Air motion and precipitation growth at a cold front

A case study of a cold front has shown that, although appreciable ascent occurred over a deep layer, practically all precipitation growth was associated with the ascent of air which originated within the friction layer ahead of the front. Doppler radar observations showed that the ascent was accomplished in two phases; first through near‐vertical line convection to between 1 and 3 km at the surface cold front, and thence through shallow‐slope convection of the same air to between 3 and 6 km. The line convection was 2‐dimensional rather than cellular and occurred in the absence of appreciable hydrostatic instability. Horizontal convergence at low levels was very intense (10−2 s−1 averaged over 500 m vertically and horizontally), so that despite the shallowness of the line convection, the updraught attained a rising speed of 8 m s−1 which was sufficient to generate hail and thunder. The subsequent slope convection produced a period of moderate precipitation behind the surface cold front, and was associated with a pronounced transverse circulation, with strong gradients of velocity separating the weak downdraughts in (and beneath) the sloping frontal zone from the overlying updraught. The overall efficiency of precipitation production was high, 60 per cent of the water vapour flux in the rising air reaching the ground as precipitation.