The dynamics of idealized katabatic flow over a moderate slope and ice shelf

Abstract

SUMMARYA non-hydrostatic numerical weather prediction model has been employed to simulate idealized katabatic flows over a moderate slope and adjoining ice shelf. The topography of Coats Land and the adjoining Brunt Ice Shelf, Antarctica, has been used; this is typical of much of the Antarctic coastline. The Regional Atmospheric Modeling System Version 4.3 has been adapted for simulations over compacted snow, most notably through changes to the multi-layer soil model. The simulations are initialized using clear-sky conditions and at rest. On the slope, a shallow katabatic flow develops, the winds becoming approximately steady on the slope by ∼12 h. The peak downslope winds are about 7 m s −1 at 30 m above the snow surface. The katabatic flow depth ranges from 50 to 100 m down the slope. Over the ice shelf the katabatic flow peters out, while a pool of cold air develops, primarily through sensible-heat loss into the surface and partially balancing the net radiative-heat loss to space. Near-surface and sounding data from the model simulations compare well with archetypal and typical katabatic flow observations, especially after some tuning of the model's turbulence parametrization.An analysis of the downslope flow dynamics shows the buoyancy force is generally balanced by the inertial force, except towards the foot of the slope where it is balanced by upslope forces related to gradients in the potential-temperature deficit and katabatic-layer height, caused by the pool of cold air over the ice shelf. Over time, the cooling of the ice shelf boundary layer leads to an apparent retreat of the katabatic flow from the ice shelf and some way up the slope. The dynamical analysis explains the surface climatology observed, such that the persistent katabatic winds of Coats Land rarely reach the Brunt Ice Shelf. The simulated katabatic flow moves from 'shooting' to 'tranquil' towards the foot of the slope. This transition acts to trigger a train of internal gravity waves which propagate energy upwards away from the katabatic flow jump. Previous studies have also found shooting to tranquil katabatic flow transitions, so to generalize these findings would suggest that internal gravitywave generation is ubiquitous around much of coastal Antarctica and Greenland.

Profiles of katabatic flow in summer and winter over Coats Land, Antarctica

“…Ball 1960;Gallée and Schayes 1992;Gallée and Pettré 1998;Heinemann 1999). This hypothesis was confirmed by a momentum budget analysis in the numerical modelling study of Renfrew (2004).…”

“…They have been seen in recent idealized modelling experiments for the Coats Land region by Renfrew (2004) and Yu et al (2005). Renfrew (2004) found internal gravity waves being triggered towards the foot of the Coats Land continental slope, close to the C2 site, as the katabatic flow went from weakly supercritical to subcritical in response to cold air building up on the Brunt Ice Shelf and damming the flow. Yu et al (2005) examined katabatic jumps in detail, finding their location to be a function of the depth and strength of this cold-air pool.…”

“…According to this surface climatology, the AWS site subject to the strongest katabatic flows was C2; while according to the idealized numerical modelling of Renfrew (2004), the area of strongest katabatic winds stretched from just below C3 to just below C2. For this reason the C2 site was chosen for the installation of the Doppler sodar wind profiling system; the aim was to observe the vertical structure and variability of katabatic flows within a relatively dense (at least for the Antarctic) regional observing network.…”

Surface layer climate and turbulent exchange in the ablation zone of the west Greenland ice sheet

“…The opposite trend in V 10m that is observed towards the ice margin is caused by (1) the greater surface roughness in the crevassed terrain at the margin during summer, limiting near surface wind speeds and (2) the piling up of cold air over the flat tundra in winter, which sets up a reverse pressure gradient force and slows down the winds in the marginal zone (Van den Broeke et al, 1994). This phenomenon has also been observed over the Antarctic ice sheet and adjacent ice shelves (Gallée and Schayes, 1992;Van den Broeke et al, 2002;Renfrew, 2004). In summer, when convection over the snow-free tundra has removed the cold air layer, wind speed differences between the sites become smaller (Figure 13(b)).…”

Generation of internal gravity waves by a katabatic wind in an idealized alpine valley

“…Katabatic flows are well-known to develop along gentle slopes of ice shelfs (e.g. Derbyshire & Wood (1994), Renfrew (2004)). For steep slopes (larger than about 10 degrees), the features of katabatic flows have also been well documented through in situ measurements, laboratory experiments or numerical simulations either on a simple slope (f.i.…”