Tuesday, January 24, 2012

A note on "forcing"

It is easy to take a hard look at the maps and determine easily what constitutes strong "forcing". Usually we see a highly dynamical setup (e.g. deepening surface low, intensifying short wave trough) and immediately point to strong "forcing" as a reason for an outbreak. So what did yesterdays outbreak look like in terms of forcing, where we can be specific and look at two metrics of forcing: 700 hPa Q-vector divergence (shaded) and thermal absolute vorticity advection (contour). The Q vector divergence is an approximation for the QG omega equation forcing function. Thermal vorticity advection is the Trenberth approximation for QG omega; though as noted by Sanders (1990) the divergence of Q vector method may be more reliable in frontogenetical forcing. So here is the map of these two forcing functions on the NAM 12 km grid:
The same convection as yesterdays post is used comparing the 36 (upper left),24 (upper right),12 (lower left) hr forecast to the model analysis (lower right). Although this plot is for 700 hPa the one at 500 hPa was similar. Clearly the front (e.g. cold front aloft) was present the forcing is not that strong according to the model analysis, though the forecasts suggest a much greater forcing than diagnosed through the data assimilation system. One could reach the same conclusion from yesterdays plot of the derived QG omega (through the harmonic method used in SUNYPak) at 500 hPa (shown again below). At most both of these plots suggest that forcing in the region of the outbreak in AR and later in MS and AL was more weak to moderate than strong. The forcing for ascent shown here is localized to the front aloft. Where there was strong forcing indicated by the direct retrieval of 500 hPa QG omega was located around Kansas City ahead of the upper low relative to its translation as a negatively tilted trough. Certainly we can say this was a strong upper low but by no means was the outbreak area under strong dynamic forcing.