Hierarchy of Models for Studying the Climatology and Strucutre of Tropical Storms

Wednesday, May 14, 2014 - 07:00
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GFDL's HiRAM atmospheric model when run at 50 or 25 km resolution provides quite realistic simulations of the spatial distribution, seasonal cycle, and interannual variability of the frequency of tropical cyclones (TCs). Using this model as a starting point, we idealize the geometry and boundary conditions in various ways to create a hierarchy of models designed to help us understand the behavior of TCs in the model and reality. We have created aqua planet models with prescribed zonally symmetric, and asymmetric SSTs, aqua planets with slab-oceans and zonally symmetric climates manipulated by adding heat to or removing heat from the ocean, and rotating radiative-convective equilibrium (RCE) models on a doubly-periodic f-plane all with the identical column physics package and horizontal resolution as in the comprehensive GCM. These studies suggest a variety of benchmark calculations that should be helpful in relating the behavior of TCs in different models In the model with a slab ocean, we can manipulate the latitude of the ITCZ with imposed oceanic fluxes, and study how TC genesis changes as this latitude is changed. There are almost no TCs when the ITCZ is centered on the equator, with the TC frequency increasing very rapidly as the ITCZ is displaced polewards. When this model is warmed with a CO2 increase, the number of TCs increases, the opposite result to that obtained with the realistically configured model ‰ÛÒ a result that can be traced to the poleward displacement of the ITCZ with warming in the aqua planet model. By prescribing SSTs in this aqua planet model, we examine how the flatness of the latitudinal structure of the SSTs affects TC frequency, and the limitations of the simplest (relative SST) hypotheses for how zonally asymmetric SSTs help control the zonal structure of genesis. The rotating RCE model suggests that the natural state to which the tropics tends, when homogeneously forced and in the presence of some environmental vorticity, is full of closely-packed long-lived TCs. The factors that strongly inhibit genesis in reality are all created by spatially inhomogeneous forcing. The storms captured in rotating RCE can survive for months, allowing detailed analysis of their structure. In particular, we have examined how decreasing ambient rotation rate increases the external scale of storms -- the average spacing between storms -- but decreases the radius of maximum winds.

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