Climate Effects/Controls on Mesoscale Processes
Other Agency – Assessment of the Severe Weather Environment in North America Simulated by a Global Climate Model
Karoly (primary – OU School of Meteorology), Marsh, Brooks
Funding Agency: NSF
Objectives
Estimate the frequency distribution of favorable conditions for severe
weather from archived highresolution data from simulations with the NCAR
CCSM3 global climate model; evaluate the modelsimulated distribution
by comparison with that from global reanalyses, including the climatological
seasonal and spatial variations, and the interannual variability.
Accomplishments
Severe thunderstorms and tornadoes are very important mesoscale weather events
in the central United States because of their high frequency and intensity
in this region, and the damage and loss of life that they cause every year.
Recently, it has been shown that the frequency of favorable conditions
for significant severe thunderstorms and tornados can be estimated for
the United States and other regions using global atmospheric re-analyses
with spatial resolution on the order of 200 km and temporal resolution
of 6 h.
Global climate models are unable to simulate severe thunderstorms and tornados because their spatial resolution is too coarse to be able to simulate such mesoscale events. However, they should be able to simulate the environmental conditions under which such severe weather develops, including abundant lower tropospheric moisture, steep mid-tropospheric lapse rates, and strong tropospheric wind shear. High space and time resolution data from control simulations with the NCAR CCSM3 global climate model archived at NCAR have been used to estimate the frequency of favourable conditions for severe weather, as simulated by the model. The climatological distribution of the severe weather environment in the model simulations was compared with that from the reanalyses, including the seasonal and geographical variations and its interannual variability. This small exploratory project has been funded by an NSF SGER.
The CCSM3 does a respectable job simulating the current severe weather environment of the United States. Comparisons with reanalysis data find that the spatial distributions are qualitatively similar, even if the quantitative values are reduced by up to a factor of two. Examination of the CCSM3’s seasonal cycle and diurnal cycle of mean CAPE once again captures the same qualitative patterns of the reanalysis data. The product of CAPE and deep layer shear, an indicator of the probability of occurrence of severe weather, shows similar results as the examination of mean CAPE, namely, that the spatial distributions are qualitatively similar with the numerical values being less by a factor of a little less than two. The exception to both the mean CAPE and the product of CAPE and deep layer shear is found over areas with warm water (e.g., Gulf of Mexico and Gulf Stream Current) where the CCSM3 is considerably higher than that of the reanalysis. This result demonstrates the possibility for future studies aimed at determining possible changes in distribution of the environments associated with possible global climate change. Patrick Marsh completed his MS thesis on this project in July 2007 and a paper arising from his research has been accepted for publication in Atmospheric Science Letters.
This project has been completed.
Publications
Marsh, P. T., H. E. Brooks, and D. J. Karoly, 2007: Assessment
of the severe weather environment in North America simulated by a global
climate model. Atmos. Sci. Lett., in press.
Marsh, P., H. Brooks, and D. Karoly, 2007: Assessment of the severe weather environment in North America simulated by a global climate model. 19th Conf. on Climate Variability and Change, San Antonio TX, Amer. Meteor. Soc.
Marsh, P., H. Brooks, and D. Karoly, 2007: Assessment of the severe weather environment in North America simulated by a global climate model. 12th Annual CCSM Workshop, Breckenridge, CO.
Annual spatial distribution of environments favorable for severe weather for both the CCSM3 (top) and reanalysis (bottom). A favorable environment for severe weather is defined to be an environment in which the product of CAPE and deep layer shear is greater than 10,000 m3s