Spatial analysis of outgoing longwave radiation trend in Iran

Climate change is considered as one of the environmental challenges in recent decades. Climatologists have evaluated the behavior and change of climatic elements to identify weather change and its importance to the structure of the Earth's weather in recent years. Trend is one of the most critical components of a series, which is very practical in climate to investigate the long-term orientation of time series. Outgoing long Wave Radiation (OLR) is one of the basic variables of weather, as well as the core component of the earth radiation budget (Whitburn et al., 2021: 1. Scherrick et al., 2018: 1), which is known as an essential parameter in applications for cloud identification and precipitation estimation. Therefore, it is necessary to study Outgoing long Wave Radiation trends at different temporal and spatial scales. Mann-Kendall test is one of the widely used non-parametric tests, which has been applied in climate studies, especially in examining the trend significance (Gauss and Trajkovic, 2013: 172. Nelson, 2001: 57). Kefayat Motlagh et al. (2018: 128) indicated that the trend of earthlight radiation is increasing by 0.4 watts per square meter in each decade, while the trend of Iran's earthlight radiation is more than three times (1.4 watts per square meter) the global trend in the same period. Sari Sarraf et al. (2015: 33) investigated the effects of global warming on the cities’ climate in the Urmia Lake basin using the Kendall method and the least-squares error. They concluded that the average rainfall in the whole region decreased by about 4 mm per year. Chu and Wang (1997: 636) examined the trend through Mann-Kendall statistics to find climate change in convective precipitation in the western Pacific and Indian Oceans from Outgoing long Wave Radiation. They found a significant decrease in OLR in the tropical central and western Pacific and a large part of the Indian Ocean, while the largest increase in OLR over time was in North Australia. In this study, the trend and changes were investigated using the non-parametric Man Kendall method, the amount of changes was determined by Sen's Slope method, and hot spots analysis was performed by Jay statistical method given. In addition, spatial and cluster analysis was performed on the average data and seasonal variation coefficient due to the importance of Outgoing long Wave Radiation at different temporal and spatial scales.

Iran, with an area of 1648195 square kilometers, is located between 25 to 40 degrees north latitude and 44 to 63 degrees east longitude. The data of the Atmospheric Infrared Sounder (AIRS), Aqua satellite, were used to measure Outgoing long Wave Radiation for a statistical period of 17 years (01/07/2002 to 01/07/2019). MATLAB, ArcGIS, and SPSS software were used for calculations and maps. First, the average monthly data maps of Outgoing long Wave Radiation were prepared, and then, the standard deviation parameter was used to show the data dispersion. Moreover, the Mann-Kendall test was used to determine the trend of outgoing long wave radiation on each cell in Iran, and the slope of the data series trend line was calculated by Sen's Slope estimator method. Spatial variations of outgoing long wave radiation were calculated over time as spatial behavior using hot spot map analysis.

According to Stefan–Boltzmann law, the Earth's surface and atmosphere emit energy in waves in proportion to their temperature. These waves propagate in the range of long wavelengths, i.e., between 4 and 100 microns given the normal surface temperature and atmosphere (Kaviani and Alijani, 2000: 94). According to the maps, the average long-wave radiation fluctuates between wet and dry seasons as well as geographical offerings, and its amount is higher in the dry season and the southern regions of the country. One of the reasons for the maximum outgoing long wave radiation of southern Iran in early spring is the angle of vertical radiation of the sun and the clear sky, which receives more energy than the latitudes, and the amount of energy output is more in the south of Iran than in the north. The outgoing long wave radiation increases gradually due to the decrease in cloudiness, and more energy is received in longer geographical areas gradually, in May and June, with the onset of summer as the day length increases. The ingoing and outgoing radiation becomes more uniform throughout the regions of Iran, except for the mountainous areas and the coasts. According to the results of cluster diagrams in the three seasons of spring, autumn, and winter, the radiation patterns of the basins are similar to the latitude and mountainous areas in Iran. The largest cluster in spring belongs to the south of the Alborz Mountains and the west of the Middle Zagros Mountains depending on latitude and sunny slopes. The highest uniformity in all seasons is located in the southwestern quarter of Iran, and Haraz heights in southern Iran are distinguished as a cold spot among the surrounding basins. Examination of the trend by the Mann-Kendall method showed no significant trend on an annual scale, but monthly and seasonal anomalies are quite evident. The descending trend of long wave radiation can be confirmed only in May and September in some parts of the country, and the dominant trend in most months of summer, autumn, and winter is increasing in most parts of the country, including northern offerings. The results of the G-statistic study also show the changes of hot spots towards northern offerings.

The trend of 17 years of outgoing long wave radiation of the earth (2003-2019) was investigated in monthly, seasonal, and annual time scales using non-parametric Men-Kendall test and hot spot statistics (G). Changes and abnormalities of long wave radiation were observed on a monthly and seasonal time scale in most parts of the country. These changes can be due to changes in the amount of energy input, cloud cover, and the type of clouds, aerosols, atmospheric compounds, such as moisture from global warming and other greenhouse gases. In addition, changes in land cover such as vegetation, forests, water resources, salinities, and sand dunes can influence the sensible heat and change of ground wave radiation due to the amount of moisture, which needs further investigation in this regard.