Characteristics of Thermals in Deep Convective Storms from Radar Observations

The dynamic and turbulent processes within convective storms are challenging to observe due to the range of scales involved and the quickly evolving nature of convection. Meanwhile, the need for such observations is rising as numerical weather prediction models are more commonly run at O(100m) grid lengths for both weather and climate research and even for operationally purposes. At such high resolution, individual updrafts and the largest turbulent eddies may be resolved, but observations to constrain these are generally lacking.

Here, we present new observational and analysis techniques to estimate the characteristics of thermals in deep convection from radar observations. Convective cores are identified as localised regions of intense rainfall in the Met Office Radarnet 5-minute rainfall product for a region in southern England. The locations and directions of these cores are used to automatically steer the Chilbolton Advanced Meteorological Radar (CAMRa), a dual-polarisation S-band radar with 25m antenna. Sets of 4 range-height indicators scans (RHIs) are performed at equal spacing across the location of the convective core. By tracking the same convective core over a period of time, 2D wind fields (range-height) are derived from consecutive RHIs using an optical flow method on the Doppler returns. Then, thermals are identified as regions with vertical winds greater than 4 m/s and at least 200m across with a lifetime of at least 2 minutes.

From multiple cores observed over separate days, we find that some thermals last up to 10 minutes. The lowest starting height is around 2km, close to the melting layer, though possibly constrained by the viewing geometry and analysis techniques. The mode of thermal diameter is 1.25km, comparable to findings from LES. Eddy dissipation rates - retrieved from the radar Doppler spectrum – are found to be greater along the edges and tops of thermals, in regions with stronger wind shear.

During the WesCon field campaign, which took place summer 2023 in southern England, the NCAS cloud scanning radar “Kepler” (Ka-band), coordinated its scanning with CAMRa, enabling a quasi-3D view of the core and its turbulent structures. Preliminary findings will be presented, as well as implications for evaluating turbulent structures in O(100m) model simulations and LES.