واکاوی سری زمانی آلبدوی سطح زمین در ایران با داده‌های مودیس

Introduction: Albedo is one of the key parameters in environmental studies that shows the ratio between the solar radiation reflected from the Land surface and the solar radiation incident on it. In Iran, many studies have been conducted on the Land surface albedo at the local and national levels. In some studies, the zoning of the Land surface albedo and the factors affecting it have been studied. In some studies, the trend of the Land surface albedo has also been studied. However, no research has been conducted on the albedo time series of Iran at the macro and national levels. Therefore, studying examining the albedo time series in Iran can be a step towards revealing its environmental changes over the last two decades.

Data and Methods: The MODIS sensor is mounted on two satellites, Terra and Aqua, which were launched by NASA on December 18, 1999, and May 4, 2002, respectively. These two satellites image the entire land surface twice a day. MODIS sensor produces the land surface albedo with appropriate spatial and temporal resolution and makes it available to researchers. In this study, in order to analyze the time series of Iran's albedo, daily albedo data of MODIS sensor in Iran were downloaded from NASA website in the period from 1/1/1379 to 29/12/1401 for 8402 days. After mosaicking the tiles using the Frequency function in MATLAB software, daily, monthly, seasonal, and annual time series of Iran's albedo were calculated.

Results and Discussion: The results of the time series on an annual scale showed that the highest albedo of Iran during the last two decades occurred in 2007 with a value of 14 percent. The lowest albedo of Iran was recorded in 2009 with a value of about 11 percent. Seasonal time series studies also showed that the highest and lowest albedo of Iran during the last two decades occurred in the winter of 2007 and the fall of 2018, respectively. The monthly time series results also indicate that the highest albedo of Iran during the last two decades occurred in January and especially in February of 2007 with a value of 28%. The lowest albedo of Iran was also recorded not in July or August but in December of 2005. This shows that the albedo of Iran has a bimodal behavior; that is, one peak is observed in the winter due to increased snow cover and one peak is observed in the summer due to increased temperature and dryness of the land surface.

Conclusion: According to scientific statements, among the phenomena of the land surface, the highest of the land surface albedo is specific to snow and the lowest of the land surface albedo is specific to water and dense vegetation. A large area of Iran is poor in terms of snow cover. On the other hand, snowfall occurs only in a short period of the year. Therefore, the average albedo of Iran over the last two decades has been recorded at about 12 percent, which is insignificant compared to the global average (30 percent). The results of the time series on a daily scale showed that the lowest albedo of Iran over the last two decades occurred on December 2, 2018, which seems to be the day when precipitation systems were active in the country and a large area of the country was covered with precipitation. This seems natural considering the widespread flood of 2018 that occurred over a large area of the country and requires a separate study. On this day, the average albedo of Iran was calculated to be only 9 percent. The time series on a daily scale also confirmed that the maximum albedo of Iran was in the winter of 2007. The results showed that on January 19, 2007, Iran's albedo reached 38 percent, which suggests that heavy snow occurred in Iran on that day, which requires a separate study.