Diurnal evoluion of properties of the sub-cloud layer over land

Abstract

Observations of temperature, humidity and velocity structure beneath fields of cumulus clouds are analysed with the needs of cloud modelhng and cloud field parameterization in mind. Sub-cloud fluxes of sensible and latent heat and their diurnal variations are determined from layer-mean profiles and from eddy-correlation measurements; the latter are especially sensitive to sampling on cloud-sized scales. The characteristic fluctuations in velocity temperature and humidity beneath clouds have time scales like the lifetime typical of small cumulus clouds: there is no evidence that such fluctuations occur only as brief pulses at cloud initiation. The magnitudes of these fluctuations exhibit well-marked diurnal patterns of change.

A model of fair-weather cumulus convection

“…Data on the observed lifting condensation level in the mixed layer are not available, and so this cannot be compared with the predicted H,. Coulman (1980) estimates the cloud cover on both days to be 318, which agrees well with the model prediction of about 0.33 on 5 April but which is much higher than the predicted value of about 0.13 on 2 April (see Figure 8). Cloud top is estimated to be 2600 m on 2 April and 30N1 m on 5 April.…”

“…At 1100 hr the model gives H, equal to 890 m while the observations indicate 960 m. At 1220 hr H, is predicted to be 1220 m and the observed level is 1250 m. When cloud is first observed at 1230 hr, the model has H, equal to 1305 m while the observed height is 1270 m. Thus the condensation level appears to be predicted satisfactorily by the model. Coulman (1980) presents two sets of aircraft soundings showing the development of a cumulus-cloud field over land On each day (2 and 5 April, 1976), one sounding is taken in the morning before cloud forms and then six soundings are made every half-hour following the onset of cloud around midday. The surface fluxes of heat and moisture are chosen for the mode1 equations such that the fluxes vary in a simple manner and the mixed-layer values of potential temperature and mixing ratio (0, and qO) are predicted reasonably well.…”