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On the twenty-first-century wet season projections over the Southeastern United States

Description of Models

>Description of Models

One global climate model and one regional climate model (nested into the global model) are used to analyze precipitation projections for the twenty-first century. The global climate model, the Community Climate System Model version 3 (CCSM3; Collins et al. 2006), is a coupled climate model whose components are summarized in Table 1. The CCSM3 integrations used in this study have 26 vertical hybrid levels, and the model uses spectral truncation of T85, which offers a horizontal resolution of approximately 1.4 by 1.4. The CCSM3 climate integration data at six-hour intervals are obtained from the Earth System Grid (http://www.earthsystemgrid.org/).

For downscaling the CCSM3 simulation and projection, we use the Regional Spectral Model (RSM) initially developed by Juang and Kanamitsu (1994) and further developed by Kanamitsu et al. (2010). The RSM at 10-km grid resolution offers a better alternative for resolving the local orographic features of the SEUS, which are poorly represented in the native resolution of the CCSM3 (Fig. 1). For example, South Florida is under water in CCSM3, and the coastlines are ill defined. Furthermore, the height gradients of the lower Appalachians in the CCSM3 are relatively weak. The 10-km grid of the RSM rectifies some of these deficiencies. In addition, the discretization of vegetation (Fig. 2) is also relatively well resolved in the RSM. The brief outline of the RSM is provided in Table 2. Further details on RSM can be found in Kanamitsu et al. (2010) and Misra et al. (2011a).

Table 1 Component models of the CCSM3
Component Model
Atmosphere Community Atmosphere Model version 3 (Collins et al. 2004, 2006)
Ocean Parallel Ocean Program version 1.4.3 (Smith and Gent 2002)
Sea Ice Community Sea Ice Model version 5 (Briegleb et al. 2004)
Land Community Land Surface Model version 3 (Oleson et al. 2004; Dickinson et al. 2006)

Table 2 Surface features and parameterizations of the RSM
Feature Description
Topography United States Geological Survey (USGS) GTOPO30 Topography
Vegetation USGS Vegetation reduced to 12 NOAH types (Loveland et al. 1995)
Land Surface Scheme NOAH with four soil levels (Ek et al. 2003)
Atmospheric boundary layer Scheme Nonlocal (Hong and Pan 1996)
Convection Scheme Arakawa Schubert (Pan and Wu 1995)
Shortwave Scheme Chou and Lee (1996)
Longwave Scheme Chou and Suarez (1994)
Cloud Scheme Relative Humidity based (Slingo 1987)

contoured topography and land/sea masks for Community Climate System Model Version 3 and Regional Spectral Model
Fig. 1 Topography (contoured and in meters) and land/sea mask for CCSM3 (left) and RSM (right). Brown denotes land and blue denotes ocean points in the native resolutions of the two models [larger image]

vegetation map used by Regional Spectral Model reduced to 12 Noah vegetation types
Fig. 2 Vegetation map used by RSM, adapted from the USGS vegetation map, reduced to 12 Noah vegetation types [larger image]

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Last updated: 03 April, 2014 @ 11:34 AM (KP)