عنوان مقاله [English]
The MODIS facility for the Earth Observing System is a key element that supports ambitious goals related to studying the Earth as a system. One of the MODIS products is high quality land surface temperature data produced in Terra and Aqua platforms. Knowledge of the LST provides information on the temporal and spatial variations of the surface equilibrium state and is of fundamental importance in many applications. Therefore, it is required to conduct a wide variety of climatic, hydrological, ecological, and biogeochemical studies. Due to the intrinsic scanning characteristics of the MODIS instrument onboard the polar-orbiting satellites, the differences in local solar time for pixels along a given scan line on the same day or for the same pixel on different days in one revisit period can be detected for 2 hours. As LST changes with local solar time, it is not possible to directly compare the LST of different pixels in the same day or of the same pixels in different days. Awareness about slope of land surface temperature is an important factor for cognition of land surface temperature behavior that can be used for increasing spatial and temporal resolution, comparability with other data, and accuracy achievement. The results will help calculate a time consistent land surface temperature.
Materials and Methods
Land surface temperature data used in this research are produced using Normalized split windows algorithm. These data haver been downloaded from MODIS website (http://reverb.echo.nasa.gov/reverb) in daily time scale for temporal range of 2002/07/08–2015/11/30. Prior to this date, Aqua MODIS data were not available. This dataset contain measurements of land surface temperature, quality assurance, view time, view angel, land surface emissivity available for day time and night time. MODIS Terra 10:30AM and Aqua 1:30PM data have been used in this study. A matrix in dimensions of 4984*1884077 is provided from this dataset for Terra and Aqua day time land surface temperature and day view time. The matrix has provided basic information for this study. The slope of land surface temperature between two observation of Terra (10:30AM) and Aqua (1:30PM) is calculated. View time and land surface temperature measurements should initially be recalled and calculate variation of land surface temperature in relation to observation time distance between Terra/Aqua measurement for each pixel of Iran for every day of time series. The land surface temperature gradient has been calculated in monthly long term mean for spatial and temporal analysis.
Results and Discussion
View time is an important parameter in analysis of land surface temperature gradient. View time distribution of land surface temperature in Iran show that Terra and Aqua view time has a high uniformity in frequency percent of observation times for land surface temperature at 10-12 and 12:30-14:30 for Terra and Aqua, respectively. The coordination of land surface temperature gradient and topography in Iran is high. Low elevation lands between Zagros and central mountain ranges with northwest to southeast direction and central hollows such as Kavir plain, Lut desert and Jazmouryan is visible with higher slope in land surface temperature in all different months of year. North and south shorelines and high elevations are the regions with smaller gradient. The gradient of less than 2 kelvin/hour in land surface temperature occurred in 85% of Iran territory. Difference between Terra and Aqua land surface temperature decreased clockwise from northeast aspects to southwest and increased from northwest to northeast. The gradient of land surface temperature has an inverse relation with slope of land and decreased one kelvin/hour from zero to 22 degree in slope.
Significant spatial and temporal variation that occurred in land surface temperature gradient of Iran is the result of variation in environmental conditions and incoming solar radiation. The gradient of land surface temperature in shorelines and mountainous regions is lower than deserts and low level elevations in all months of year. The gradient of zero to ±1 and +1 to +2 kelvin/hour in land surface temperature has covered great areas of Iran in different months. Zero to -1 kelvin/hour gradient spread can be seen in winter season in high level elevations of Alborz and Zagros mountains. Decrease in land surface temperature gradient with increase land slope can reflect relation between land surface temperature and elevation. The results show that small amounts of land surface temperature gradient are the characteristic of major parts of Iran. In these conditions we can produce a time consistent land surface temperature data for each pixel of every day in available time series for a time between MODIS Terra and Aqua observations.