simulating the consequences of lake urmia’s drying up on downfall in northwest of iran

1. introduction considering the importance of lake urmia in the weather of northwest of iran, the present study, using dynamic modelling and simulation, investigates the consequences of the drying up of lake urmia on the climate parameter of downfall. 2. study area lake urima, considered the biggest pond in western asia, with a surface area of 5 thousand and 822 square kilometers, is the second biggest lake in iran, after the caspian sea, and the second highly saturated saline lake in the world. the drainage basin of lake urmia constitutes nearly 3 percent of the entire area of iran. in recent years, climate issues are considered to be the main culprit in water crisis engulfing northwest of iran. in some studies, climate is estimated to have been the cause of environmental crisis in lake urmia somewhere between 60 to 65 percent. studies indicate that the aggregate seasonal and annual rainfall and snowfall in northwest of iran is declining. it has, also, been shown that the sharp decline in rainfall and snowfall throughout the year in northwest of iran, especially in winter, is the main reason for the decline, on an annual basis, in this period (charbgu, 2011). 3. material and methods in the study, for simulation using regcm, it was paired the lake’s model and was executed with a resolution of 10 kilometers for a three-year period (2001-2003). for the initial input of the model, digitized data were used. lateral boundary condition data were obtained from the reanalyzed data with 2.5 degrees of horizontal separation, supplied by national center for environmental prediction (ncep) /national center for atmospheric research (ncar). the data, gathered in six-hour intervals, includes geopotential height, parallel (u) and meridian (v) components of wind, temperature, relative humidity, surface pressure and vertical speed. data regarding the initial conditions include the surface temperature of the sea, gathered weekly, with a spatial horizontal separation of one degree, provided by national oceanic and atmospheric administration (noaa). the data for surface specification includes the topographical data gathered by national cartographic information center in the usa, with a horizontal separation of 30 seconds, and land cover data together with the data regarding soil humidity and texture, with a spatial separation of 30 seconds. in order to investigate the influence of lake urmia on the amount of downfall in north-west of iran, the paired model of the lake was executed first using the real condition and then by replacing the surface of lake urmia with a semi-desert and salt marsh. in order to investigate the importance of lake urmia’s existence and its effect on region’s climate for a three-year period (2001-2003), regional climate model (regcm 4.3) with grell parameterization in initial settings (with the lake) and simulated settings (without the lake) was executed. since the aggregate daily or hourly rainfall or snowfall in a region forms its annual and seasonal amount, and on the other hand, depicting the pattern of rainfall or snowfall on a smaller scale is of importance for studying drought and wet year cycles, daily rainfall or snowfall was analyzed using grell parameterization. in this parameterization, two ascending and descending major convective streams for clouds are considered. in fact, it is one of the cogent parameterizations for calculating rainfall and snowfall. 4. results and discussion the map reveals that in case lake urmia is dried up, the eastern section of the lake will experience a more dramatic decline in the amount of downfalls. as can be seen, of the three kernels of decline in downfall, one is located in proximity to the shabestar, tabriz and marand counties, the second kernel is in the middle of the eastern section of the lake, in azarsharh county, and finally the third kernel is discernable in south-west of the lake, in maragheh county. the greatest decrease in downfall, in simulated settings, occurs in the northeastern kernel, above shabestar, marand and tabriz, which is much higher compared to other kernels, to the extent that in case of the lake’s disappearance, the entirety of western and northern sections of east azerbaijan province will experience a decrease in the amount of downfall. in other regions, too, the amount of downfall will gradually become zero. finally, it can be said that downfall during winter, in simulated settings, has decreased and the decrease of downfall in spring is much more severe and extensive, in comparison with other seasons. 5 conclusion the map indicates that in case the lake disappears, jolfa, maraghe, tabriz, shabestar, azarsharh, and marand counties will experience a decrease in the amount of downfall. this decrease will have dire consequences for the provinces of east and west azerbaijan, during the rainy season of spring. these consequences are much more salient for east azerbaijan. the extent of the decrease in downfall around the lake, in case of its disappearance, will impact all the counties around it, and similar to the pattern of spring, will affect east azerbaijan province more than before. with regards to the maps obtained through grell parameterization, it can be said that this parameterization is capable of simulating the amount of downfall in northwest of iran. the results yielded by grell parameterization were validated by the data gathered through observation, and the percentage of error and skewness were analyzed using the data gathered by stations. after analyzing the aggregate seasonal three-year downfall (2001-2003), the outputs of the model in initial settings and after eliminating the lake, and their comparison with the real amount of downfall in lake urmia, revealed that regcm4.3, in connection with the lake’s model, is capable of simulating the amount of snowfall and rainfall. although the model has simulated more downfall than real situation (especially in spring), its conformity with the downfall recorded in stations of tabriz, jolfa, and maraghe is satisfactory and in fact, regcm has been able to perfectly simulate the pattern of monthly downfall with a high correlation of 0.9. the average percentage of error and skewness of downfall, compared to the observed amounts in synoptic weather stations in iran for the months of the year 2003, were calculated using relations 1 and 2, and it was revealed that simulation for the amount of north-west downfall using grell parameterization, has the highest amount of skewness in spring (85 millimeters) and lowest amount of skewness in summer (-2 millimeters). model’s high amount of skewness in spring is indicative of a higher-than-real estimation compared to the actual data for northwest region. the pie chart, too, depicts model’s skewness and its ability to simulate downfall in the region, to the extent that winter, with a skewness of 15%, spring with 58%, autumn with 26% and finally summer with a 1% were simulated. it merits a mention that the reason for model’s low degree of simulation in summer is that; generally, downfall in this season is minimal (3%). the findings reveal that in case lake urima dries up, the amount of downfall in the eastern half of the lake and especially in east azerbaijan will decrease, posing serious challenges to north-west of iran.