Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2009. / Cataloged from PDF version of thesis. / Includes bibliographical references (p. 133-142). / In this thesis, we study observationally the variation of upper level cloud fraction with sea surface temperature in tropical oceanic regions. We also explore the consequences of a cloud feedback, arising from variations in the coverage of thin upper level clouds, in the climate of the Archean using a simple radiative-convective model that includes the effect of the variations in the area of thin cirrus clouds. First, we look at the variation in area of upper level clouds using TRMM VIRS (Tropical Rainfall Measuring Mission, Visible Infrared Sounder) data. We quantify the effect of methodological choices on the magnitude of the observed correlations between upper level cloud cover and SST. We discuss several methodological choices that might contribute to the relatively small signals found by previous researchers (- 0%/K to 2%/K) , namely, the classification of cloudy regions into convective updrafts and anvil, the use of cloud weighted SST, and the truncation and sampling error with respect to the evolution of mesoscale convective systems. All of these contribute to some extent to the weakness of signal. We show that the coarse sampling of orbital satellites can introduce biases in the estimation of the signal. Our observational evidence shows a negative correlation between the upper level cloud fraction normalized by convection and sea surface temperatures. We also study the relation between local sea surface temperature (SST) and convective precipitation fraction and stratiform rainfall area from radar observations of precipitation, using data from the Kwajalein atoll ground-based radar as well as the precipitation radar on board of the Tropical Rainfall Measurement Mission (TRMM satellite). / (cont.) We find that the fraction of convective precipitation increases with SST at a rate of about 6 to 12%/K and the area of stratiform rainfall normalized by total precipitation decreases with SST at rates between -5 to -28%/K. These relations are observed to hold for different regions over the tropical oceans and also for different periods of time. Correlations are robust to outliers and to undersampled precipitation regions. Kwajalein results are relatively insensitive to the parameters in the stratiform-convective classification algorithm. Quantitative differences between the results obtained using the two different radars could be explained by the smoothing in the reflectivity of convective regions due to the relatively large pixel size of the TRMM precipitation radar compared to the size of the convective clouds. We discuss how this observations can be interpreted as an increase in the efficiency of precipitation with temperature in mesoscale convective systems in the tropics. Finally, we explore the consequences of a negative feedback mediated through tropical cirrus clouds in the climate of the Archean, and the possibility that this feedback could explain the so-called faint young sun paradox. We assume that rates of change in the area of detrainment cloud (from the observational estimates) are characteristic also for thin cirrus clouds that have a net positive cloud radiative forcing. We find that global mean surface temperatures above freezing can indeed be found for luminosities larger than about 0.8 (corresponding to - 2.9 Ga and nearly complete tropical cirrus coverage). / (cont.) For luminosities smaller than 0.8, even though global mean surface temperatures are below freezing, tropical mean temperatures are still above freezing, indicating the possibility of a partially ice-free earth for the Early Archean. While it is feasible for tropical cirrus to completely eliminate the paradox, it is similarly possible for tropical cirrus to reduce the amounts of other greenhouse gases needed for solving the paradox and therefore easing the constraints on CO 2 and CH4 that appear to be in disagreement with geological evidence. / by Roberto F. Rondanelli. / Ph.D.
| Identifer | oai:union.ndltd.org:MIT/oai:dspace.mit.edu:1721.1/55161 |
| Date | January 2009 |
| Creators | Rondanelli, Roberto F |
| Contributors | Richard S. Lindzen., Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences., Massachusetts Institute of Technology. Dept. of Earth, Atmospheric, and Planetary Sciences. |
| Publisher | Massachusetts Institute of Technology |
| Source Sets | M.I.T. Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | 142 p., application/pdf |
| Rights | M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission., http://dspace.mit.edu/handle/1721.1/7582 |
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