Numerical Modelling of Katabatic Flow Over a Melting Outflow Glacier

Abstract

A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, 'a state of the art' mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 • C while the air temperature warms adiabatically as it descends the slope. This provides a 'self enhanced' negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large 'curvature term' in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of nonlocal processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.

A non-uniqueness problem in the identification of power-law spectral scaling for hydroclimatic time series

“…Arendt et al 2002) and affect local-to-mesoscale meteorology, by generating katabatic winds for example (e.g. Söderberg and Parmhed 2006). Of particular interest to us, however, is that they lie at the heart of such continental-scale "water towers" (e.g.…”

Improving mixing length‐scale for stable boundary layers

“…Nappo and Rao (1987), Oerlemans (2001), Grisogono (2003), Renfrew and Anderson (2006) or Söderberg and Parmhed (2006). Almost needless to say, the whole model set-up is prepared so that it would simulate an idealized but still generalized katabatic flow regime pertaining to e.g.…”

“…Grisogono et al, 1998;Mahrt, 1998;King et al, 2001;Söderberg and Parmhed, 2006;Mauritsen et al, 2007;Steeneveld et al, 2007;Banta, 2008). Most numerical weather prediction (NWP) models either produce too much mixing by artificially enhancing vertical diffusion (needed for other purposes, e.g.…”

Clima, sociedad, riesgos y ordenación del territorio

“…Este fenómeno físico tiene amplio reconocimiento en climatología e identifica vientos característicos generadores de sombras pluviomé-tricas a sotavento de cadenas montañosas transversales a las direcciones dominantes de los flujos, que reciben nombres bien conocidos: Fohen (Alemania y Austria), Chinook (Canadá y EEUU), Zonda (Argentina), Santa Ana (Suroeste EEUU), Canterbury-nor'wester' (Nueva Zelanda), ''berg wind" (Sudáfrica), "fagüeño" (Aragón) etc. Entre sus efectos se citan situaciones relacionadas con olas de calor (Gaffin, 2002;Takane et al, 2015), polución atmosférica (Natale et al, 1999;Li et al, 2015), riesgo y propagación de incendios (Sharples et al, 2010;Jin et al, 2014), deshielo (Soderberg y Parmhed, 2006;Pedersen et al, 2015), entre otros. El calentamiento adiabático por descenso de la masa de aire describe en términos físicos el fenómeno, pero su distribución espacial, intensidad y frecuencia dependen de las características topográ-ficas y de las condiciones de circulación atmosférica (Seluchi et al, 2003).…”