A Meteorological Experiment in the Melting Zone of the Greenland Ice Sheet

Preliminary results are described from a glaciometeorological experiment carried out in the margin (melting zone) of the Greenland ice sheet in the summers of 1990 and 1991. This work was initiated within the framework of a Dutch research program on land ice and sea level change. Seven meteostations were operated along a transect running from the tundra well onto the ice sheet. At the ice edge, humidity, temperature, and wind profiles were obtained with a tethered balloon. On the ice sheet, 90 km from the edge, a boundary-layer research unit, including a sound detecting and ranging system (SODAR) and a radio acoustic sounding system (RASS), was established. Although focusing on the relation between surface energy balance, glacier mass balance, and ice flow, the experiment has also delivered a unique dataset on the dynamics of the atmospheric boundary layer around the warm tundra-cold ice sheet transition. Unexpected behavior was found for the surface albedo during the melt season. Lowest values are not found close to the ice edge, which is usual for glaciers, but higher on the ice sheet. Meltwater accumulation due to inefficient surface drainage was found to be the cause for this. The wind regime is dominated by katabatic flow from the ice sheet. The katabatic layer is typically 100-200 m thick. Close to the ice edge, the flow exhibits a very regular daily rhythm, with maximum wind speed in the afternoon. Farther on the ice sheet, the regime changes, and wind speed reaches maximum values in late night/ early morning

Turbulence characteristics of the stable boundary layer over a mid-latitude glacier. Part II: pure katabatic forcing conditions.

Observations obtained over a glacier surface in a predominantly katabatic flow and with a distinct wind maximum below 13-m height are presented. The data were collected using a 13-m high profile mast and two sonic anemometers (at about 2.5-m and 10-m heights). The spectra at frequencies below that of the turbulence range appear to deviate considerably from the curves obtained by Kaimal and co-workers during the 1968 Kansas experiment. The characteristics of these deviations are compared to the observations of others in surface-layers disturbed by any kind of large-scale outer-layer (or inactive) turbulence. In our case the disturbances are likely to be induced by the high mountain ridges that surround the glacier. Moreover, the deviations observed in the cospectra seem to result from an, as yet, unspecified interaction between the inactive outer-layer turbulence and the local surface-layer turbulence. Near the distinct wind maximum turbulence production ceased while turbulence itself did not, probably the result of turbulence transport from other levels. Consequently, we studied the local similarity relations using σ(w) instead of u(*) as an alternative velocity scale. Well below the wind maximum, and for relatively low stability (0 0.2), and near or above the wind maximum, the boundary-layer structure conforms to that of z-less stratification suggesting that the eddy size is restricted by the local stability of the flow. In line with this we observed that the sensible heat fluxes relate remarkably well to the local flow parameters

Wet canopy evaporation from a Puerto Rican lower montane rain forest: the importance of realistically estimated aerodynamic conductance

Rainfall interception (I) was measured in 20m tall Puerto Rican tropical forest with complex topography for a 1-year period using totalizing throughfall (TF) and stemflow (SF) gauges that were measured every 2-3days. Measured values were then compared to evaporation under saturated canopy conditions (E) determined with the Penman-Monteith (P-M) equation, using (i) measured (eddy covariance) and (ii) calculated (as a function of forest height and wind speed) values for the aerodynamic conductance to momentum flux (