Date of Award

January 2014

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Atmospheric Sciences

First Advisor

Xiquan Dong

Abstract

A large degree of uncertainty in global climate models (GCMs) can be attributed to the representation of clouds and how they interact with incoming solar and outgoing longwave (Earth emitted) radiation. In this study, the simulated total cloud fraction (CF), cloud water path (CWP), top-of-atmosphere (TOA) radiation budgets and cloud radiative forcings (CRFs) from 28 CMIP5 AMIP models are evaluated and compared to multiple satellite observations from CERES, MODIS, ISCCP, CloudSat, and CALIPSO. The multimodel ensemble mean CF (58.6 %) is, on global average, under estimated by nearly 7 % compared to CERES-MODIS (CM) and ISCCP results, with an even larger negative bias (16.7 %) compared to the CloudSat/CALIPSO result. The CWP bias is similar in comparison to the CF result; the multimodel ensemble mean is under estimated (16.4 gm−2) when compared to CM. The model simulated and CERES EBAF observed TOA reflected shortwave (SW) and outgoing longwave (LW) radiation fluxes, on average, differ by 1.6 and −0.9 Wm−2, respectively, and is contrary to physical theory. The global averaged SW, LW, and net CRFs form CERES EBAF are −47.2, 26.2, and −21.0 Wm−2, respectively, indicating a net cooling effect due to clouds on the TOA radiation budget. Global biases in the SW and LW CRFs from the multimodel ensemble mean are −1.1 and −1.3 Wm−2, respectively, resulting in a greater net cooling effect of 2.4 Wm−2 in the model simulations. A further investigation of cloud properties and CRFs reveals the GCM biases in atmospheric upwelling (15 °S − 15 °N, ocean-only) regimes are much less than their downwelling (15 ° − 45 °N/S, ocean-only) counterparts. Sensitivity studies

have shown that the magnitude of SW cloud radiative cooling increases significantly with increasing CF at similar rates ( −1.20 and −1.31 Wm−2 %−1) in both regimes. The LW cloud radiative warming increases with increasing CF but is regime dependent, demonstrated by the different slopes over the upwelling and downwelling regimes (0.81 and 0.22 Wm %−1, respectively). Through a comprehensive error analysis, we found that CF is a primary modulator of warming (or cooling) in the atmosphere. The comparisons and statistical results from this study may provide helpful insight for improving GCM simulations of clouds and TOA radiation budgets in future versions of CMIP.

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