ASSESSMENT OF CLOUD COVER CHARACTERISTICS OVER CALIBRATION TEST SITES USING MODIS CLOUD MASK PRODUCTS

Spaceborne remotely sensed data can provide the spatial, spectral, and temporal coverage for earth monitoring. Radiometric calibration is indispensable for the accurate higher level products derived from satellite optical sensors, and the long-term radiometric calibration has a crucial role. Especially, vicarious calibration is currently the most accurate method to conduct the radiometric calibration of satellite optical sensors. Various organization and projects have been conducting the vicarious calibration experiments for satellite optical sensors. Railroad valley playa is the most popular calibration site, and we have been also acquiring the vicarious calibration data for Terra ASTER there from the past. Recently, RadCalNet (Radiometric Calibration Network) is open to the public, which is an initiative of the CEOS WGCV. Railroad valley is also one of the RadCalNet sites, and our calibration site is located at the adjacent area. Understanding the cloud cover characteristics over calibration test sites is very important for vicarious calibration and automated facilities measurement. This research evaluates the clear sky ratio for Railroad valley playa using Terra an Aqua MODIS cloud mask product (MOD35 L2 and MYD35 L2) since 2000.


INTRODUCTION
Many researches related with environmental monitoring use data obtained by satellite optical sensors, and radiometeric accuracy is very important for deriving the biophysical parameters.Therefore, radiometric calibration has a key role in the accuracy of higher level products obtained by satellite optical sensors.Vicarious calibration is one of techniques of radiomteric calibration for satellite sensors (Slater et al., 1996).
The reflectance factor on the ground, temperature, atmospheric parameters, and so on are obtained for estimeting the top of atmosphere (TOA) reflectances when target satellite sensor flies over the specific site (Desert, Salt Lake, and so on).It is desirable to have much data obtained over the calibration sites as possible due to the repeat cycle of the satellite.Hence, recently, the Infrared and Visible Optical Sensors subgroup of the Committee on Earth Observation Satellite (CEOS) Working Group on Calibration and Validation (WGCV) set up the radiometric test sites network, the Radiometric Calibration Network (Rad-CalNet, https://www.radcalnet.org),which provides the TOA reflectances from 380nm to 2500nm at a 10 nm spectral sampling interval (Czapla-Myers et al., 2016).
We have conducted vicarious calibration experiments for Terra ASTER sensor (Yamaguchi et al., 1998) in the dry lake beds and salt lake every year since 1995 (Figure 2).Railroad valley playa is one of our main calibration sites, which is located at 38.505 • N, 115.690 • W, 1436 [m] altitude in east-central Nevada, US (Figure 1).The Radiometric Calibration Test Site (RadCaTS) at Railroad Valley, Nevada was developed by the University of Arizona, which is one of RadCalNet sites.RadCaTS is located at the adjacent area of our calibration site.Many calibration groups in various satellite mission have used Railroad valley playa as calibration site (MODIS, ASTER, GOSAT, MISR, OCO, Land-sat, Setinel, and so on) (Thome et al., 2003;Kuze et al., 2011;Bruegge et al., 2014;Czapla-Myers et al., 2017).
Calibration sites need to be homogeneous and flat, and it is preferable that the characteristic of surface reflectance from visible to short-wavelength infrared region is stable.In addition to the logistics and accessibility, the weather and atmospheric conditions of the sites are also important.The objective of this research is to understand the cloud cover trend over the calibration site, Railroad valley playa, by using MODIS cloud mask (MOD35 L2 and MYD35 L2) products.are daily global Level 2 products, and provide 48-bit (6 byte) field in each 1-km pixel (Strabala, 2005, Ackerman, 2010).The version of all products we used is collection 6.1, which is the latest MODIS cloud mask product.There are four types in "Unobstructed FOV Confidence Flag" of these products, which are Cloudy, Uncertain, Probably Clear, and Confident Clear (Table 1).This research define "Confident Clear" and "Probably Clear" as clear-sky, and we simply calculatete the clear sky ratio r(t) as below, We extracted the cloud flag values over our measurement sites in Railroad valley playa.

RESULTS
Figure 3 shows mean values of clear sky ratio r(t) using MOD35 L2 in 2000-2018 and MYD35 L2 in 2002-2018 for each day.Figure 4 and Figure 5 depict every 5 years mean values of clear sky ratio r(t) from 2004 to 2018 in the morning and afternoon, respectively.Table 2 shows the summary of clear sky ratio r(t) statistics in four seasons.
Highest clear sky ratio, r(t) (> 0.900) in the morning is t = 176 ∼ 261, which means that date on which we conduct the vicarious calibration experiment over Railroad valley playa should be set to arround Jun 25∼Sep 18.On the other hand, there are no highest r(t) in the afternoon, and higher r(t) (> 0.850) in the afternoon is t = 170(Jun 19) ∼ 268(Sep 25).r(t) of MOD35 L2 is slightly higher than MYD35 L2, that difference is approximately 6.5%.Although vicarious calibration is very accurate calibration techniques for satellite optical sensors, this needs various resources (human, time, cost, and so on).The results in this research is very useful information for vicarious calibration plan.And, the methodology of clear sky ratio using MODIS cloud mask products in this study could be also applicable to assessnebt of validation sites.

Figure 1 .
Figure 1.Location of Railroad Valley Playa Bit fields within first bite of the 48-bit MODIS cloud mask (MOD35 L2, Day Of Year (DOY, t=3∼ 363 or 364) r(t) : Clear sky ratio at t N (t) : The number of clear sky days ("Probably Clear" or "Confident Clear" in MOD35 L2 or MYD35 L2 flag) in five days(t − 2 ∼ t + 2) All products are downloaded from the Level-1 and Atmosphere Archive & Distribution System (LAADS) Distributed Active Archive Center (DAAC), NASA, and 10,838 MOD35 L2 and 9,721 MYD35 L2 products were processed.
Figure 4 and Figure 5 indicate that past trend of clear sky ratio is almost same as the current trend.

Figure 3 .
Figure 3. Mean value of clear sky ratio r(t) over Railroad Valley