Seminar

Abstract

The cloud fraction of shallow non-precipitating cumulus residing at the lifting condensation level (LCL) increases in the afternoon, most evident in airborne lidar observations from EUREC4A. This cannot be easily explained by afternoon increases in the ocean sea surface temperature. Observations from the German aircraft HALO platform and from the R/V Ronald H. Brown are used to search for an explanation. Three hypotheses are investigated: 1) afternoon increases of the ocean sea surface temperature help support buoyancy fluxes that lift air parcels to saturation, as seen in tropical regions under low wind speeds; 2) shortwave absorption of the sub-cloud layer helps deepen the sub-cloud layer, so that its mixed-layer height can reach the LCL; 3) clouds form where the sub-cloud layer is already moist, and shortwave absorption further stratifies the lower one km during the afternoon, so that the clouds only remain while forced from above. Analysis to date suggests #3 is the closest to the correct explanation. If so, then the next question is to identify why the daytime cloud layer is more or less moist in some places, and how this relates to moisture redistribution from shallow circulations, or, moisture transport occurring at larger scales. We do not yet have all the answers to the questions we are raising, so expect the presentation to foster more discussion as opposed to a summary of firm findings.

---

Dr. Paquita Zuidema is a Professor in the Department of Atmospheric Sciences at the University of Miami's Rosenstiel School of Marine and Atmospheric Science. She received her BS from the University of Illinois in Champaign-Urbana, and her PhD from the University of Colorado in Boulder. Dr. Zuidema then joined NOAA as a National Research Council postdoctoral fellow, and later as a CIRES scientist in the then Environmental Technology Laboratory, now the Physical Sciences Laboratory. Dr. Zuidema's research focuses on the relationship of warm, shallow clouds to the large-scale environment, with an emphasis on the connection to radiation. She is now the chair of the Atmospheric Sciences Department at the University of Miami.