نشریه نیوار
پیاپی 58-59 (پاییز و زمستان 1384)

Changes in heavy and extreme precipitation and increase in frequency of extreme events such as flood and drought is one of the results of global warming. Based on recent report of Untied Nations Environmental Programme (UNEP), frequency of flood and other extreme climatic events are doubled and their hazards of them increase to $ 150 milion every year.These impacts will cause a major variation on society and ecosystem. Then in recent decades, survey on precipitation variation and other extremes such as snow, hail, heavy precipitation and ect. have a key role in policy of agricultural planning and water resources management. Therefore, countries have begun to study monthly and seasonal–scale climate variabilty and also variation of extreme values. Studies on monthly and annual precipitation have been done in Iran. National and regional investigation on extreme values of temperature and precipitation variation have begun on the basis of daily data and new extreme indices. In this paper, we try to introduce some of indices for precipitation and temperature and to investigate their variation in some stations. A few indices such as, no. of frost days (FD), Percent of time Tmin greater than 90th percentile of daily minimum temperature (Tn90), for Consecutive Dry Days (CDD), No. of days with precipitation equal or greater than 10 mm(R10), Maximum 5 day precipitation total (RX5day), Simple Daily Intensity Index (SDII) and Fraction of annual total precipitation due to events exceeding the 1961-90 95th percentile, are presented for 5 stations in 1951-2003. The result shows that the analyzed stations have significantly fewer frost days per year. There is a statistically significant increasing in warm nights but the result for precipitation indices shows a mixed pattern of positive and negative changes which are needed to be considered in agriculture planning and water resource management.

Solar declination angle is one of the most important geometrical characteristics of earth-sun system. In this study, we applied 13 of recent years formulae to compute solar declination angle for 5 selective years of 1970, 1980, 1986, 1989, and 2001. Algorithms of Michalsky (1988) and Walraven (1978), Bourges (1985) formulae have been found as most precise ones respectively. Michalsky (1988) as most precise one of those three shows maximum difference of 0.003 degree on 3 rd March 1980. Well known Spencer and WMO (1984) formulae rank 5th and 6th. These two are not recommended for very precise computation of solar coordinates, comprehensive study of solar radiation and applied purposes of solar energy. Spencer formula that uses Fourier expansion shows an error of about 1 degree. Cooper (1969) and Pisimanis (1987) formulae show highest error and rank 12th and 13th.

This paper presents preliminary results of the numerical simulation of weather systems over Iran using the mesoscale model of Eta and compares the results with that of MM5 modeling system. It is to be mentioned and stressed that the comparison made here is in no way comprehensive and does not show anything about the weaknesses or strengths of any of the two mentioned models. The experiments were designed to see the consistency of the Eta model outputs over Iran being compared with that of MM5 and the experiment was based upon using an almost arbitrary and already set up configuration of the models. In this case study, a very active weather system, which occurred over Iran and affected the region from 25 to 28 March 2003 was selected for simulation. Main outputs of the two models including mean sea level pressure, geopotential heights at 500 and 850 hPa levels and cumulative precipitation were both examined with a verifying analysis. In general a fairly good agreement and consistency of the Eta model results were seen. Results of simulations made by the two models for geopotential height at 850 hPa level showed a reasonable agreement among themselves and also with the verifying analysis. A 24-hour forecasts of precipitation with MM5 showed a fairly good agreement with the corresponding observation but in case of Eta a slight under prediction in the west and over prediction in the North of Iran is quite evident.

Convenient statistical post-processing approaches such as Perfect Prognosis Method (PPM), Model Output Statistics (MOS) and Kalman Filtering are introduced. Furthermore, by using PPM, surface minimum and maximum temperature prediction equations have been developed while measures and causes of errors have been recognized. Developing of these equations are based upon the linear regression analysis between 850-700 hPa and 700-500 hPa thickness as predictor (independent variable) and maximum temperature as predictand (response ariable), from the period 1986-1995. Obtained results express significant correlation between independent and response variables. In implementing of those regression models, thickness quantities from MM5 numerical model output have been used. But, due to the significant error of MM5 on predicting altitude of 850 hPa level over Tehran, one can not use 850-700 hPa thickness as predictor variable. By substituting 700-500 hPa thickness in computations, the difference between predicted and real values decreases highly.

Precipitation is an important element of atmosphere which has great influence on economy and life of people in every region which its insufficienty has much effect on agriculture, animal husbandry and industrial products. It causes a lot of damages and may destroy above sectors. Most parts of country of Iran are in desert, because of periodic distribution of precipitation, which caused, many regions and many provinces of Iran are drought prone. Drought is one of the phenomena that brings about tragedy, one of the ways for adoption to drought is its intensity and monitoring based on indices that determine its intensity and duration in a specific area. In this paper, we have studied and analyzed drought in Kohkiloie and BoyerAhmad province (intensity, duration, trend and area) using SPI, five time scales (3,6,12,24 and 48 months), percent of normal precipitation index and Z standard index were used. The results show that frequency of short range drought (3 months) in the selected stations has been 13 in the period 1983-2002 but that in long range droughts are lesser. The duration also in long range droughts (24 and 48 months) are longer (80 months).Therefore to return to normal status, hydrological droughts (long range) that related surface and sub-surface water need long time but for (short range) agricultural droughts, daily precipitation is compensated until to be normal. Severe droughts also in the years of 1993 and 1999 covered more than 80% surface area of the region.

Access to the daily records enables users to conduct many analyses that are not possible with just the use of 10 days totals or monthly totals. In this study dates of start of the rain are extracted by taking daily rainfall data as the basic unit at Karaj Agrometeorological Station during the period of 1970-1996 and summarize the records in ways that have direct applications in agricultural planning. Three alternative definitions for this date are considered in this paper which gain three statistical series. With application one, definition for date of end of the rain, we also got three series for length of the rains. Mean dates of start of the rain are calculated on 19 Nov, 7 Dec and 14 Dec respectively. With regard to sowing date of wheat in Karaj region and comparison the coefficients of variation in the series, the first definition was accepted. Fitting normal distribution to the climatologically series, provided us statistical forecasting.