Abstract
Abstract. Idealized numerical simulations of thermally driven flows over various valley–plain topographies are performed under daytime conditions. Valley floor inclination and narrowing valley cross sections are systematically varied to study the influence of along-valley terrain heterogeneity on the developing boundary layer structure, as well as horizontal and vertical transport processes. Valley topographies with inclined valley floors of 0.86° increase upvalley winds by a factor of about 1.9 due to smaller valley volumes (volume effect) and by a factor of about 1.6 due to additional upslope buoyancy forces. Narrowing the valley cross section by 20 km per 100 km along-valley distance increases upvalley winds by a factor of about 2.6. Vertical mass fluxes out of the valley are strongly increased by a factor between 1.8 and 2.8 by narrowing the valley cross sections and by a factor of 1.2 by inclining the valley floor. Trajectory analysis shows intensified horizontal transport of parcels from the foreland into the valley within the boundary layer in cases with inclined floors and narrowing cross sections due to increased upvalley winds.
Highlights
Driven flows are well known phenomena under fair weather conditions over complex terrain
Recent idealized simulations confirmed these values (Wagner et al, 2014a) and demonstrated that the vertical transport can be up to 8 times larger over a valley compared to a plain depending on the reference surface through which vertical transport is assessed and which is associated with different definitions of the boundary layer height
Over the foreland a convective boundary layer and a plain-to-mountain circulation develops in all simulations using a valley–plain topography
Summary
Driven flows are well known phenomena under fair weather conditions over complex terrain. Several authors have investigated mechanisms which induce thermally driven flows and have developed analytical models and basic concepts to describe the formation of upslope and upvalley winds. The importance of thermally driven flows for the planetary boundary layer (PBL) over complex terrain and their contribution to horizontal and vertical transport processes has been examined in several observational and modelling studies in the past (e.g. Henne et al, 2004; Weissmann et al, 2005; Weigel et al, 2007; Wagner et al, 2014a, b). Measurements and numerical modelling showed that vertical moisture transport over a valley can be 3–4 times larger than over flat and homogeneous terrain during a summer day with fair weather conditions (Weigel et al, 2007). Recent idealized simulations confirmed these values (Wagner et al, 2014a) and demonstrated that the vertical transport can be up to 8 times larger over a valley compared to a plain depending on the reference surface through which vertical transport is assessed and which is associated with different definitions of the boundary layer height
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