Water‐vapour flux and flux‐divergence over Southern England: Summer 1954

Abstract Average zonal (eastward) and meridional (northward) transfers of water vapour for the period June‐August 1954 are computed at four aerological stations in southern England ‐ Liverpool, Hemsby, Crawley and Camborne‐at levels corresponding to the earth's surface, 950, 900, 850, 800, 750, 650, 550, 450, and 350 mb. In the zonal direction average transfers are eastwards and reach a maximum of 400–500 g (cm 100 mb sec) −1 below 900 mb. In the meridional direction both northward and southward transfers occur and their vertical distribution varies from station to station. Largest meridional transfers are of the order of 100 g (cm 100 mb sec) −1 and normally occur below 800 mb. Both zonal and meridional transfers become small near 350 mb. Vertically integrated eastward and northward transfers are respectively: Liverpool 1461, ‐13; Hemsby 1541, 226; Crawley 1701, 132; Camborne 1781, ‐42 g (cm sec) −1. . Average zonal eddy‐transfers are normally eastward and amount to only a small percentage of the total zonal transfer. Average meridional eddy‐transfers are northward, amount to a large percentage of the total meridional transfer and in some cases are oppositely directed. Vertically integrated eastward and northward eddy‐transfers are respectively: Liverpool 63, 73; Hemsby 81, 58; Crawley 97, 27; Camborne 113, 125 g (cm sec) −1. Although directed roughly from high to low values of average specific humidity, average eddy‐transfer vectors at a given (geometric or pressure) level bear little quantitative relation to average (horizontal or isobaric) specific humidity gradients at the same level. Assuming the above transfers to vary linearly between stations, computation of the net horizontal outflow of water vapour from the region shows marked convergence of water vapour from the surface to 850 mb with divergence above this. For the three months' period this amounts to a net inflow of 31 g cm −2 at low levels and a net outflow of 23 g cm −2 at higher levels. Increase in storage amounts to approximately 1 g cm −2 , thus leaving a surplus of precipitation over evaporation of approximately 7 g cm −2. Estimation of precipitation from isohyetal charts and evapotranspiration by empirical methods yields a corresponding surplus of 6 g cm −2. . Computation of the large‐scale vertical transfer of water vapour (using instantaneous kinematically‐computed vertical velocities) at 900, 800, 700 and 600 mb showed that at these levels large‐scale vertical eddy‐transfer of water vapour is negligibly small. Using mean vertical velocities and specific humidity averaged over the area at 950, 850, 750, 650, 550 and 450 mb, computations yield large‐scale upward transfers of water vapour for the three months' period amounting to 17, 23, 13, 6, 2, 0 g cm −2 respectively at the above levels. This flux is shown to be inadequate for water‐balance requirements and it is concluded that a considerable part of the upward transfer is carried by turbulent and convective systems of essentially smaller scales.