Validating MODIS Cloud Mask Based on a Regional Cloud Mask of AVHRR

Document Type : Full length article


1 PhD Candidate in Satellite Climatology, Department of Physical Geography, Tarbiat Modares University, Tehran, Iran

2 Professor of Climatology, Department of Physical Geography, Tarbiat Modares University, Tehran, Iran

3 Assistant Professor of Climatology, Department of Physical Geography, Tarbiat Modares University, Tehran, Iran

4 Professor of Space Physics, Institute of Geophysics, University of Tehran, Tehran, Iran


Cloud plays an important role in the study of radiation balance and greenhouse gases due to existing water vapor as the greenhouse gases of the atmosphere. Clouds can reflect sun radiated in the top of the atmosphere based on their thickness and density. Iran country has different regions in regards to cloudiness. For example, the north of Iran has a cloudy sky in most days of the year, while cloudy conditions are low in the central regions of Iran. One of the most important datasets for cloud detection is satellite data. The two main sensors that can be used in meteorology are Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard Terra and Aqua Platforms and Advanced Very High Resolution Radiometer (AVHRR) aboard National Oceanic and Atmospheric Administration (NOAA) that were used in this paper. Various cloud detection algorithms are applied to different satellite observations over various land surfaces. AVHRR level-1 data is used to introduce a regional cloud detection scheme for two regions in Iran which have different geographical characteristics. Most cloud mask algorithms were developed globally using different spectral tests and have not examined and validated at regional scale. For example, MODIS cloud mask (MCM) algorithm can be applied in five group tests in visible, near infrared, and thermal spectral regions. The goal of the study is to validate the MCM based on AVHRR data and radiative transfer model simulation of clear sky brightness temperature (BT).  
Materials and methods
We have executed the analysis in two regions of Guilan, located in southern coasts of Caspian Sea with maritime climate as well as in Khohgiluyeh Va Boyerahmad in west Iran with mountainous climate. We have employed five spectral threshold tests for the regions. Radiative Transfer for Television and Infrared Observation Satellite (TIROS) Operational (RTTOV) simulation of clear sky BT was also applied for clear sky BT thresholding. Snow have detected by NearIR(1.6) /VIS(0.6) test ranging values below and equal to 0.2 with a BT test at 12 ranging -8.16 to 11.84 degrees Celsuis. The reflectance test was also performed. Fourth test was a NearIR (0.9)/VIS (0.6) ranging from 0.6 to 1.3 over cloudy area and finally a clear sky BT test was applied to the areas. 
Results and discussion
These all tests were applied to five dates with the highest positive anomalies (Dec 2003, Oct 2006, Jan 2008 and Nov2011) and the highest negative anomaly (Dec 2010) during 2001-2015. The results showed that although an extended area have values below and equal to 0.2, BT test for cloudy region have values below -8.16 degrees Celsuis. According to the results of the fourth test, the test indicated the values between 0.6 and 1.3 over Guilan and 0.7 and 1.3 over Khohgiluyeh Va Boyerahmad fot detecting the clouds. The fifth test revealed interesting results due to different properties over low and high skin temperature area so that an area with values of skin temperature higher than 5 degrees Celsuis experienced clear sky BT colder than cloudy sky temperature and vice versa. This test showed that the values of T12μm below -2.5 C can detect clouds in both cases (skin temperature below 5 C and greater than 5 C) over Khohgiluyeh Va Boyerahmad. The values of 5 and -13.68 C can be used for the thresholding over the area with skin temperature greater and below 5 C on Guilan province, respectively. The results demonstrated that these tests can well detect clouds over more than 90 percent of the areas. With applying these tests over 90 and 91 points of Guilan and Khohgiluyeh Va Boyerahmad, the results showed good agreement between new cloud mask and MCM and weather stations total cloud cover data.
This study has addressed the cloud mask properties by MCM using AVHRR imagery data and weather stations statistic data. Results of the radiative transfer model revealed interesting effects on cloud detection. is the results of this research, as the first attempt to introduce a regional cloud mask over Iran, suggested the use of SEVIRI data with higher temporal resolution over two regions. The radiative transfer simulation results can compare the radiative transfer model in this research. It is recommended to use more geographic areas and more different dates for such researches.  


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