63 Impact of Island Morphology on Convective Cloud Formation

Monday, 23 January 2017
Udaysankar S. Nair, University of Alabama, Huntsville, AL; and E. Foshee and C. Phillips

It has been estimated that convection over the maritime region contributes to ~40% of the latent heating in the global tropics, with substantial portion of it occurring in the vicinity of relatively small islands (van der Molen et al., 2006).  On the other hand, continental convection contributes only ~20% of the latent heating in the global tropics.  Thus, in the maritime continent, small islands have a climatic impact that is disproportionate to their areal extent (van der Molen et al., 2006; Quain 2008).  In an island setting, sea breeze circulation is an important forcing factor for cloud and precipitation formation, which in turn is modulated by the proportion of island area that comes under the influence of sea breeze.  The nature of the sea breeze circulation is dependent on island size, aspect ratio, orientation, nature of terrain and land cover. 

Developing a conceptual understanding of the role of island morphology on cloud and precipitation formation is important, especially in context of large scale (greenhouse gas forcing) and regional climate forcing (land use and land cover change, aerosols).  Idealized numerical model simulations were conducted for this purpose, where systematic variations in size, aspect ratio and orientation of the island are imposed.  The Ocean Land Atmosphere Model (OLAM) with unstructured grid is utilized in the idealized experiments.  In the idealized experiments, cloud formation over a circular island with radius comparable to sea-breeze propagation distance is simulated, which produced shallow boundary layer clouds and no precipitation.  When the island area is doubled (island radius is twice the sea-breeze propagation distance), simulations produce deeper and more persistent cloud formation and also localized rainfall in excess of 3.5mm.  Increase in the aspect ratio of the island geometry from 1 to ~2.7 (circle to ellipse) while keeping the surface area constant, resulted in organized cloud formation oriented along the major axis.  Similar to circular shape, cloud formation over the elongated islands become more vigorous as the total island area increase. However, clouds are less deep and produce less precipitation ( ~0.4mm) compared to circular island on same surface area. 

As the island size increases, the area impacted by the sea-breeze decreases. Whereas convection forms along the advancing edge of the sea-breeze circulation, it is suppressed in regions of descent behind the sea-breeze front.  As the size of the island increases, area available for convection to develop unimpeded by sea breeze increases and leads to higher rainfall.  Elongation of island shape results increased convergence along the sea-breeze front, both due to interactions with large scale flow patterns and also due to interactions of multiple sea-breeze fronts.  Note that the above discussed results are for a subset of the island morphology and topographical parameter space considered in the study.  The presentation will also include results from experiments conducted for a more comprehensive parameter space and implications for regional and large scale climate will be discussed.

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