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Xiong, XZ, Storvold R, Stamnes K, Lubin D.  2004.  Derivation of a threshold function for the Advanced Very High Resolution Radiometer 3.75 mu m channel and its application in automatic cloud discrimination over snow/ice surfaces. International Journal of Remote Sensing. 25:2995-3017.   10.1080/01431160310001619553   AbstractWebsite

The distinct contrast between the reflectance of solar radiation in Advanced Very High Resolution Radiometer (AVHRR) channel 3 (3.75 mum) by clouds and by bright surfaces provides an effective means of cloud discrimination over snow/ice surfaces. A threshold function for the top-of-atmosphere (TOA) albedo in channel 3 (r(3)) is derived and used to develop an improved method for cloud discrimination over snow/ice surfaces that makes explicit use of TOA r(3) . Corrections for radiance anisotropy and temperature effects are required to derive accurate values of r(3) from satellite measurements and to utilize the threshold function. It has been used to retrieve cloud cover fractions from National Oceanic and Atmospheric Administration (NOAA)-14 AVHRR data over the Arctic Ocean and over the North Slope of Alaska (NSA) Atmospheric Radiation Measurement (ARM) site in Barrow, Alaska. The retrieved cloud fractions are in good agreement with SHEBA (Surface HEat Budget of the Arctic Ocean) surface visual observations and with NSA cloud radar and lidar observations, respectively. This method can be utilized to improve cloud discrimination over snow/ice surfaces for any satellite sensor with a channel near 3.7 mum.

Xiong, XZ, Li W, Lubin D, Stamnes K.  2002.  Evaluating the principles of cloud remote sensing with AVHRR and MAS imagery over SHEBA. Journal of Geophysical Research-Oceans. 107   10.1029/2000jc000424   AbstractWebsite

[1] A rigorous discrete ordinates radiative transfer formulation has been applied to two Advanced Very High Resolution Radiometer (AVHRR) images extracted from telemetry collected by the CCGS Des Groseilliers satellite tracking system during SHEBA to estimate cloud optical depth and effective radius of the cloud droplet size distribution. The two cases, from 2 and 3 June 1998, were chosen for analysis because (1) the images contained mostly liquid water clouds and (2) contemporaneous MODIS Airborne Simulator (MAS) overflight imagery was available for these AVHRR overpasses. The objective is to apply the same detailed radiative transfer formulation to both the MAS and AVHRR data so that the quality of the retrievals from the latter can be evaluated. Retrievals of cloud optical properties from MAS are assumed to be more reliable, because (1) all MAS channels have direct radiometric calibration, (2) the higher spatial resolution of MAS (50 m nadir versus 1.1 km nadir with AVHRR) should yield smaller uncertainties related to partially cloudy pixels in a given study area, and (3) effective droplet radius can be retrieved directly from the MAS 1.62-mum m channel without additional uncertainties involved with subtracting a thermal radiance component. Examination of the retrievals from both sensors in these two cases reveals considerable spatial variability (more than a factor of 2) in cloud optical depth, on a variety of scales ranging from tens of meters to tens of kilometers, even for relatively uniform liquid water clouds. Retrievals of cloud effective droplet radius from AVHRR are generally consistent with those from MAS, suggesting that AVHRR can be reliably used to estimate this quantity. However, AVHRR-based retrievals of cloud optical depth are subject to large errors that result from small uncertainties in the absolute radiometric calibration of AVHRR channel 2. Using recalibration coefficients from one of the more robust AVHRR postlaunch calibration efforts, the cloud optical depths that we retrieved from NOAA 14 AVHRR channel 2 are consistently 30-50% larger than those obtained from MAS. The intercomparison of MAS and AVHRR retrievals of cloud optical depth also revealed errors with AVHRR due to partial cloud cover, and these errors are not immediately apparent when examining the AVHRR retrievals alone. If the AVHRR retrievals are averaged to spatial resolutions of order 10-30 km, they appear to become more stable for use in applications such as atmospheric energy budget calculations.

Xiong, XZ, Stamnes K, Lubin D.  2002.  Surface albedo over the Arctic Ocean derived from AVHRR and its validation with SHEBA data. Journal of Applied Meteorology. 41:413-425.   10.1175/1520-0450(2002)041<0413:saotao>;2   AbstractWebsite

A method is presented for retrieving the broadband albedo over the Arctic Ocean using advanced very high resolution radiometer (AVHRR) data obtained from NOAA polar-orbiting satellites. Visible and near-infrared albedos over snow and ice surfaces are retrieved from AVHRR channels 1 and 2, respectively, and the broadband shortwave albedo is derived through narrow-to-broadband conversion (NTBC). It is found that field measurements taken under different conditions yield different NTBC coefficients. Model simulations over snow and ice surfaces based on rigorous radiative transfer theory support this finding. The lack of a universal set of NTBC coefficients implies a 5%-10% error in the retrieved broadband albedo. An empirical formula is derived for converting albedo values from AVHRR channels 1 and 2 into a broadband albedo under different snow and ice surface conditions. Uncertain calibration of AVHRR channels 1 and 2 is the largest source of uncertainty, and an error of 5% in satellite-measured radiance leads to an error of 5%-10% in the retrieved albedo. NOAA-14 AVHRR data obtained over the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp are used to derive the seasonal variation of the surface albedo over the Arctic Ocean between April and August of 1998. Comparison with surface measurements of albedo by Perovich and others near the SHEBA ice camp shows very good agreement. On average, the retrieval error of albedo from AVHRR is 5%-10%.

Xiong, XZ, Lubin D, Li W, Stamnes K.  2002.  A critical examination of satellite cloud retrieval from AVHRR in the Arctic using SHEBA data. Journal of Applied Meteorology. 41:1195-1209.   10.1175/1520-0450(2002)041<1195:aceosc>;2   AbstractWebsite

This study examines the validity and limitations associated with retrieval of cloud optical depth tau and effective droplet size r(e) in the Arctic from Advanced Very High Resolution Radiometer ( AVHRR) channels 2 (0.725-1.10 mum), 3 (3.55-3.93 mum), and 4 (10.3-11.3 mum). The error in r(e) is found to be normally less than 10%, but the uncertainty in tau can be more than 50% for a 10% uncertainty in the satellite- measured radiance. Model simulations show that the satellite- retrieved cloud optical depth tau(sat) is overestimated by up to 20% if the vertical cloud inhomogeneity is ignored and is underestimated by more than 50% if overlap of cirrus and liquid water clouds is ignored. Under partially cloudy conditions, tau(sat) is larger than that derived from surface-measured downward solar irradiance (tau(surf)) by 40%-130%, depending on cloud-cover fraction. Here, tau(sat) derived from NOAA-14 AVHRR data agrees well with tau(surf) derived from surface measurements of solar irradiance at the Surface Heat Budget of the Arctic Ocean (SHEBA) ice camp in summer, but tau(sat) is about 2.3 times tau(surf) before the onset of snowmelt. This overestimate of tau(sat) is mainly due to the high reflectivity in AVHRR channel 2 over snow/ ice surfaces, the presence of partial cloud cover, and inaccurate representation of the scattering phase function for mixed-phase clouds.