Observations of boundary layer structures from CSIP

The mechanisms responsible for the initiation of precipitating convection in the maritime climate of the United Kingdom are not well understood. Whilst much research has been undertaken in other parts of the world on the larger,long-lived convective systems, there has been little focus on the smaller but often locally intense systems experienced in the UK. The goal of the Convective Storm Initiation Project (CSIP) is to understand where and how convective clouds form and develop into showers and thunderstorms. The CSIP field campaigns were based around the Chilbolton radar, which is situated in the low undulating terrain of southern England. The precise location of the initiation of convective cells is often controlled by spatial inhomogeneities in the evolving boundary layer. These perturbartions include horizontal variations in temperature and moisture generated by land/sea contrasts and orographic features. In addition, fine-scale structures in the vertical profile can lead to variations in convective inhibition and affect cumulus development and the resultant precipitation.

In this talk, we examine some of the boundary-layer structures observed during CSIP and how they affected convective initiation and development. One of the unique aspects of CSIP was the ability to observe the clear-air convective field and shallow cumulus clouds with the 1275 MHz radar through the process of Bragg scatter. In range-height indicator (RHI) mode, thermals and cumulus clouds appear as 'mantle' echoes surrounding a region of lower return. Low-level fields of refractivity were also retrieved from this radar by implementing the Fabry technique, whereby changes in the phase of ground targets are related to the refractive index of the air. The refractive index is dominated by humidity therefore these maps can reveal inhomogeneities in surface moisture. Data from the network of automatic weather stations, serial radiosonde ascents and aircraft flights have also been used to map the structure of the boundary layer.