The cloud-topped marine boundary layer as observed during the Atlantic Tradewind Experiment (ATEX) has been idealized for study as part of the 5th GCSS Boundary-Layer Cloud Working Group intercomparison study (to take place in Madrid Spain in August 1998). The case is characterized by a classic Tradewind structure wherein cumulus clouds with bases near 700m detrain under the base of a strong trade inversion near 1500m. Observed cloud fractions fluctuate between 30 and 80 % with a strong diurnal cycle. Objectives of the intercomparison are three fold: (1) to understand what processes regulate cloud fraction in the tradewind regions, (2) to assess the ability of LES to represent such processes, and (3) to evaluate the fidelity of operational boundary layer parameterizations in an ATEX like environment. In this talk we focus on the first two objectives; the third objective is discussed in a companion talk (Cuxart et al., abstract modified).
Preliminary comparisons show that LES is capable of representing the ATEX boundary layer, in which cloud fractions are typically around 0.5. In contrast to a previous intercomparison of clouds observed during BOMEX (in which cloud fractions were significantly lower), a thin layer of enhanced cloud-fraction develops at the base of the trade inversion. A key difference between the ATEX and BOMEX simulations is that ATEX took place upstream of BOMEX and is thus characterized by cooler SSTs leading to a cooler PBL and a stronger trade inversion. The high relative humidity that develops at the base of the trade inversion is favorable for the development of stratiform clouds in this layer. Although cloud fractions derived from LES are sensitive to grid resolution, LES does show a significant sensitivity to various physical processes. In particular we find that both the effect of longwave cooling from the top of an incipient stratiform layer, as well as the magnitude of the entrainment drying (which for a given entrainment rate is largely controlled by the strength of the jump in total water across the trade inversion) play an important role in determining cloud fraction.