Parameterization of momentum and energy flux associated with mountain wave across the Assam - Burma hills

Abstract

An attempt has been made to parameterize the energy flux and momentum flux associated with orographically excited internal gravity waves (IGW) across the Assam Burma Hills (ABH) in an idealized air stream. For this a two-dimensional meso-scale model has been developed and used to study the momentum/energy flux associated with vertically propagating orographically excited internal gravity waves for an idealized air stream characteristic, where both wind and stability of the prevailing flow remain invariant with height. Here the barrier is assumed to be extending semi infinitely in the cross-prevailing flow direction.

The above model is then applied to the ABH to compute the fluxes across it. The study shows that both the fluxes are independent of height, the energy flux is upward and momentum flux is downward. Following salient results are found from the study.

Wave momentum flux is vertically downward and wave energy flux is vertically upward for a vertically propagating mountain wave across the ABH.

It is found that the directions of vertical fluxes across the valley between the two ridges of the ABH are opposite to those across the two ridges and across the entire ABH.      

This study also shows that a long valley (length of the valley exceeds the sum of the half widths of individual ridges, separated by the valley) acts as a source in the atmospheric momentum budget and as a sink in the atmospheric energy budget.

From the cases studied, it is found that the downward momentum flux (averaged over a length of 100 km along the flow) across the entire ABH varied between 4.5 – 10.0 Nm^-2 and that across the two ridges varied between 2.0 - 7.0 Nm² and upward momentum flux across the valley varied between 0.5 - 1.2 Nm^-2. It is also found that the upward energy flux (averaged over a length of 100 km along the flow) across the entire Assam- Burma hill varied between 45.0 - 180.0 Wm^-2 and that across the two ridges varied between 20.0 - 125.0 Wm^-2 and downward energy flux across the valley varied between 5.0 - 21.0 Wm^-2.

For the ABH, magnitude of both the fluxes increases with height but decreases with half with of the individual ridges.