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A feasibility study of the estimation of net solar radiation at the sea surface using NOAA-9 AVHRR data

University of British Columbia

A feasibility study is presented of the use of a simple physical parameterization to estimate sea-surface net solar flux from radiance measurements taken by the Advanced Very High Resolution Radiometer (AVHRR) on board National Oceanic and Atmospheric Administration (NOAA) polar orbiting satellites. The scheme, which is based on Gautier's parameterization for geostationary satellite data, is revised for application to AVHRR data at high latitudes. The revised technique employs relationships among cloud optical and microphysical parameters, cloud broadband radiative properties, and the AVHRR channel radiances derived using a 24-band δ-Eddington radiative transfer model. The δ-Eddington model is used to test the sensitivity of the daily mean surface net flux to variations in ozone amount, column water vapor amount, surface albedo, cloud optical thickness, cloud fraction and droplet size. Over typical ranges of these variabilities, the uncertainty of daily mean surface net solar flux is less than 6 W m⁻² for the clear case. For the cloud case, the main uncertainty of surface net flux is due to the variations in cloud optical depth and cloud fraction. The typical sensitivities in cloud optical depth and cloud amount introduce variations in the surface net flux of 17 W m⁻²and 9 W m⁻², respectively. Based on these sensitivity studies, a modification of Gautier's simple physical model is proposed. This modification calculates cloud optical depth and droplet effective radius from the top of the atmosphere (TOA) upward fluxes in AVHRR channel 1 and 3, and then uses these derived parameters, with solar zenith angle, to estimate cloud albedo and absorptance. To compare the original and the modified Gautier model, the Slingo δ-Eddington model is used to simulate the satellite and pyrometer measurements in the real world. Simulation studies under standard midlatitude summer conditions show that the original model may produce surface net flux errors of over 30 W m⁻², mainly due to the poor estimation of cloud broadband absorptance. In contrast, the modified model produces errors of less than 3 W m⁻². However, both models produce errors of similar magnitude when the standard atmospheric condition is perturbed. This shows the need to accurately estimate atmospheric parameters. In recognition of 1) the large sensitivity of the satellite measurements to the atmospheric scattering, 2) the low sensitivity of the surface net flux to the sea surface albedo found from these sensitivity studies, an alternative clear sky retrieval scheme is proposed. Several existing techniques for estimating radiatively important cloud and atmospheric parameters by means of remote sensing are applied to the AVHRR data collected during the First ISCCP (International Satellite Cloud Climatology Project) Regional Experiment in marine stratocumulus clouds in the western Pacific. The results are compared with in situ measurements made by the aircraft and radiosonde during the same experiment. The preliminary comparisons show promise in retrieving these parameters from the AVHRR data.

Text

2010-11-05

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http://hdl.handle.net/2429/29855

Atmospheric Science

University of British Columbia

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