Stable Boundary Layer in Complex Terrain. Part I: Linking Fluxes and Intermittency to an Average Stability Index

Abstract

Average heat and momentum fluxes observed by a network of surface stations during the Hudson Valley Ambient Meteorology Study (HVAMS) were found as functions of a spatially representative bulk Richardson number Ri br . Preferential sites were identified for the occurrence of strong turbulence under mesoscale stability conditions common to all stations. Locally sensed turbulence intermittency depends on the mesoscale flow stability. Nearly continuous turbulence with few long-lived intermittent events occurs when Ri br , Ri cr , the critical gradient Richardson number. Less-continuous mixing associated with a larger number of events occurs when Ri cr , Ri br , 5, with the weakest turbulence and fewer events observed for Ri br Ri cr . It was found that the need to allow for extra mixing above the conventional critical bulk Richardson number in numerical weather prediction models is primarily a consequence of spatial averaging in a heterogeneous landscape and is secondarily the result of turbulence above Ri cr at locations with ''nonideal fetch.''

Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

“…These factors include the inherent horizontal heterogeneity [15,47], the corresponding local advection processes [57,61], slope effects on the available radiant energy [62,63], the potential failure of the vertically-constant-flux assumption in the SL [64], and others.…”

“…Besides limiting the applicability of scaling approaches, horizontal heterogeneity in mountainous terrain also leads to significant small-scale variability of turbulence characteristics [47,58,84], particularly in stable boundary layers. This aspect is discussed in greater detail in Section 2.5.…”

“…The heat budget at one point may be affected by upstream thermal heterogeneity, while changes in the slope roughness and angle may alter the wind speed and therefore the intensity of turbulence downstream, possibly even causing flow separation [58] (dashed white line in Figure 4). Similarly, mixing in the SBL is affected by terrain curvature, with convex areas being sheltered and therefore less turbulent [47,84].…”

“…A summary of the insights emerging from these observations and from the related theoretical and numerical studies can be found in a few monographs and review articles [15,[43][44][45]. More recently, results from field campaigns and long-term observational programmes, focusing specifically on exchange processes over heterogeneous terrain, have appeared in the literature (i-Box [46], HVAMS [47]). Building on this bulk of knowledge, we synthesize the current understanding of exchange processes in the atmosphere over mountainous terrain and propose some possibilities for future research.…”

On the Vertical Exchange of Heat, Mass, and Momentum Over Complex, Mountainous Terrain

“…They also point to the rare, but significant, influence of meso-scale gravity waves on boundary layer characteristics. Over moderate terrain, Medeiros and Fitzjarrald (2014) attribute the missing drag to averaging over their non-homogeneous terrain. Focusing on the small-scale end of the gravity wave spectrum, Sun et al (2015) give a detailed review of the wave-turbulence interaction in the stable boundary layer.…”