Multi-scale flow and turbulence in complex terrain under weak synoptic conditions

Mountain weather has a remarkable range of phenomena, which are signified by two flow types: modifications to the background (∼100 km, synoptic-scale) flow driven by regional pressure gradients and thermal circulations (valley/slope flows, mesoscale) generated by local heating/cooling of the ground surface. Current state-of-the-art mesoscale models typically employ a grid resolution of ∼1 km, with the assumption that over such scales there is sufficient spatial homogeneity to represent heat, mass and momentum transport within a grid cell. Nevertheless, because of topographic heterogeneities, differential heating/cooling and flow interactions, the flow within a mesoscale model grid element can be highly inhomogeneous, and this exemplifies the difficulty of mathematically representing (parameterizing) sub-grid processes of a slopescale grid. The aim of the MOUNTAIN TERRAIN ATMOSPHERIC MODELING AND OBSERVATIONS (MATERHORN) PROGRAM was to conduct fundamental research to improve weather predictions for mountainous terrain, and improvements in understanding of sub-grid scale processes and their parameterizations were a significant part of it. To this end, field and laboratory studies were conducted and theoretical formulations were developed, which are described in this thesis. A suite of flow diagnostic techniques were used. At the core of the work are the subtopics of upslope flow separation in mountainous terrain, multi-scale interactions of slope and valley flows and measurement of turbulence in katabatic flows. Owing to the vast range of scales involved, new cutting edge techniques were developed and deployed for process identification. These include tower mounted threedimensional hot-film combo probes, consisting of sonic anemometers co-located with hot-film anemometers. The combo probes follow mean winds using a feedback control loop and use a Neural Network to calibrate the hot-films in-situ. Once calibrated, these probes can measure from mesoscale flow down to the Kolmogorov scale. Also deployed were three scanning Doppler LiDARs in coordination to visualize the velocity structure and to obtain three-dimensional velocity virtual towers up to 300 m AGL. Turbulence in slope and valley flows, upslope flow separation, flow collisions and interactions between different types of flow are discussed in this thesis, with particular emphasis on quantitative results of consequence for numerical modeling

Separation of upslope flow over a plateau

\u3cp\u3eA laboratory study was conducted in order to gain an understanding of thermal convection in a complex terrain that is characterized by a plateaued mountain. In particular, the separation of upslope (anabatic) flow over a two-dimensional uniform smooth slope, topped by a plateau, was considered. The working fluid was homogeneous water (neutral stratification). The topographic model was immersed in a large water tank with no mean flow. The entire topographic model was uniformly heated, and the width of the plateau, the slope angle, and the heating rate were varied. The upslope velocity field was measured by the Particle Tracking Velocimetry, aided by Feature Tracking Visualizations in order to detect the flow separation location. An analysis of the resulting flow showed a quantitative similarity to separating the upslope flow over steeper slopes, in the absence of a plateau when an effective angle that incorporates the normalized plateau width, the slope length, and the geometric slope angle, was used. Predictions for the dependence of the separation location and velocity on the geometry and heat flux were presented and compared with the existing data.\u3c/p\u3

Separation of upslope flow over a plateau

A laboratory study was conducted in order to gain an understanding of thermal convection in a complex terrain that is characterized by a plateaued mountain. In particular, the separation of upslope (anabatic) flow over a two-dimensional uniform smooth slope, topped by a plateau, was considered. The working fluid was homogeneous water (neutral stratification). The topographic model was immersed in a large water tank with no mean flow. The entire topographic model was uniformly heated, and the width of the plateau, the slope angle, and the heating rate were varied. The upslope velocity field was measured by the Particle Tracking Velocimetry, aided by Feature Tracking Visualizations in order to detect the flow separation location. An analysis of the resulting flow showed a quantitative similarity to separating the upslope flow over steeper slopes, in the absence of a plateau when an effective angle that incorporates the normalized plateau width, the slope length, and the geometric slope angle, was used. Predictions for the dependence of the separation location and velocity on the geometry and heat flux were presented and compared with the existing data

CASPER: Coupled Air-Sea Processes and Electromagnetic Ducting Research

The article of record as published may be found at http://dx.doi.org/10.1175/BAMS-D-16-0046.1The objective of CASPER is to improve our capability to characterize the propagation of radio frequency (RF) signals through the marine atmosphere with coordinated efforts in data collection, data analyses, and modeling of the air–sea interaction processes, refractive environment, and RF propagation.Office of Naval Research (ONR) Multidisciplinary University Research Initiative (MURI) programOffice of Naval Research Multidisciplinary University Research InitiativeUS Naval Research Laboratory (ONR)grant N0001416WX00469program element 61153N (WU BE023-01-41-5461C04)Office of Naval Research Multidisciplinary University Research Initiative grant N0001416WX00469US Naval Research Laboratory program element 61153N (WU BE023-01-41-1C04

Assessment of Planetary Boundary-Layer Schemes in the Weather Research and Forecasting Mesoscale Model Using MATERHORN Field Data

The study was aimed at understanding the deficiencies of numerical mesoscale models by comparing predictions with a new high-resolution meteorological dataset collected during the Mountain Terrain Atmospheric Modelling and Observations (MATERHORN) Program. The simulations focussed on the stable boundary layer (SBL), the predictions of which continue to be challenging. High resolution numerical simulations (0.5-km horizontal grid size) were conducted to investigate the efficacy of six planetary boundary-layer (PBL) parametrizations available in the advanced research version of the Weather Research and Forecasting model. One of the commonly used PBL schemes was modified to include eddy diffusivities that account for enhanced momentum transport compared to heat transport in the SBL, representing internal wave dynamics. All of the tested PBL schemes, including the modified scheme, showed a positive surface temperature bias. None of the PBL schemes was found to be superior in predicting the vertical wind and temperature profiles over the lowest 500 m, however two of the schemes appeared superior in capturing the lower PBL structure. The lowest model layers appear to have a significant impact on the predictions aloft. Regions of sporadic flow interactions delineated by the MATERHORN observations were poorly predicted, given such interactions are not represented in typical PBL schemes