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

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

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

MULTI-SCALE OROGRAPHIC FORCING OF THE ATMOSPHERE LEADING TO AN EROSION EVENT

A satellite image of blowing dust is compared to a simulation of winds during a major erosion event in Iceland. There is large spatial variability in the wind speed and this variability is attributed to the topography. The atmosphere responds particularly stongly to the mountains because of a low-level inversion which is a result of synoptic-scale descent from the Greenland ice cap. The simulation is a part of the new MM5-based forecast system in Iceland (HRAS) and comparison with the patterns revealed by the dust image indicates that all main features of the flow are correcty reproduced by the forecast system. This case study indicates that local enhancment of the wind may be important for erosion

THE RESPONSE OF PRECIPITATION TO OROGRPAHY IN SIMULATIONS OF FUTURE CLIMATE

Precipitation pattern in Iceland in relatively high–resolution simulations of the future climate is investigated. In the winter and in the autumn ther is much greater increase in precipitation over the mountain slopes than elswhere, indicating that a future climate may feature a topographic precipitation gradient that is different than in the current climate. The results are an encouragement to go to even higher resolutions in climate simulations, rather than to apply the so–called delta change to estimate changes in precipitation climate

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

STATISTICS OF FORECAST ERRORS AND OROGRAPHY

We have compared differences between radiosonde observations in SW-Iceland and 48 hour forecast by a numerical weather prediction model over a period of five years (2000-2004). Temperature and height of the pressure levels of 925, 850 and 500 hPa were compared in search for systematic errors. In the overall mean, the predictions have little error and very limited bias. There are however slight seasonal variations and indications of situations where the model does relatively poorly. At 500 hPa there is a cold bias in the forecasts in late winter, but no such bias in the autumn and early winter. At the lowest level there is a tendency of a cyclonic bias in the forecasted wind direction in northeasterly winds and in westerly flow, there is a warm bias in the forecasts. Both of these systematic low-level errors are attributed to non-resolved orography; the bias in the wind direction is most likely due to an underestimation of the deviation of the flow by the mountains and the warm bias appears to be associated with an underestimation of the accumulation of low level cold air upstream of Iceland

OROGRAPHIC IMPACT ON THE PREDICTED CHANGE IN THE FREQUENCY OF CONDITIONS FOR WET SNOW ICING IN FUTURE CLIMATE

Conditions for wet snow icing are estimated in terms of mean daily values of temperature, precipitation and wind speed. On the basis of this estimation, calculations are made of the change in frequency of events of wet snow icing in simulations of future climate. In coastal areas of Iceland, a large reduction in the number of wet snow icing events is predicted for the period 2071-2100, but at an elevation of 400-700 m.a.s.l., there is a different result: in Southwest-Iceland, there is only a moderate decrease in the number of events, while in Northeast-Iceland, there is a large increase in the frequency of wet snow icing events. The decrease of the number of wet snow icing events is associated with fewer days with temperatures close to 0°C, while in the case of an increase in precipitation, the impact of the temperature increase is overrun by an increase in frequency of heavy precipitation in the mountains

THE KVÍSKER 2002 PRECIPITATION RECORD

The precipitation record of Kvísker, SE-Iceland (293 mm/24 hrs) is investigated. Kvísker is located downstream of a 2119 m high mountain and observations indicate that strong wind upstream of the mountain is a key factor in generating the maximum precipitation intensity within the 24 hours period. A numerical simulation of the flow suggests that there are two distinct maxima in the precipitation intensity. One is situated close to the mountain top and is directly associated with the updraft and high concentration of liquid cloud water. The second maximum is located downstream and is associated with localized downdrafts and horizontal convergence of the spillover rain