ESRL/PSD Seminar Series

Influence of Increasing Surface Humidity on Winter Warming at High Altitudes through the 21st Century

Imtiaz Rangwala
Assistant Professor, CU/CIRES Center for Science & Technology Policy Environmental Studies Program


This presentation will review the late 20th century climate change over the Tibetan Plateau. Studies, including this one, suggest an elevation dependent warming on the plateau, i.e. higher warming at higher elevation. I will present analyses of observed climate variables (1961-2000) and a GCM output to discuss some of the mechanisms responsible for this phenomenon. In particular, I will focus on the localized greenhouse effect of water vapor increases in the boundary layer, which will be referred as surface water vapor feedback (SWVP). SWVP appears to be the major cause of surface warming during winter, particularly at higher elevations, in the model. This is supported by greater increases in downward longwave radiation relative to outgoing longwave radiation at surface during winter. This difference is greater at higher elevations. Seasonally, observations show largest warming trend in winter when there are much greater increases in the minimum temperatures relative to the maximum temperatures. These differences are also greater at higher elevations. Cloud cover, which can significantly influence the downward longwave flux, show decreasing trends in both observations and the modeled results. Between 1950 and 2100, the model shows an elevation dependent warming trend during winter and spring. We find that (1) increases in downward longwave radiation influenced by increases in surface specific humidity during winter, and (2) increases in absorbed solar radiation influenced by decreases in snow cover extent during spring are, in part, the reasons for a large warming trend over the plateau in the model.

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