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 wi nd case), both with cold air advection. We perform a scale analysis in orde r to identify the main transporting scales of the turbulent fluxes and to i nvestigate the mesoscale variability of the sensible heat and moisture flux . The example of a mesoscale moisture front indicates the importance of sub grid mesoscale variability. The turbulent fluxes of sensible heat and moist ure are affected significantly by this mesoscale variability. This mesoscal e 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 sensibl e heat and moisture. The average vertical mesoscale flux divergence is smal l compared to the turbulent flux for the two cases investigated. However, t he vertical mesoscale flux can be large locally. The budgets indicate the importance of horizontal advection for the NOPEX r egion. The time evolution of sensible heat and moisture during 14 June appe ars to be dominated by large scale horizontal advection. A comparison betwe en the area-averaged surface flux from aircraft data and surface flux stati ons for the 13 June is not conclusive because of fractional cloud cover. Fo r the strong wind case on 14 June, the surface fluxes obtained from aircraf t data and from surface flux stations disagree, which appears to be related with the pronounced mesoscale moisture variability. (C) 1998 Elsevier Scie nce B.V. All rights reserved.