spatial distribution of mean annual and mean seasonal rainfall in both IS0902 and TRMM_3B42 from 1998 to 2006 shows two main rainfall patterns along the Caspian Sea and over the Zagros Mountains. Scatter plots of annual average rainfall from IS0902 versus TRMM_3B42 for each 0.25°×0.25° grid cell over the entire country (25°–40° N, 45°–60° E), along the Caspian Sea (35°–40° N, 48°–56° E), and over the Zagros Mountains (28°–37° N, 46°–55° E) were derived. For the entire country, the Caspian Sea region, and the Zagros Mountains, TRMM_3B42 underestimates mean annual precipitation by 0.17, 0.39, and 0.15 mm day−1, respectively, and the mean annual rainfall spatial correlation coefficients are 0.77, 0.57, and 0.75, respectively. The mean annual precipitation temporal correlation coefficient for IS0902 and TRMM_3B42 is ~0.8 in the area along the Zagros Mountains, and ~0.6 in the Caspian Sea and desert regions.
Abstract:This investigation represents temporal and spatial analysis of drought events over Iran for a 42 -year period (1963 -2005). At first, the severity of drought occurrences over Iran has been identified using SPI (Standardized Precipitation Index) at various time scales (i.e., 3, 9, and 12 -month) for 50 synoptic stations. Then the spatial distribution of annual and seasonal drought severity using SPI -12 months and SPI -3 months respectively have been derived. Seasonal drought has been determined for spring, autumn, and winter using SPI -3 months for May, Nov and Feb respectively. Maps of annual and seasonal drought showed occurrence of drought in most parts of Iran mostly in following years, 1985, 1990, 1998, 1999, 2000, and 2001. Based on the frequency of each drought severity category at different time scales (SPI -3, 9, and 12 months), time series of number of stations experienced drought were derived. They showed that mild drought has been occurred continuously in about 85% -95% of stations from 1998 through 2001 using SPI -9 months and SPI -12 months. But SPI -3 months showed mostly 99% of stations were affected by drought continuously from 1998 through 2002. Finally, spatial distribution of each drought severity categories for different time -scales were derived.
In order to assess the performance of operational cloud seeding operations and achieve desirable results, application of numerical cloud model is one of the most important tools. In this paper, effects of silver iodide (AgI) cloud seeding on hail fall have been examined using one dimensional time dependent numerical cloud model. We included 32 microphysical processes with six water substances including water vapor, cloud droplet, cloud ice, rain, snow, and hail (graupel) for natural precipitation and 3 microphysical processes for cloud seeding parameterization. The model has been executed and it is concluded that as AgI introduced into the cloud resulting in graupel/hail mixing ratio reducing about %53. Heterogeneous nucleation of AgI enhanced the cloud ice, therefore snow increased and rainfall enhanced by melting of snow and cloud ice. On the other hand, cloud ice consumed to produce snow and they did not grow up to reach graupel/hail size, consequently the processes that related to growth of graupel/hail decreased strongly.
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