The interaction of katabatic winds and mountain waves

The variation in the oft-observed, thermally-forced, nocturnal katabatic winds along the east side of the Rocky Mountains can be explained by either internal variability or interactions with various other forcings. Though generally katabatic flows have been studied as an entity protected from external forcing by strong thermal stratification, this work investigates how drainage winds along the Colorado Front Range interact with, in particular, topographically forced mountain waves. Previous work has shown, based on measurements taken during the Atmospheric Studies in Complex Terrain 1993 field program, that the actual dispersion in katabatic flows is often greater than reflected in models of dispersion. The interaction of these phenomena is complicated and non-linear since the amplitude, wavelength and vertical structure of mountain waves developed by flow over the Rocky Mountain barrier are themselves partly determined by the evolving atmospheric stability in which the drainage flows develop. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by both turbulence and dynamic pressure effects. The effect of turbulent interaction is to create changes to katabatic now depth, katabatic flow speed, katabatic jet height and, vertical thermal stratification. The pressure effect is found to primarily influence the variability of a given katabatic now through the evolution of integrated column wave forcing on surface pressure. Variability is found to occur on two scales, on the mesoscale due to meso-gamma scale mountain wave evolution, and on the microscale, due to wave breaking. Since existing parameterizations for the statically stable case are predominantly based on nearly flat terrain atmospheric measurements under idealized or nearly quiescent conditions, it is no surprise that these parameterizations often contribute to errors in prediction, particularly in complex terrain

The interaction of katabatic winds and mountain waves

The variation in the oft-observed, thermally-forced, nocturnal katabatic winds along the east side of the Rocky Mountains can be explained by either internal variability or interactions with various other forcings. Though generally katabatic flows have been studied as an entity protected from external forcing by strong thermal stratification, this work investigates how drainage winds along the Colorado Front Range interact with, in particular, topographically forced mountain waves. Previous work has shown, based on measurements taken during the Atmospheric Studies in Complex Terrain 1993 field program, that the actual dispersion in katabatic flows is often greater than reflected in models of dispersion. The interaction of these phenomena is complicated and non-linear since the amplitude, wavelength and vertical structure of mountain waves developed by flow over the Rocky Mountain barrier are themselves partly determined by the evolving atmospheric stability in which the drainage flows develop. Perturbations to katabatic flow by mountain waves, relative to their more steady form in quiescent conditions, are found to be caused by both turbulence and dynamic pressure effects. The effect of turbulent interaction is to create changes to katabatic now depth, katabatic flow speed, katabatic jet height and, vertical thermal stratification. The pressure effect is found to primarily influence the variability of a given katabatic now through the evolution of integrated column wave forcing on surface pressure. Variability is found to occur on two scales, on the mesoscale due to meso-gamma scale mountain wave evolution, and on the microscale, due to wave breaking. Since existing parameterizations for the statically stable case are predominantly based on nearly flat terrain atmospheric measurements under idealized or nearly quiescent conditions, it is no surprise that these parameterizations often contribute to errors in prediction, particularly in complex terrain

Revisiting the Local Scaling Hypothesis in Stably Stratified Atmospheric Boundary Layer Turbulence: an Integration of Field and Laboratory Measurements with Large-eddy Simulations

The `local scaling' hypothesis, first introduced by Nieuwstadt two decades ago, describes the turbulence structure of stable boundary layers in a very succinct way and is an integral part of numerous local closure-based numerical weather prediction models. However, the validity of this hypothesis under very stable conditions is a subject of on-going debate. In this work, we attempt to address this controversial issue by performing extensive analyses of turbulence data from several field campaigns, wind-tunnel experiments and large-eddy simulations. Wide range of stabilities, diverse field conditions and a comprehensive set of turbulence statistics make this study distinct

Simulation of katabatic flow and mountain waves

It is well-known that both mountain waves and katabatic flows frequently form in the severe relief of the Front Range of the Rocky Mountains. Occasionally these phenomena have been found to occur simultaneously. Generally, however, the large body of literature regarding them has treated each individually, seldom venturing into the regime of their potential interaction. The exceptions to this rule are Arritt and Pielke (1986), Barr and Orgill (1989). Gudiksen et al. (1992), Moriarty (1984), Orgill et al. (1992), Orgill and Schreck (1985). Neff and King (1988), Stone and Hoard (1989), Whiteman and Doran (1993) and Ying and Baopu (1993). The simulations overviewed here attempt to reproduce both atmospheric features simultaneously for two case days during the 1993 ASCOT observational program near Rocky Flats, Colorado

The mutual evolution of mountain waves and katabatic flow

Typically, katabatic flows have been studied in their least complicated or idealized state. Further, these flows are generally regarded as having simple forcing and non-turbulent structure due to the strong atmospheric stability they are bedded within. Somewhat analogously, mountain waves and their effects have been mostly studied in their idealized state, i.e. for constant upstream flow and stability. Even in the numerous cases where these two atmospheric phenomena have been studied in their realistic state, seldom has their mutual interaction been considered. One exception that includes numerical modeling is Gross (1990). The express purpose of this work is to examine how each of these phenomena interact with each other in an evolving nocturnal atmosphere. This work is motivated by observations from the Atmospheric Studies in Complex Terrain (ASCOT) Program which clearly indicate non-idealized behavior in katabatic flows. Although numerous idealized simulations were also completed, discussion here focuses on the most realistic simulations of the case night 3--4 September 1993. This night was dominated by clear skies and light near surface winds. A high pressure system to the southwest of Colorado caused northwesterly flow at {approximately} 7 m s{sup {minus}1} upstream of the Rockies with a Froude number of 0.45 overnight. ASCOT observations indicated that katabatic and mountain wave flow were occurring

Assessing the interaction of mountain waves and katabatic flows using a mesoscale model

This paper has two main purposes. The first is to evaluate the interaction of two common complex terrain meteorological phenomena, katabatic flow and mountain waves. Although occasionally investigated together, generally, the large body of literature regarding them has treated each individually. The second purpose is to show the reader the utility of extracting high time resolution data sets of (1) standard meteorological variables, and (2) seldom used, components of the model equations. Using such time series, significant variability is found in the evolving, clear sky, nocturnal boundary layer, when meteorological variability is generally considered to be at its lowest point diurnally. The approach is to use results from three, 3-d, realistic topography simulations produced by the Regional Atmospheric Modeling System (RAMS). RAMS is a primitive equation mesoscale model formulated in {sigma} coordinates. The model is set up with five nested grids that focus on Eldorado Canyon, which is embedded in the Front Range slope of Colorado. On the finest grid {Delta}x = {Delta}y = 400 m and {Delta}z = 20 m for the lowest 400 m above ground level (AGL). The three simulations were: (1) a realistic simulation; (2) the same as (1) but without radiative forcing (referred to as mountain wave only or MWO) and (3) the same as (1) but without boundary nudging and no initial winds (referred to as katabatic flow only or KFO). The case night is 3--4 Sep 1993 from the Atmospheric Studies in Complex Terrain (ASCOT) 1993 field program near Rocky Flats, Colorado. Both mountain waves and katabatic flows were occurring on this night

Dynamical processes in undisturbed katabatic flows

Idealized analytical investigations of katabatic slope flows have usually sought to simplify the analysis by either assuming a particular force balance amenable to analytical solution or using integral (or bulk) models. In each case, steady state conditions are evaluated, with occasional exception. Historically, the modeling of idealized katabatic flows has focused analysis of model time where steady state conditions have been achieved. To investigate the true dynamics of evolving undisturbed katabatic flow, the Regional Atmospheric Modeling System (RAMS) is used. As described in Pielke et al (1992) RAMS is a prognostic numerical model that contains the three-dimensional primitive equations in terrain-following, non- hydrostatic, compressible form. In addition to standard variables, RAMS was configured to output the various components of the governing equations with high temporal resolution. Each of the simulations used idealized 2000m high mountain topography of a given slope (1{degree}, 2.5{degrees},5{degrees}, or 10{degrees}) on either side of the peak. In the 3-d simulations this mountain becomes an infinite north-south ridge (cyclic boundary conditions in the N-S direction). Vertical grid spacing was set to 20m for the first 500m {delta}z increases to a maximum of 400 m over 72 grid points to 10.5 km. Horizontal grid spacing was 500 m and the number of east-west grid points was 701, 301, 201 and 201 for the 1 {degree}, 2.5{degrees}, 5{degrees} and 10{degrees} mountains, respectively. Only results from the homogeneous with a vertical structure as follows: 0.0 m s{sup -1} to 3000 m AGL, standard atmospheric {theta} lapse rate of 2.5 K km {sup - 1} to 3000 m AGLl, standard atmospheric {theta} lapse rate of 3.4 K Km {sup -1} above that. The simulations ran for 12 hours after model sunset ({similar_to}1800 MST) so that only longwave radiative effects were active