Wednesday, 9 July 2014
Accurate estimates of the Top of the Atmosphere (TOA) and surface radiative budgets are needed to understand the energy flow within the earth system. Earlier studies indicate that change of hydrological cycle is driven by the change of the net surface irradiance. The net shortwave irradiance is a critical component in estimating heat balance of snow and ice covered surfaces. One of uncertainty sources in estimating surface irradiances using satellite-based passive sensors is retrieved cloud properties. Cloud detection using a passive sensor over bright snow and ice surfaces is difficult. Lower-level clouds overlapping with higher-level clouds are usually not detected. If upper layer clouds are optically thin, overlapping clouds might lead to an error to determine the cloud top height. In this study, we evaluate International Satellite Cloud Climatology Project (ISCCP) cloud properties used in the NASA/GEWEX Surface Radiation Budget (SRB) process using CALIPSO and CloudSat data. In addition, surface irradiances from SRB are compared with those included in the CALIPSO-CloudSat-CERES-MODIS (CCCM) product to assess the impact of cloud difference in computing surface irradiances. We use SRB releases 3.1 (longwave) and 3.0 (shortwave), and CCCM release B1 from 2007 for the comparison.
Irradiances are computed in a 1°×1° grid every 3 hours in SRB. CALIPSO and CloudSat observations from CCCM are gridded, averaged and collocated with closest 3 hourly SRB hour box in ±15 minutes time. Additionally, cloud properties such as cloud fraction, cloud type and cloud optical depth are averaged for both datasets and evaluated the origin of the differences.
The result shows that the global annual for all the 1:30 pm overpass average of TOA irradiances are;18.4±11.1 Wm-2 for reflected shortwave, and -2.3±2.81 Wm-2 for outgoing longwave. Similarly, the global annual average of the net (downward minus upward) irradiance differences are relatively small [bias±σ], 3.8±14.1 Wm-2 for shortwave Possible explanation of the difference is the assumptions used for cloud properties (cloud fraction and cloud optical thickness) and cloud vertical distribution (cloud overlap), which affect the estimation of the shortwave and longwave irradiances.
The total net surface irradiance difference is 2.8±4.1 at the TOA, 3.4±2.1 Wm-2 and -0.6±5.5 Wm-2 at the irradiance absorbed by the atmosphere. Much larger differences, up to ±30 Wm-2, are observed in over Tropics and Polar regions.
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