Formulation of a Bulk Boundary Layer Model with Partial Mixing and Cloudiness
Abstract
The classical cloud-topped mixed-layer model is generalized to allow for arbitrary fractional cloudiness and incomplete mixing. The boundary-layer depth and turbulence kinetic energy (TKE) are prognostically determined. The large turbulent eddies that contain most of the TKE and are primarily responsible for the fluxes are modeled as convective circulations, with ascending and descending branches. By assuming that the ventilation and entrainment layers at the lower and upper edges of the PBL are dominated by small-scale turbulence in quasiequilibrium, boundary conditions are developed for the rising and sinking branches of the convective circulations, and also for the scalar variances associated with the convective circulations. The convective mass flux and the fractional area covered by updrafts are diagnosed by the model. Fractional cloudiness occurs when the ascending branches are saturated and the descending branches are not. A modified bulk formula is used in which the square root of the TKE takes the place of the wind speed. The advantages of this approach are discussed. The entrainment rate is also assumed to be proportional to the square root of the TKE; the proportionality factor depends on the inversion Richardson number, and also on an additional parameter that represents the effects of evaporative cooling when clouds are present. The ventilation mass flux is similarly parameterized. Instead of using a conventional bulk formula in which the wind speed is multiplied by a transfer coefficient, we use a modified bulk formula in which the square root of the TKE takes the place of the wind speed.
Document Details
- Document Type
- Technical Report
- Publication Date
- Jan 01, 1990
- Accession Number
- ADA245704
Entities
People
- David A. Randall
- Qingqiu Shao
Organizations
- Colorado State University