Convection and Appalachian cold-air damming

Abstract

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 moisture and 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 (1) clarify and quantify the influence of the CAD cold dome on the convective environment (specifically on instability, shear, and lift in the lower troposphere), (2) determine what influence the wedge front has on the location, structure, and intensity of convection, and (3) compile the research findings into a useful form for application to operational forecasting. We hypothesize that increased vertical wind shear at the periphery of a CAD cold dome provides a more favorable environment for strong rotating convection than would otherwise exist. Although the cold dome stabilizes the lower troposphere, we hypothesize that added wind shear due to the presence of the cold dome can compensate for the loss of instability with the ingredients needed for the development of strong rotating convection along the wedge front. In order to test this we (1) identified cases of convection along or near a wedge front, (2) isolated notable events in the case spectrum, and (3) performed numerical experiments (with CAD and with removal of CAD) on a notable event to isolate and quantify the influence of a wedge front on convection. A dataset of active wedge-front convection events was assembled. A representative event, characterized by convection with a relatively strong and definite cold dome, took place on 20 March 2003. The WRF model was 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. Initial simulations confirm that the presence of the cold dome decreased instability, but increased low-level shear (strong deep-layer shear was present with and without the cold dome). Additional simulations at higher spatial and temporal resolution were performed to determine the detailed mechanisms at work on the convective scale. Although the wedge front was analyzed to lead to convection with more discrete cell structures and some splitting into right- and left-moving cells, the convection without the presence of the cold dome was surprisingly more intense. This is attributed to increased surface-based moisture and instability and associated cell updraft strength with the particular case in a strong deep-layer sheared environment. It is speculated that the wedge front would be more significant in triggering strong rotating convection with a case of high instability and weak deep-layer shear. A conceptual model created from real case data is presented to allow applicability to operational forecasting.