THE HEAT SOURCE OF THE FOEHN

Twelve years of upper air data and surface observations across Iceland are used to investigate the connection between the difference of surface air temperature upstream and downstream of the Icelandic highlands and several parameters of the airflow, such as precipitation, static stability and wind speed. The data does not support the well-known concept of the heat of the foehn being a result of latent heating. In the winter, warm air, descending from above the upstream boundary layer appears to be responsible for the downslope heating. There is little correlation between the upstream wind speed and the upstream-downstream temperature difference. This is explained by weak winds contributing to low level upstream blocking and descent from upper levels in the lee, while strong winds contribute directly to vertical mixing through mechanical turbulence. The annual cycle of temperature difference between the upstream and the downstream slopes indicates that in the summer, solar heating over the downstream slopes is responsible for a substantial part of the heating of the foehn

QUASI-GEOSTROPHIC FLOW PAST MOUNTAINS

Blocked flow at moderate Rossby numbers is studied and an analytic expression is derived for the vertical velocities on each side of the wake: W = (Uo h) / (f Lx Ly) Where Uo is the incoming wind speed, h is the mountain height, f is Coriolis parameter, Ly is mountain half-width and Ly is a length scale of the vertical mortion downstream of the edges of the mountains. To illustrate the use of the above expression, a collection of flows over Iceland is presented and the calculated vertical velocities are compared to the number of days with precipitation in two different regions

CASES OF LARGE FORECAST ERRORS OVER ICELAND

Forty-eight hour numerical forecasts during a period of 5 years are studied with emphasis on cases of false alarms and missed windstorms at 850 hPa. The overall performance of the forecast system is very good. Windstorms from the southwest are very well predicted, there are a few false alarms in southeasterly winds and northeasterly windstorms tend to be underestimated by the forecast model. The false alarms are in many cases associated with fronts, where a slight shift of a position of the weather system in time may give a large difference in the forecasted and observed winds. Yet, the true value of the forecast may be high. We attribute an underestimation in the wind speed in northeasterly windstorms to non-resolved orography, leading to an underestimation of the corner effect SW-Iceland, and possibly to winds that are generated by a pressure gradient at the western side of the Iceland wake