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  • Author or Editor: Kenneth Mitchell x
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Alan Basist
,
Don Garrett
,
Ralph Ferraro
,
Norman Grody
, and
Kenneth Mitchell

Abstract

A comparison between two satellite-derived snow cover products demonstrates the strengths and weakness of each procedure. The current NESDIS operational product is subjectively derived from visible satellite imagery. The analysis is performed once a week, using the most recent clear view of the surface. The experimental product is objectively derived from daily microwave measurements observed by polar-orbiting satellites. The operational product uses a high albedo in the visible spectrum to identify snow cover, whereas the experimental product uses a high albedo in the visible spectrum to identify snow cover, whereas the experimental product uses a passive microwave scattering signature.

Comparisons between the operational and experimental product show good agreement in the extent and distribution of snow cover during the middle of the winter and summer seasons. However, the agreement weakness in the transition seasons and along the southern edge of the snowpack. The analysis suggests that the operational procedure is better at observing snow under a densely vegetated canopy, whereas the experimental procedure is better over rugged terrain and persistent cloud cover. The experimental product is also better at observing rapid fluctuations in the snowpack, since it has higher temporal resolution and can see through nonprecipitating clouds.

An integration of the two products, currently under development by NOAA/Office of Research and Application and the National Meteorological Center, would represent true snow cover better than either single procedure. However, it would probably introduce discontinuity into the 5-yr time series of the current operational product, which is the longest record of snow cover over the Northern Hemisphere. Averaging the experimental product between two consecutive weeks effectively brings the two datasets into closer agreement throughout the global time series. However, this technique does not resolve the regional biases between the two datasets into closer agreement throughout the global time series. However, this technique does not resolve the regional biases between the two datasets. Surface observations would help identify the source of these biases; unfortunately, these reports are severely limited over many areas.

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Fanglin Yang
,
Kenneth Mitchell
,
Yu-Tai Hou
,
Yongjiu Dai
,
Xubin Zeng
,
Zhuo Wang
, and
Xin-Zhong Liang

Abstract

This study examines the dependence of surface albedo on solar zenith angle (SZA) over snow-free land surfaces using the intensive observations of surface shortwave fluxes made by the U.S. Department of Energy Atmospheric Radiation Measurement (ARM) Program and the National Oceanic and Atmospheric Administration Surface Radiation Budget Network (SURFRAD) in 1997–2005. Results are used to evaluate the National Centers for Environmental Prediction (NCEP) Global Forecast Systems (GFS) parameterization and several new parameterizations derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) products. The influence of clouds on surface albedo and the albedo difference between morning and afternoon observations are also investigated. A new approach is taken to partition the observed upward flux so that the direct-beam and diffuse albedos can be separately computed. The study focused first on the ARM Southern Great Plains Central Facility site. It is found that the diffuse albedo prescribed in the NCEP GFS matched closely with the observations. The direct-beam albedo parameterized in the GFS is largely underestimated at all SZAs. The parameterizations derived from the MODIS product underestimated the direct-beam albedo at large SZAs and slightly overestimated it at small SZAs. Similar results are obtained from the analyses of observations at other stations. It is also found that the morning and afternoon dependencies of direct-beam albedo on SZA differ among the stations. Attempts are made to improve numerical model algorithms that parameterize the direct-beam albedo as a product of the direct-beam albedo at SZA = 60° (or the diffuse albedo), which varies with surface type or geographical location and/or season, and a function that depends only on SZA. A method is presented for computing the direct-beam albedos over these snow-free land points without referring to a particular land-cover classification scheme, which often differs from model to model.

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