Pressure Gradient Errors in CAM-SE

Wednesday, May 6, 2015 - 07:00
Add to Calendar

In models like CAM which use terrain following coordinates, there can be large numerical artifacts in regions of rough topography related to how one numerically computes horizontal gradients. The "pressure gradient" term, in terrain following coordinates decomposes into two large terms which ideally would cancel, but instead when small relative errors in each term fail to exactly cancel, the result is large errors creating spurious sources of divergence leading to anomalously large vertical velocities over steep terrain. It is conjectured that these anomalous velocities are the main contribution to the precipitation biases seen in CAM over steep terrain. Hence one of our tasks in ACME is to improve the numerics used to compute the horizontal pressure gradient. This subject has been extensively studied and there are several techniques in the literature which will be tested in ACME. Before embarking on this work, we will first establish in ACME how much of the precipitation biases seen over steep terrain is related to anomalous vertical velocity, as opposed to other effects such as the physical parameterizations triggered by surface roughness. This poster will our experimental setup and preliminary results looking at the relationship between precipitation and vertical velocity as a function of topography and surface roughness.