The development of tornadic storms near a surface warm front in England

Strong tornadoes are a relatively rare phenomenon in the United Kingdom, even though its tornado count is one of the highest in Europe (Dotzek, 2003). A likely reason for this is that strong latent instability is an infrequent occurrence, because steep mid-tropospheric lapse rates originating from the Sahara and the Spanish highlands rarely overspread the British Isles. Here, we present a case-study of the development of tornadic storms in the more common situation of relatively low instability, that, as a result of a combination of mesoscale factors, allowed for the development of multiple tornadoes. One of them produced F2 damage in downtown Birmingham (Marshall and Robinson, 2006). The data used for the analysis was collected partly within the field campaign of the Convective Storm Initiation Project (Browning et al., 2007) and partly by the operational network of the U.K. Met. Office.

The analysis indicates that the zone that could support tornadic storms was very narrow and was found directly on the cold side of a surface warm front. It was characterized by high thermal buoyancy in the lowest few kilometers of the atmosphere for air parcels ascending from the surface. This was a result of a local maximum of low-level moisture. This maximum was caused by strong evapotranspiration and weaker downward mixing of drier air atop the boundary layer, compared to the warm side of the front. Additionally, weaker downward mixing of horizontal momentum ensured that a veering wind profile with height persisted in this zone, creating substantial storm-relative helicity for a right-moving storm. In contrast, turbulent mixing was larger on the warm side of the front where horizontal convective rolls were visible as cloud streets.

In addition to the observation-based analysis, we have used the COSMO model (Steppeler et al., 2003) to simulate the mesoscale environment and storm development. The grid spacing of the model was reduced to 1.1 km, compared with the operationally used 2.8 km. This required an adjustment of the mixing length used for the parametrization of boundary-layer turbulence in order to produce realistic behavior of boundary-layer moisture and convective initiation. The resulting simulation develops storms containing rotating convective updrafts that exhibit low-level rotation. The low-level rotation is maximized when storms cross the aforementioned zone on the cold side of front.

References:

Browning, K.A. and coauthors, 2007: The Convective Storm Initiation Project

Bulletin of the American Meteorological Society, 88, 1939—1955.

Dotzek, N., 2003: An updated estimate of tornado occurrence in Europe, Atmospheric Research, 67, 153—161.

Marshall, T.P. and S. Robinson, 2006: The Birmingham, UK Tornado: 28 July 2005, 23rd Conference on Severe Local Storms, St. Louis, MO, USA.

Steppeler, J., G. Doms, U. Schättler, H.W. Bitzer, A. Gassmann, U. Damrath, G. Gregoric, 2003: Meso-gamma scale forecasts using the nonhydrostatic model LM, Meteorology and Atmospheric Physics, 82, 75—96.