A field study of nocturnal stratocumulus: I. Mean structure and budgets

摘要

AbstractObservations of anticyclonic stratocumulus obtained with ground‐based and balloon‐borne equipment during the night of 19/20 November 1976 at Cardington, Bedford, UK, are described in relation to the synoptic situation.The mean structure of the atmosphere from ground to about 300m above cloud top is discussed. Of particular interest are the large temperature and humidity ‘jumps’ observed within a layer 1‐10m thick at cloud top, and the thermally stable surface layer about 200m thick in the presence of an overcast sky and surface wind speed of 3–4 ms−1. The mean liquid water content profile is accounted for in terms of turbulence theory, but the cause of observed large variations in total cloud water content is not identified.The locally steady conditions allowed estimates to be made of the local budgets of heat and water for the boundary layer (cloud top to ground) as a whole. In particular, the shape of the temperature inversion above cloud top allowed a quantitative estimate of cloud top entrainment rate to be made.Although the local water budget appears to be in approximate balance, only about one‐third of the radiative loss from cloud top is accounted for locally. It is not possible to account for this deficit in terms of the observations; it is suggested it could result in a downstream mean cooling of the boundary layer of about 0.5K per 100km.The observed dispersal of cloud downstream cannot be explained by the local budget deficit (which would tend to increase cloud). It is estimated that a local subsidence rate of 2–4cms−1through the cloud would be required to disperse it on the observed time scale. Potential sources of evidence for