The Possible Role of Land Surface in the Rapid Intensity Changes of Tropical Cyclones

Thursday, 21 April 2016: 11:00 AM
Miramar 1 & 2 (The Condado Hilton Plaza)
Saiprasanth Bhalachandran, Purdue University, West Lafayette, IN, India; and S. Subramanian, R. Nadimpalli, S. Gopalakrishnan, K. K. Osuri, F. D. Marks Jr., U. C. Mohanty, and D. Niyogi

The possible role of land surface in the rapid intensity changes of tropical cyclones

Saiprasanth Bhalachandran, Purdue University, West Lafayette, IN; and Subashini. S, S. Gopalakrishnan, Frank Marks, Krishna Osuri, U.C. Mohanty., and D. Niyogi

The study objective is to address the possible influence of land surface on the rapid intensity changes of tropical cyclones (TCs) prior to landfall.

Two very severe cyclones off the North Indian Ocean - Phailin and Lehar, made landfall within a span of five weeks (October-November 2013) and yet had vastly contrasting dissipating characteristics as they approached landfall. On approaching land, Phailin intensified to a very severe cyclone from its severe cyclone stage within 18 hours whereas Lehar decayed from a very severe cyclone in to a depression within the same time span. Previous studies have noted that moderate to weak vertical shear in a storm environment can create alternative pathways for the entrainment of dry air in to the moist envelope of the vortex. While this potential interplay of shear and dry air is acknowledged, there is limited understanding of how land surface processes affect the upper air conditions, and potentially contribute to the introduction of shear and dry air in to the storm environment. Recognizing that land could potentially act as a source of relatively warm, dry and dusty air and that the horizontal temperature gradients over land as well as land-sea contrast might contribute towards thermally induced wind shear in the TC environment, we hypothesize that the influence of land over the TC environment could well extend whilst the TC is still offshore.

Experiments will be conducted using HWRF wherein the land surface variables are varied and the consequent low-level moisture influx, hourly evolution of shear (winds at 850 hPa – 500 hPa), perceptible water (vertically integrated specific humidity between 850 hPa – 500 hPa) and intensity changes, monitored. In addition to the intensity changes, the potential impact of land surface on the rainfall associated with offshore cyclones will be addressed and compared to the observations using TRMM precipitation radar reported in prior studies.

Over the land, the role of surface roughness gradients on the landfalling storm characteristics will be examined. There have been instances where TCs such as Emma (2006) and Phyan (2009) have passed through regions of distinct roughness lengths. We hypothesize that the heterogeneities caused by roughness length variation influences the organization and hydrometeorology associated with the storm. While greater gradients in roughness length associated with intense urbanization, mountainous regions and dense forests, are zones of higher local convergence (higher buoyant activity/Ekman pumping), they can also be regions of greater dissipation. In order to understand the storm behavior, results from numerical experiments using the idealized version of HWRF will be presented. Finally, to establish the scale of influence, the size and translational speed of the landfalling storm, as well as the degree of heterogeneity over land will be varied and the resultant impact on storm intensity and precipitation, addressed.

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