MAP Newsletter no.15 Editorial

It has been approximately a year and a half since the completion of the MAP SOP. The first results are starting to come in and were reported at the recent MAP Science Meeting in Schliersee. The editors of the MAP Newsletter have asked me to give my impressions of our progress and problems in reaching the MAP science objectives. Impressions being what they are, I have made no attempt to discuss here each MAP objective and each line of research being pursued.

One of the emerging themes that struck me was that the split between “dry” and “wet” phenomena was not clear- cut in many cases. For example a number of papers at the Schliersee meeting showed that, for cases of moist southerly flow, moist processes on the south side of the Alps define the characteristics of the medium in which waves (e.g. Foehn and lee waves) on the north side propagate. There were, however, a number of papers concerning gap flow, Foehn, and gravity waves where the effects of moisture could be safely neglected.

Measurements in the Wipptal show fairly convincing evidence for hydraulic-jump-like features in gap flow; theoretical work using the shallow water equations (SWE) generally supports this interpretation. On the other hand, primitive equation models of continuously stratified flow past a ridge with a gap in it are starting to address some of the larger-scale influences. It is my view that we are several steps away from a satisfactory understanding. One of the problems is an old one for mountain meteorologists: Do the SWE capture the essence of the behavior of a vertically unbounded continuously stratified fluid in the circumstances under study? And if so, why?

Newer problems to be addressed are the importance of cold-air advection and drainage effects which were observed during gap flow events.

Prediction of the precise location and timing of mountain- induced gravity-wave activity remains an illusive goal, in part due the uncertainty of effects associated with the existence of a blocked layers in the valleys between mountain ridges. Unknown at the present are the effects of uncertainties in the initial/boundary conditions used in making the wave predictions. I think it’s time to consider making probabilistic forecasts (presumably through ensembles) of gravity-wave events. Similar remarks apply to Foehn prediction. MAP contributions to the latter are still in the potential-to-realized state. The data sets are impressive and some good cases were observed. I look forward to some definitive answers to questions such as “How is the downstream cold-air pool removed?”.

The PV banner work is showing the existence of wakes behind major Alpine peaks, more or less as predicted by models. It is my view that the only quantity that was measured with the required accuracy is the horizontal wind component perpendicular to the mean Alpine ridge (and hence the vertical component of vorticity, assuming the parallel wind component is small). I look forward to seeing results of the model vorticity budgets in the near future. My own personal experience during the SOP was with precipitation studies. I spent seven weeks at the POC in Milano and was personally involved with events IOP4 - IOP15. I spent countless hours staring at radar images from the Lago Maggiore Target Area (LMTA) and I can count on the fingers of one hand the number of times I saw a convective cell. With the exception of IOP2a and other isolated events, the precipitation over the LMTA had a predominantly stratiform character. This conclusion was reinforced by a number of papers given at the Schliersee meeting. Another personal observation from my time in Milano during the SOP was that, although we expected southerly winds during the strong precipitation events, low-level winds at Milano always had a distinct easterly component. Analysis of observations and models of several IOPs indicate that the easterly component is due to flow deflection by the Alpine Massif and that the blocking can exert a strong influence on precipitation in the LMTA. Forecasting of the properties of the blocked layer (and hence the precipitation) was often difficult.

There were a number of papers looking at the effects of a conditionally unstable flow impinging on an Alps-like obstacle, motivated by unstable soundings observed in Corsica (upwind) during heavy rain events. To reconcile these studies with my remarks in the previous paragraph, I think that the instability hardly ever reached the LMTA. It is my opinion that these idealized studies are more relevant to the Maritime Alps, Ligurian coastal mountains, and the Apennines, rather than the Alps north and west of the Po Valley. Heavy convective activity was observed by aircraft and radars in those locations. A related subjective impression I had was that all models typically under-predicted precipitation in the Po Valley; my guess is that the coastal convection has an effect on the flow in the wake of the Apennines (i.e. the Po Valley) that is not well-handled by the models.

In summary, I am encouraged and gratified by the contributions we MAPsters are making to the science of mountain meteorology. The SOP data set contains a wealth of information yet to be exploited, and the team at the MDC is making it easier for us to do so. I’m confident that improved understanding and predictive skill will soon follow.

Rich Rotunno NCAR Boulder, Colorado, USA