A new mass flux parametrization for the convective boundary layer based on a mass flux repusentation of thermals
We have developed a mass flux parameterization of the convective boundary layer based on an idealisation of thermal cells (Hourdin et al., J. Atm. Sci., 59:1105-1123, 2002). When introduced in the LMD atmospheric general circulation model (LMDZ), this parameterization of meso-scale motions inhibits the K-diffusion in the mixed layer. The K-diffusion is then active in the surface layer only. In convective situations, buoyant parcels issued from this unstable surface layer are used to compute vertical velocities in the thermals. The width of updrafts is determined based on physical considerations on the geometry of thermal cells or rolls. The upward flux, product of the velocity and width, is compensated by a subsiding motion. This new model was validated systematically by comparison to the results of LES simulations of the dry atmospheric boundary layer (Ayotte et al., Bound.-Layer Meteor., 79:131-175, 1996) and tested in a one-column version of LMDZ. More recently, this parameterization was shown to significantly improve the diurnal cycle of surface concentrations of radon over Europe, as simulated with the 3D GCM, using the zoom capability ("Z" of LMDZ) to reach a resolution of 50 by 50 kilometer square over Europe. In particular, during spring and summer, the parameterization reproduces well the rapid decrease of surface concentrations often observed in the morning, associated to the rapid growth of the convective layer. This parameterization is now developed further in order to simulate cloudy boundary layers. The first step consists in introducing water condensation in the CAPE computation. It may then be necessary to introduce an additional downward flux in the presence of cloud top entrainment instability. This cloud parameterization will be evaluated systematically by comparison with observations at the "Site Instrumental de Recherche par Teledetection Atmospheric", close to Paris.
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003