Regional surface fluxes over the NOPEX area

The regional surface fluxes of sensible heat and moisture are estimated for two days during NOPEX '94 by means of aircraft measurements. A box flight pattern is analysed for 13 June (a weak wind case) and 14 June (a strong wind case), both with cold air advection. We perform a scale analysis in order to identify the main transporting scales of the turbulent fluxes and to investigate the mesoscale variability of the sensible heat and moisture flux. The example of a mesoscale moisture front indicates the importance of subgrid mesoscale variability. The turbulent fluxes of sensible heat and moisture are affected significantly by this mesoscale variability. This mesoscale variability is usually subgrid in numerical models and therefore must be parameterized. The source of the mesoscale moisture variability appears to be related to a cold front. Mesoscale flux divergence is explicitly included into the budget of sensible heat and moisture. The average vertical mesoscale flux divergence is small compared to the turbulent flux for the two cases investigated. However, the vertical mesoscale flux can be large locally. The budgets indicate the importance of horizontal advection for the NOPEX region. The time evolution of sensible heat and moisture during 14 June appears to be dominated by large scale horizontal advection. A comparison between the area-averaged surface flux from aircraft data and surface flux stations for the 13 June is not conclusive because of fractional cloud cover. For the strong wind case on 14 June, the surface fluxes obtained from aircraft data and from surface flux stations disagree, which appears to be related with the pronounced mesoscale moisture variability. (C) 1998 Elsevier Science B.V. All rights reserved.