The sensitivity of dependence of large scale flow characteristics in a four-level model atmosphere on a simulated planetary boundary layer

Abstract

A four-level, quasi-geostrophic, β plane, general circulation numerical model is developed, initially without a simulated boundary layer, and the sensitive dependence of large scale flow characteristics on the numerical choice of essential parameters (non-adiabatic heating funnctions, static stability values, and internal turbulent stresses) is noted in a number of long period (100 day) integrations. It is particularly interesting to note that unless the values of turbulent stress coefficients are sufficiently large, the characteristic structures of cyclone waves and eddy kinetic energy functions are lost and these become random flow patterns and functions.

In order to obtain a lower limit estimate of the sensitivity of dependence of large scale flow model characteristics on the presence of a planetary boundary layer, a simple constant ‘height’ (from 1 000 to 900 mb) ‘Ekman boundary layer’ is then introduced, an analytic solution of the linearised boundary layer equations being matched to the numerical model through the vertical velocity field at the model grid points. The effect of this introduction on the structure of the meridional circulation is very marked, and the magnitude of this circulation is seen to depend quite sharply on the numerical value chosen for the model boundary layer eddy viscosity. It is also found that the most realistic large scale flow characteristics are reproduced with a boundary layer present, when the free atmosphere turbulence (vertical) stress coefficients are reduced to quite small values.