Burghard Brümmer

Atmospheric boundary layer rolls observed by the synthetic aperture radar aboard the ERS-1 satellite

Two synthetic aperture radar (SAR) images acquired by the European Remote Sensing Satellite ERS-1 over the Jade-Weser estuary in the German Bight of the North Sea on January 2 and 20, 1992, are analyzed. The images show sea surface manifestations of atmospheric boundary layer rolls. This is inferred from the orientation of the quasi-periodic sea surface patterns which are aligned approximately with the wind direction, from the ratio of the wavelength of the patterns to the height of the boundary layer, and from the conditions encountered in the atmospheric boundary layer as measured quasi-simultaneously by radiosondes. The atmospheric boundary layer rolls were generated by a dynamic instability on January 2 and by a thermal instability on January 20. For the first time, quantitative estimates of variations of the wind velocity at the sea surface associated with the atmospheric rolls are extracted from a spaceborne radar SAR image. It is shown that wind velocities derived from SAR image intensity variations are in agreement with theoretical estimates.

Boundary Layer Mass, Water, and Heat Budgets in Wintertime Cold-Air Outbreaks from the Arctic Sea Ice

Abstract Eleven cold-air outbreaks from the Arctic sea ice to the open water of the Fram Strait and the Norwegian Sea have been monitored by aircraft during the field campaigns ARKTIS 1991 and ARKTIS 1993. Budgets of mass, water vapor, water, and heat in the atmospheric boundary layer are computed for boxes that are located at different distances from the ice edge ranging from the marginal ice zone to several hundred kilometers downstream. Averaged over all cold-air outbreaks, the large-scale flow is divergent near the ice edge and convergent at larger distances from the ice edge. Regardless of divergence, the large-scale flow exports everywhere water vapor, water, and heat from an atmospheric box within the boundary layer. In the case of the water vapor budget this export and the loss by condensation in clouds are compensated by evaporation from the sea surface. Both the condensation in clouds and surface evaporation increase in downstream direction, as does their ratio from about 0.4 near the ice edge t...

Boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice

During the field experiment ARKTIS 1993 ten cases of boundary-layer modification in wintertime cold-air outbreaks from the Arctic sea ice in the Spitsbergen region were observed by aircraft over a distance ranging from about 50 km over the ice to about 300 km over the water. The modification depends decisively on the initial conditions over the ice, the boundary conditions at the bottom and top of the boundary layer and on the conditions of the large-scale flow. The modification of the bulk boundary-layer characteristics in relation to these conditions is presented.Besides the air-sea temperature contrast, the most important role for the boundary-layer modification is played by the stability on top of the boundary layer and by the divergence of the large-scale flow. According to the high variability of these conditions the observed boundary-layer modifications were very variable ranging from 100 to 300 m thick boundary layers with air temperatures between -32 and -22 °C over the ice to thicknesses between 900 and 2200 m and air temperatures between -15 and -5 °C after 300 km fetch over the open water. In most cases the large-scale flow was anticyclonic and divergent over the ice and changed to cyclonic and convergent over the water and an ice-sea breeze was superimposed on it.The sensible and latent heat fluxes are the dominant terms in the surface energy budget over the open water and ranged between 200 and 700 W m-2 whereas the net longwave radiation is the dominating term over the ice with the heat fluxes only about 10 W m-2.

Observations And Modelling Of The On-Ice And Off-Ice Air Flow Over The Northern Baltic Sea

Two cases of on-ice and off-ice air flow characterizing the opposite weather situations over the ice-edge zone in the northern Baltic Sea are analysed on the basis of aircraft observations, and modelled using atwo-dimensional mesoscale model. The stable boundary layer (SBL) during theon-ice flow exhibited little thermal modification, but a low-level jet (LLJ) was generated at the 250-m high top of the SBL. In the model, the LLJ was associated with inertial oscillations in space, while the baroclinicity explained the shape of the wind profile well above the SBL. Although the observed LLJ was most pronounced over the ice, the modelling suggests that it was not generated by the ice edge but by the coastline some 400 km upwind of the ice edge, where a much more drastic change in the thermal stratification and surface roughness took place. The generation, maintenance, and strength of the LLJ were very sensitive to the parameterization of turbulent mixing in the SBL. In the case of the off-ice flow, the modification of the air mass and the development of a convective boundary layer (CBL) both over the ice and open sea were reasonably well modelled. Sensitivity runs suggested that it was essential to take into account the effects of subgrid-scale leads, a forest in the archipelago (which was crossed by the air flow), and water vapour condensationinto ice crystals. The heat flux from leads was particularly important for the heatbudget of the CBL, and the observed growth of the CBL was partly due to theeffective mixing over the rough and relatively warm forest.

Roll Convection within an Arctic Cold-Air Outbreak: Interpretation of In Situ Aircraft Measurements and Spaceborne SAR Imagery by a Three-Dimensional Atmospheric Model

Abstract Atmospheric roll convection within an Arctic cold-air outbreak was observed over the Greenland Sea during the ARKTIS 1993 experiment on 24 March 1993 by in situ aircraft measurements and synthetic aperture radar (SAR) imagery from the first European Remote Sensing satellite (ERS-1). Inside a boundary layer heated from below, two kinds of rolls were observed, one aligned parallel and the other perpendicular to the mean wind direction. The wind-parallel rolls occupied the entire boundary layer, whereas the wind-perpendicular rolls were confined to a region around the top of the boundary layer, where a strong vertical shear in the downstream wind component was observed. A three-dimensional numerical model has been applied to simulate the observed convective pattern. It is shown that the model does not reproduce the observed pattern when using a height-constant geostrophic wind profile. However, when adjusting the vertical wind profile to the one measured from the aircraft, the model reproduces buoya...

Wintertime roll and cell convection over Greenland and Barents Sea regions: A climatology

Wintertime cold-air outbreaks from the polar ice and land surfaces over the open sea lead to organized convective patterns (OCP) in the atmosphere, visible as cloud streets and cellular cloud structures on satellite images. Large amounts of energy are transferred from the ocean to the atmosphere and convective organized flows contribute substantially to the vertical fluxes of heat, moisture and momentum in the boundary layer. However, little is known about the frequency of OCP occurrence. The paper is aimed to fill this gap and presents a climatology of OCP occurrence over the Greenland Sea and Barents Sea. The study is based on daily NOAA satellite images of 10 winters (November till March) for the years from 1985/1986 to 1994/1995. It covers the area from 70°N to 82.5°N and from 20°W to 60°E and is subdivided into 40 subareas with a grid size of 10 degrees in longitude and 2.5 degrees in latitude. OCP occur in more than 50% of the time averaged over the 10 winters. Complete absence of OCP is observed in less than 5% of the time. OCP occur most frequently over the Westspitsbergen current and around the border between the Greenland Sea and the Barents Sea. Cloud streets are the dominating OCP mode close to the ice edge, whereas cellular structures dominate at farther distances. Variability of OCP occurrence may be large on all timescales. A relation between ice extent and frequency of OCP occurrence is present occasionally.

Boundary-layer observations over water and arctic sea-ice during on-ice air flow

Observations made on 8 and 9 May 1988 by aircraft and two ships in and around the marginal ice zone of the Fram Strait during on-ice air flow under cloudy and cloud-free conditions are presented.The thermodynamic modification of the air mass moving from the open water to the ice over horizontal distances of 100–300 km is only a few tenth of a degree for temperature and a few tenth of a gram per kilogram for specific humidity. This is due to the small temperature differences between sea and ice surfaces. During the day, the ice surface is even warmer than the sea surface. The stably stratified 200–400 m deep boundary layer is often topped by a moisture inversion leading to downward fluxes of sensible as well as latent heat.The radiation and energy balance at the surface are measured as functions of ice cover, cloud cover and sun elevation angle. The net radiationRNis the dominating term of the energy budget. During the day, the difference ofRNbetween clear and overcast sky is only a few W/m2 over ice, but 100–200 W/m2 over water. During the night,RNover ice is more sensitive to cloud cover.The kinematic structure is characterized by strong shears of the longitudinal and the transversal wind component. The profile of the latter one shows an inflection point near the top of the boundary layer. Dynamically-driven roll circulations are numerically separated from the mean flow. The secondary flow patterns have wavelengths of about 1 km and contribute substantially to the total variances and covariances.

Temporal and spatial variability of surface fluxes over the ice edge zone in the northern Baltic Sea

Three land-fast ice stations (one of them was the Finnish research ice breaker Aranda) and the German research aircraft Falcon were applied to measure the turbulent and radiation fluxes over the ice edge zone in the northern Baltic Sea during the Baltic Air-Sea-Ice Study (BASIS) field experiment from 16 February to 6 March 1998. The temporal and spatial variability of the surface fluxes is discussed. Synoptic weather systems passed the experimental area in a rapid sequence and dominated the conditions (wind speed, airsurface temperature difference, cloud field) for the variability of the turbulent and radiation fluxes. At the ice stations, the largest upward sensible heat fluxes of about 100 Wm�2 were measured during the passage of a cold front when the air cooled faster (�5 K per hour) than the surface. The largest downward flux of about �200 Wm�2 occurred during warm air advection when the air temperature reached +10�C but the surface temperature remained at 0�C. Spatial variability of fluxes was observed from the small scale (scale of ice floes and open water spots) to the mesoscale (width of the ice edge zone). The degree of spatial variability depends on the synoptic situation: during melting conditions downward heat fluxes were the same over ice and open water, whereas during strong cold-air advection upward heat fluxes differed by more than 100 Wm�2. A remarkable amount of grey ice with intermediate surface temperature was observed. The ice in the Baltic Sea cannot be described by one ice type only.

The Atmospheric Boundary Layer In An Arctic Wintertime On-Ice Air Flow

A warm on-ice air flow from the open water over the Arctic sea ice in the Fram Straitwas, for the first time, systematically measured on 12 March 1998 by aircraft in thelowest 3 km over a 300-km long distance. The air mass modification and the processesinvolved are discussed.Over the water, air temperature was lower than water temperature so that a convectiveboundary layer (CBL) was present as initial condition. As soon as the CBL passed theice edge, a shallow stable internal boundary layer (IBL) was formed. In the residual CBL, turbulence and pre-existing convective clouds dissolved within about 20 km. Within about the same distance, due to the transition from unstable to stable stratification, the influence of surface friction increased in the IBL and decreased above the IBL with consequent generation of a low-level jet at IBL top. The IBL was strongly stratified with respect to both temperature and wind. The wind shear was around 0.1 s-1 so that the Richardson number in the IBL was subcritical and turbulence was generated. The IBL top grew to about 145 m over 230 km distance. The growth of the IBL was not monotonic and was influenced by (a) inhomogeneous ice surface temperatures causedby both different ice thickness and changes in the cloud conditions, and (b) leads in theice deck. At the front side of the on-ice flow, the air mass boundary between the warmair and the cold Arctic air was sharp (12 K over 10 km) at low levels and tilted withheight. Observations suggest that the stratified IBL was lifted as a slab on top of thecold air.

The Iceland–Lofotes pressure difference: different states of the North Atlantic low-pressure zone

Abstract The extended North Atlantic low-pressure zone exhibits two pressure minima in the long-term winter mean: the primary one west of Iceland and the secondary one near Norwegian Lofotes Islands. Based on the ERA-40 data set and on wintertime monthly sea level pressure (SLP) anomalies at both places, the states of co-and antivariability are investigated. The covariability represents states of a strongly or weakly developed North Atlantic low-pressure zone The difference between these two states represents the NAO pattern. The antivariability is defined by an Iceland—Lofotes difference (ILD) index, which is positive (negative) when the anomaly in the Lofotes area is higher (lower) than that in the Iceland area. An ILD pattern is calculated as difference between SLP composites for high and low ILD indices. The ILD pattern extends horizontally beyond the two centers and affects other prominent Northern Hemisphere pressure centres: Aleutian low; Siberian high and Azores high. The pattern extends into the stratosphere and shows significant impacts on surface air temperature, Arctic sea ice concentration and sea ice motion.