Session 7B.4 Convection and Appalachian cold-air damming

Wednesday, 3 June 2009: 8:45 AM
Grand Ballroom West (DoubleTree Hotel & EMC - Downtown, Omaha)
Adam K. Baker, North Carolina State Univ., Raleigh, NC; and G. M. Lackmann

Presentation PDF (330.0 kB)

In the southeastern U.S., cold-air damming (CAD) is a common occurrence to the east of the Appalachian Mountains, especially during the cool season. During CAD events, a shallow surge of cold air can extend as far southward as central Georgia and persist for several days before erosion occurs. In addition to an influence on the climatology of winter weather, in some situations CAD can also significantly influence the environment for convective storms. When sufficient instability is present in the ambient atmosphere, the shear environment at the periphery of the cold dome has been hypothesized to enhance the possibility of severe convection. Operational forecasters are aware of this influence, and place great importance on the shear and instability environments along or near the cold-dome boundary, or “wedge front”. The objectives of this research are to clarify and quantify the CAD cold dome influence on convection and the convective environment. Specifically, our goal is to isolate conditions in which the presence of the cold dome sufficiently alters convective storm structure and intensity. Accordingly, we examine changes in stability, shear, and lower-tropospheric lift associated with the CAD wedge front through use of numerical experiments with the Weather Research and Forecasting (WRF) model.

A dataset of active wedge-front convection events was assembled. A representative event, characterized by a relatively strong cold dome, took place on 20 March 2003. The WRF model is used to simulate this event with (i) unmodified terrain and (ii) flattened terrain in order to quantify differences in the convective environment in the presence or absence of CAD.

Simulations indicate that the cold dome significantly alters lower-tropospheric lift and shear. While higher resolution model simulations and observational analyses are needed to determine the detailed mechanisms at work on the convective scale, these results indicate that the region of convective triggering is determined by the location of the wedge front, and that shear increases of up to 30 kts and storm relative helicity increases of up to 600 m2s-2 are attributable to the presence of the cold dome. Ultimately the aim is to gain insight as to how the presence of a wedge front should be considered in forecasting convection in an operational setting.

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