a Linear Analysis of Monsoon Base States Associated with Banded Convection.
The response of a linearized system of anelastic perturbation equations to varied wind, temperature, moisture, and diabatic heating profiles representative of base states found during the Arabian Sea monsoon is investigated as an eigenvalue problem. The diabatic heating profile for deep convection is parameterized using the Anthes scheme which is driven by a 1DSS cloud model. Shallow non-precipitating convection is parameterized using the Betts two layer cloud model. The base state wind profiles are fit with a least squares curve whose order may vary. Eddy diffusion coefficients are determined from the cloud model using a Smagorinsky -Lilly type formulation. Moisture is accessed into the linear model via either boundary layer moisture accession or the vertically-integrated advection over the cloud depth. Sensible warming, a consequence of environmental subsidence, and cooling in the cloud layer below 2 km, a result of the moist downdrafts, are included in the calculations. Results suggest the shapes of the convective heating and horizontal wind profiles are crucial in determining the onset of the longitudinal stationary mode as the precipitation efficiency is increased. Stationary longitudinal modes are associated with a minimum of flux Richardson number in the wavenumber domain. Including sensible heat transports in the cumulus parameterization shifts the convective heating maximum to a higher level and delays the onset of the stationary mode. The stationary mode onset (critical point) is characterized by sharp reductions in the flux Richardson number and phase speed for the longitudinal mode. For deep convection, the shear-parallel momentum flux remains down-gradient through the stationary mode onset while the shear-perpendicular momentum flux becomes up-gradient at the onset of the stationary mode. Analysis of the energy transfer spectra suggests that kinetic energy conversion is down-scale for both longitudinal and transverse modes prior to the critical point but becomes up-scale for transverse modes after the onset of stationarity.
- Pub Date:
- Physics: Atmospheric Science