عنوان مقاله [English]
Water vapor is an essential climate element. Through latent heat exchanges, the water vapor is considered as the principal way of energy transport through the global atmosphere and it is also a dominant greenhouse gas (Held and Soden, 2000). Over the oceans, warmer surface temperatures may lead to increased evaporation and, therefore, greater specific humidity, but an approximately constant relative humidity (RH); the greater concentration of water vapor will, in turn, warm the surface further, since water vapor is a potent greenhouse gas (GHG) (IPCC, 2007; Dessler and Sherwood, 2009). As temperature rises, the atmosphere’s capacity to hold water vapor is also increased. As the water vapor can be transported vertically through convection and subsidence, and horizontally by atmospheric circulation, changes in surface absolute moisture can cause changes in moisture aloft (McCarthy and Toumi, 2004). The Clausius-Clapeyron relation represents exponential increases in the atmosphere’s water holding capacity with increasing T at approximately 7% K-1 (Manabe and Wetherald, 1967; Allen and Ingram, 2002; Trenberth et al., 2005).
Knowledge about changes in water vapor in the upper troposphere and lower stratosphere is important because it can result in strong alterations in radiative forcing, hydrological cycle, precipitation intensity, human activities and biosphere. The aim of this study is to analyze spatiotemporal variation of atmospheric humidity of Iran during 1979-2013.
Material and Methods
In conduction of this study, we have used specific humidity and relative humidity gridded monthly data of European Centre for Medium Range Weather Forecasts (ECMWF) from 1/1979 to 12/2013. The data with spatial resolution of 0.125 degree have been selected. According to the spatial resolution of the data, 9965 pixels are located in political boundary of Iran. Annual specific humidity and relative humidity for each pixel are also calculated. The variation of Iran’s air temperature from 1000 to 200 hPa is analyzed. Two non-parametric tests of Mann-Kendal and Sen Estimator are used to decide about significance of trend and slope of trend, respectively. The cosine of latitude is considered as data weight to estimate area mean annual specific humidity and relative humidity. The following equations are also used to calculate area mean annual specific humidity:
where, is weighted specific humidity (Q) mean in year j , and Qi,j is specific humidity mean on pixel i in year j.
Results and Discussion
The results of this study show that atmospheric humidity (specific humidity) and relative humidity experienced significant trend during study period. In lower troposphere, the specific humidity and relative humidity trends are negative over most regions of the country. In spatial view, the highest decrease in the rate is observed on the areas between 34 to 36 northern latitudes. During the study period, Semnan, Shahrod and Gorgan experienced the highest decrease in the rate of specific humidity and relative humidity. In contrast to most regions of the country, positive trend of specific humidity is observed over coastal regions of southern seas and southwestern parts of Caspian Sea. The relative humidity trend is not significant statistically over these regions. According to the Iran area mean in 1000 and 924 hPa, the decreasing rate in specific humidity is 0.04 and 0.05 g.kg-1.decade-1, respectively. The area mean decrease in relative humidity is about 1.61 and 1.62, respectively, in 1000 and 925 hPa.
Based on the available evidence, specific and relative humidity is decreased in lower troposphere over Iran but in middle and upper troposphere the trend is not significant. From 1979 to 1998, analysis of zonal mean annual specific and relative humidity show positive anomaly while during the period from 1999 to 2013 the anomalies are negative. Over the oceans, warmer surface temperatures will likely lead to increased evaporation and, therefore, greater specific humidity, but an approximately constant relative humidity (RH). However, over the land area the increased temperature results in less higher humidity and decrease in relative humidity. In some small regions of country include coastal regions and southwestern parts of Caspian Sea the trend of specific humidity is positive. The accumulation of humidity in the atmosphere of coastal regions and southwestern parts of Caspian Sea will alter the radiation balance, by an increase in long‐lived greenhouse gases. Water vapor is itself a greenhouse gas. This increase in humidity causes additional warming. The decreasing trends in specific and relative humidity over Iran particularly in land areas are fairly consistent with the increase trends observed in the troposphere temperature. The results of area mean temperature in 9 levels show that temperature trend is positive in lower troposphere. In conclusion, the warming observed in Iran is generally accompanied by a decrease in the moisture content of the lower troposphere during 1979–2013. Further work should be carried out to investigate controls on subtropical moisture and their possible connections to the pole ward displacement of Hadley cell and subtropical jet stream characteristics and on regional precipitation and/or cloud changes.