AtmosphericEnvironment Vol. 21, No.9, pp. 1905-1913, 1987. Printed in Great Britain 0004-6981/87 $3.00+0.00 ~ 1987 Pergamon Journals Lid. NUMERICAL MODEL EVALUATION OF THE EXTENSION OF THE CRITICAL DIVIDING STREAMLINE HYPOTHESIS TO MESOSCALE TWO-DIMENSIONAL TERRAIN RAYMOND W. ARRITT Cooperative Institute for Research in the Atmosphere, Colorado State University, Fort Collins, Colorado 80523, U.S.A. RICHARD T. McNIDER K.E. Johnson Environmental and Energy Center, University of Alabama in Huntsville, Huntsville, Alabama 35899, U.S.A. and ROGER A. PIELKE Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado 80523, U.S.A. First received 16 June 1986 and in final form 9 February 1987) Abstract-A two-dimensional nonlinear numerical model is used to study the extension of the critical dividing streamline hypothesisto largerscale terrain than considered in mostprevious observational studies. Upstream blocking in the absence of thermal forcing is found to bereasonably well described for air quality assessment purposes by the critical dividing streamlinehypothesis. The effect of rotation (i.e. the Coriolis force)is to decrease the extent of blocking in marginal cases. Influence of the Coriolis force is expected to increase as the length scaleof the topography increases. It is also found that thermally induced nocturnal slope winds may compromise the dividing streamline hypothesis asa result of their entrainment mass flux. We suggest that slope flows have not played a substantial role in previous tracer studiesover small-scale terrain due to the dependence of slope flows on length scale and ambient thermal stratification. Tracer studies around mesoscaleterrain would help to clarify the importance of rotational and diabatic effects. .. Key word index: Air pollution, dividing streamline,mountain meteorology, numerical modeling, upwind stagnation. I. INTRODUCTION atmospheric tracer studies have mainly been con- ducted around small-scale terrain. An example is Cinder Cone Butte,a site used in the CTMD program (Strimaitis et al., 1983, 1985). This is a nearly conical hill rising about 100 m above the surrounding terrain and having a radius of about 350m. There is concern as to whetherthe dividing stream- line hypothesiscan betransferred from the laboratory and small-scale field studies to mesoscale terrain (e.g. Hovind et al., 1979). In this paper,we usea mesoscale numerical model to examine the application of the critical dividing streamline hypothesis to two- dimensional terrain which is an order of magnitude larger than that considered in mostprevious dispersion studies. A numerical model provides a suitable ap- paratus for such an investigation, as it permits the selective activation of physical processes. We begin by altering the numerical model so that it reflects the simplifications of the critical dividing streamline hypothesis, and compare the numerical modelresultsto the predictions of the dividing stream- line hypothesis. Next, we add turbulence and the Coriolis force to the numericalmodel formulation, and The need for air quality modelsappropriate for use in complex terrain is increasingly recognized. The Complex Terrain Model Developmentprogram of the U.S. Environmental Protection Agency is pursuing the development of these models as next-generation air quality assessment tools (Schiermeier, 1984; Egan, 1984). Such models typically do not integratethe time- dependentequations of motion, but instead rely on analytic parameterizations of topographic effects. The critical dividing streamlinehypothesis is an example of one such representationfor the effectsof topography which is being considered for use in statutory air quality models. This formulation (described in section 2) proposes a simple guide for determining whether upwind stagnation occurs for stably stratified flow over terrain, and allows the depth of the stagnantlayer to be estimated. The critical dividing streamlinehypothesishas been validated in laboratory experiments (e.g. Hunt and Snyder,1980; Lee et al., 1984) and atmospheric tracer studies (e.g. Rowe, 1980;Strimaitis et al., 1983). The