MWFM - Mountain Wave Clouds over Jan Mayen

One of the goals of the MWFM 2.0 initiative is to allow the model to accurately simulate and forecast the "ship wave" patterns that emanate from circular three-dimensional topography. One of the best prototypes for this kind of mountain wave activity is provided by Jan Mayen, a small Arctic island chain ~550 km north-east of Iceland (see map). The topography is dominated by Mount Beerenberg , a large quasi-circular volcanic mountain above which mountain wave clouds are often photographed .

Indeed, mountain wave cloud patterns emanating from Mount Beerenberg can be observed from space. They were first observed in high resolution satellite images by Gjevik and Marthinsen [1978], who noticed their "ship wave" shapes and attempted to model them using simple ray methods. The circular nature of Mount Beerenberg means that it acts rather like a rock in a stream, causing bow-shaped waves to radiate away from it as the atmospheric fluid flows over and around this obstacle to the flow.

Figure 1: False color image of mountain wave cloud patterns emanating from Jan Mayen on 25th. January, 2000 at 1627Z. The orange-white regions are polar stratospheric cloud decks formed by cold temperatures on this day.

During the SOLVE mission, high resolution false color NOAA/AVHRR images were were made available in the Scandinavian region to aid forecasting and flight planning (thanks to Andreas Dörnbrack and Robert Meisner of DLR, Oberpfaffenhofen, Germany). We observed a variety of interesting mountain wave cloud patterns emanating from Mount Beerenberg in these images. One of the best examples occurred on January 25, 2000 (shown opposite), when a spectacular ship-wave pattern of mountain wave clouds was weakly seen emanating in the raw images. A digitally enhanced enlarged version of the image is shown in Figure 1. We see mountain waves penetrating large distances downstream of the island (outlined in white) in this image. It is clear that tracing these waves back to their "point source" here clearly implicates Mount Beerenberg as the source. There was also strong north-easterly flow on this day, consistent with the downstream penetration of the patterns in this direction on this day. This gave rise to spectacular mountain wave-induced polar stratospheric clouds (PSCs) over northern Scandinavia on this day as well. The occurrence of wave-induced PSCs on this day was forecast by both the MWFM 2.0 and by the MM5 model operated by Andreas Dörnbrack at DLR. The MWFM 2.0 maps also forecast a broad region of extremely cold mean temperatures in the stratosphere over Jan Mayen on this day, which leads to predictions of large-scale polar stratospheric cloud decks being formed. Indeed, PSC decks are evident in the image above as the white-orange regions in the satellite image. Waves were forecast to be nonturbulent, allowing for safe ER-2 flights from Kiruna for the pilots on this day.


Mountain wave clouds from Jan Mayen on 25th. February, 2000 at 1816Z.	Mountain wave clouds from Jan Mayen on 28th. February, 2000 at 1353Z.
Figure 2

Images on other days revealed different patterns, which showed the importance of surface weather and flow features on the development and nature of the wave cloud patterns over Mount Beerenberg. Figure 2 opposite show two other images in late February. On the 25th. February, we see a developing compact polar low system with southerly flow generating wave cloud patterns to the north of Mount Beerenberg. Three days later the flow is easterly and the mountain-induced ship-wave patterns extend long distances to the west of the island.

A primary goal of our MWFM 2.0 development initiative is to model and forecast these types of wave fields better. Recently, we have developed and tested a sophisticated new algorithm that captures the features of three-dimensional mountain waves radiated from circular orography very well [Broutman et al., 2000]. Further development and testing of these ideas, and incorporation of aspects into the MWFM, are planned.