Estimating mass and momentum fluxes in a line of cumulonimbus using a single high‐resolution Doppler radar

Abstract

AbstractWe present a method to derive the vertical‐wind field at a scale of 1–2 km from a high‐resolution Doppler radar, by performing vertical scans in the plane of the mean wind and tracking turbulent features on a scale of several hundred metres in the radial‐velocity field between consecutive scans. The method is applied to a line of cumulonimbus clouds observed by the high‐resolution Chilbolton radar in southern England. The storm was not a squall line, since it was propagating parallel rather than perpendicular to its length. A sequence of scans was taken over an hour‐long period; from these scans we derive the u and w components of the wind. Updraughts of up to 15 m s−1 are found.From these observations, the mass flux of a ‘typical’ storm cell was estimated: over the 50 min lifetime of the cell, the total mass of air transported from the boundary layer to the upper troposphere was found to be around 50 megatonnes. The profile of hour‐averaged momentum flux against height was then calculated and compared to the vertical shear of the mean horizontal wind. It was found that above 4.5 km, the momentum flux was down‐gradient, with an equivalent eddy diffusivity of around 103 m2s−1. Below this height, however, the flux was counter‐gradient, with an equivalent eddy diffusivity of around − 103m2s−1. We propose a mechanism to explain this behaviour, involving the prevalence of precipitation cooling and descent on the downshear side of convergence lines.A synthesis of the major wind flows in the storm reveals that, despite superficial appearance, the storm did not fit the usual multi‐cell paradigm, in which gust fronts play a key roll in triggering new cells downwind. Rather, the convergence lines associated with the convection were more akin to warm frontal surfaces that remain tied to the base of each cell, new cells tending to form upwind of older cells. Copyright © 2008 Royal Meteorological Society