Radiation Budgets in the Mackenzie River Basin: Retrieval from Satellite Observations and an Evaluation of the Canadian Regional Climate Model

Abstract

The Canadian Global Energy and Water Cycle Experiment (GEWEX) Enhanced Study (CAGES) was a 14-month period in 1998?99 during which there were enhanced measurements of water vapor, precipitation, snow cover, radiation, snowmelt, evaporation, stream discharge, and other variables in the Mackenzie River basin (MRB). These measurements provided the initialization and validation fields for modeling and remote sensing studies. During the CAGES period there were only limited broadband satellite observations of radiation fluxes over the MRB from the second flight module of the Scanner for Radiation Budget (ScaRaB) instrument. A more extensive dataset of solar radiation fluxes at the top of the atmosphere (TOA) and at the surface was derived for CAGES from the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR) observations. The derived solar radiation fluxes at the surface were evaluated against the surface measurements in the basin. The results show that mean differences are generally less than 10 W m?2 at each site. Solar fluxes at the TOA and at the surface, and longwave (LW) fluxes at the TOA from the Canadian Regional Climate Model (CRCM) are compared with those from satellite measurements and retrievals. It is found that the CRCM overestimated the TOA reflected solar flux consistently by an average of 33 W m?2 during summer months, while during months when the surface is mainly covered by snow, the CRCM agrees with satellite retrievals well, with a mean difference of 2.1 W m?2. The differences in the basin monthly mean atmospheric absorption between the CRCM and the satellite retrievals for 9 months during the CAGES period vary from ?11 to 12 W m?2. The overestimation of the TOA-reflected fluxes in the model during the summer months is mainly at the expense of an underestimation of the net surface solar fluxes. A comparison of cloud amount from the CRCM and the satellite retrievals shows an overprediction by the CRCM, which is responsible for the overestimation of the TOA-reflected flux during the summer months. A comparison of outgoing LW flux from the CRCM with satellite measurements in winter months during the CAGES period shows the CRCM fluxes to be 6?12 W m?2 less than the measured fluxes, which is at least partly due to the underestimation of surface temperature in the model.