Regional-scale flows in complex terrain: an observational and numerical investigation

Fall 1990.Also issued as author's dissertation (Ph.D.) -- Colorado State University, 1990.Includes bibliographical references.An observational program has been conducted to obtain information concerning thermally driven flows in complex terrain on meso-(3 to meso-a scales (100 - 500 km). Data were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984-1988. These field experiments have been called the Rocky Mountain Peaks Experiments (ROMPEX). The observations from ROMPEX have been supplemented with data from other remote surface networks, special soundings, upper-air observations, and radar and lightning strike information to provide an adequate description of the flows and weather of interest. The observations have shown the development of a recurrent "regional-scale" circulation system across the Colorado mountain barrier, operating on a diurnal time scale. The basic structure of the flow system consists of a daytime inflow phase toward the mountains along the Continental Divide, and a nocturnal outflow away from this high terrain. Long-term averages show this circulation system to be the dominant wind pattern at several high altitude stations, revealing its climatological significance. Attention has been focused upon the nocturnal phase of the circulation system along the western slope of the mountain barrier. Here, the winds are particularly strong and from a southeasterly direction, which is generally counter to the upper-level winds, and onset abruptly in early evening with steady flow thereafter. Soundings have shown this nocturnal current to be shallow and within a distinct stable air mass. Convective storms are found to enhance this southeasterly flow regime. Numerical simulations have been performed with the Colorado State University Regional Atmospheric Modelling System (CSU-RAMS) to provide further insight into the physical mechanisms forcing the observed regional-scale circulation system. The model simulations include both idealized two- and three-dimensional experiments, as well as a three-dimensional case study experiment using actual data for the initialization. The three-dimensional simulations use two-way interactive grid nesting and a realistic representation of topography over the region of interest. The idealized three-dimensional experiment showed that thermal forcing over realistic topography in conditions of negligible, or weak ambient flow, is capable of producing many of the flow features observed throughout the diurnal cycle. his experiment further showed how the deep mountain-plains solenoid along and above the Front Range crest evolves in late afternoon into a shallower density current, which then propagates westward over the mountains of the western slope. This unexpected flow phenomenon is the primary process responsible for the strong nocturnal southeasterly winds found in observations. Sensitivity experiments show that the particular terrain configuration through an east-west cross-section of the Colorado mountains is important to the generation of this unusual circulation. The strong thermal gradient produced by differential heating of the topography is the primary driving force in the density current evolution. Coriolis influence maintains the steady nocturnal south-southeast winds over the western slope. Additional experiments show that the diurnally evolving regional-scale circulation system over the Colorado Rocky Mountains is a robust feature which can occur over a range of ambient flow and stratification conditions. Soil moisture experiments reveal that wet soil along the eastern slope and dry along the western slope aids the development of the westward propagating density current. The diurnal evolution of the circulation system on the case study day was in fair agreement with many of the observed circulation features. This experiment also revealed that synoptic-scale forcing can influence the development of the regional-scale circulations in preferential regions along the eastern slope of the mountain barrier. As a result of the numerical experiments four phases of the thermally forced regional-scale diurnal circulation system have been identified. These consist of a daytime mountain boundary layer development phase, a late afternoon transitional phase, an evening propagating density current phase, and a latte night adjustment phase.Sponsored by the National Science Foundation ATM-8610796; the Air Force Office of Scientific Research F49620-85-C-0077DEF; the National Aeronautics and Space Administration NAGW601; and the Army Research Office DAAL03-86-K-0175

Numerical simulation of explosive volcanism and its effects on the atmosphere

This is the final report of a one-year, Laboratory Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). The objective of this project was to begin work on combining two modeling approaches in order to advance the state-of-the-art in simulating and predicting explosive volcanic eruption dynamics and their effects. The authors began applying the CFDLIB family of codes for the near field (high temperature, velocity, and particle concentration) region of an explosive eruption. The authors also applied the RAMS meteorological code to model the far-field dynamics of eruption clouds and ash fallout. Initial test runs were conducted in preparation for full-scale simulations that would eventually couple the two models for the most comprehensive volcano simulation tool to date. Eventual applications include aviation hazards, risk assessment, and extension to atmospheric collateral effects of conventional and nuclear weapons

Numerical Investigation Into Effects of Complex Terrain on Spatial and Temporal Variability of Precipitation

This study is part of an ongoing research effort at Los Alamos to understand the hydrologic cycle at regional scales by coupling atmospheric, land surface, river channel, and groundwater models. In this study the authors examine how local variation of heights of the two mountain ranges representative of those that surround the Rio Grande Valley affects precipitation. The lack of observational data to adequately assess precipitation variability in complex terrain, and the lack of previous work has prompted this modeling study. Thus, it becomes imperative to understand how the local terrain affects snow accumulations and rainfall during winter and summer seasons respectively so as to manage this valuable resource in this semi-arid region. While terrain is three dimensional, simplifying the problem to two dimensions can provide some valuable insight into topographic effects that may exist at various transects across the Rio Grande Valley. The authors induce these topographic effects by introducing variations in heights of the mountains and the width of the valley using an analytical function for the topography. The Regional Atmospheric Modeling System (RAMS) is used to examine these effects

Evaluation of precipitation predictions in a regional climate simulation

The research reported here is part of a larger project that is coupling a suite of environmental models to simulate the hydrologic cycle within river basins (Bossert et al., 1999). These models include the Regional Atmospheric Modeling System (RAMS), which provides meteorological variables and precipitation to the Simulator for Processes of Landscapes, Surface/Subsurface Hydrology (SPLASH). SPLASH partitions precipitation into evaporation, transpiration, soil water storage, surface runoff, and subsurface recharge. The runoff is collected within a simple river channel model and the Finite element Heat and Mass (FEHM) subsurface model is linked to the land surface and river flow model components to simulate saturated and unsaturated flow and changes in aquifer levels. The goal is to produce a fully interactive system of atmospheric, surface hydrology, river and groundwater models to allow water and energy feedbacks throughout the system. This paper focuses on the evaluation of the precipitation fields predicted by the RAMS model at different times during the 1992--1993 water year in the Rio Grande basin. The evaluation includes comparing the model predictions to the observed precipitation as reported by Cooperative Summary of the Day and SNOTEL reporting stations

Pathways for the Oxidation of Sarin in Urban Atmospheres

Terrorists have threatened and carried out chemicalhiological agent attacks on targets in major cities. The nerve agent sarin figured prominently in one well-publicized incident. Vapors disseminating from open containers in a Tokyo subway caused thousands of casualties. High-resolution tracer transport modeling of agent dispersion is at hand and will be enhanced by data on reactions with components of the urban atmosphere. As a sample of the level of complexity currently attainable, we elaborate the mechanisms by which sarin can decompose in polluted air. A release scenario is outlined involving the passage of a gas-phase agent through a city locale in the daytime. The atmospheric chemistry database on related organophosphorus pesticides is mined for rate and product information. The hydroxyl,radical and fine-mode particles are identified as major reactants. A review of urban air chernistry/rnicrophysics generates concentration tables for major oxidant and aerosol types in both clean and dirty environments. Organic structure-reactivity relationships yield an upper limit of 10-1' cm3 molecule-' S-* for hydrogen abstraction by hydroxyl. The associated midday loss time scale could be as little as one hour. Product distributions are difficult to define but may include nontoxic organic oxygenates, inorganic phosphorus acids, sarin-like aldehydes, and nitrates preserving cholinergic capabilities. Agent molecules will contact aerosol surfaces in on the order of minutes, with hydrolysis and side-chain oxidation as likely reaction channels

Validation of Coupled Atmosphere-Fire Behavior Models

Recent advances in numerical modeling and computer power have made it feasible to simulate the dynamical interaction and feedback between the heat and turbulence induced by wildfires and the local atmospheric wind and temperature fields. At Los Alamos National Laboratory, the authors have developed a modeling system that includes this interaction by coupling a high resolution atmospheric dynamics model, HIGRAD, with a fire behavior model, BEHAVE, to predict the spread of wildfires. The HIGRAD/BEHAVE model is run at very high resolution to properly resolve the fire/atmosphere interaction. At present, these coupled wildfire model simulations are computationally intensive. The additional complexity of these models require sophisticated methods for assuring their reliability in real world applications. With this in mind, a substantial part of the research effort is directed at model validation. Several instrumented prescribed fires have been conducted with multi-agency support and participation from chaparral, marsh, and scrub environments in coastal areas of Florida and inland California. In this paper, the authors first describe the data required to initialize the components of the wildfire modeling system. Then they present results from one of the Florida fires, and discuss a strategy for further testing and improvement of coupled weather/wildfire models

Coupled Weather and Wildfire Behavior Modeling at Los Alamos: An Overview

Over the past two years, researchers at Los Alamos National Laboratory (LANL) have been engaged in coupled weather/wildfire modeling as part of a broader initiative to predict the unfolding of crisis events. Wildfire prediction was chosen for the following reasons: (1) few physics-based wild-fire prediction models presently exist; (2) LANL has expertise in the fields required to develop such a capability; and (3) the development of this predictive capability would be enhanced by LANL`s strength in high performance computing. Wildfire behavior models have historically been used to predict fire spread and heat release for a prescribed set of fuel, slope, and wind conditions (Andrews 1986). In the vicinity of a fire, however, atmospheric conditions are constantly changing due to non-local weather influences and the intense heat of the fire itself. This non- linear process underscores the need for physics-based models that treat the atmosphere-fire feedback. Actual wildfire prediction with full-physics models is both time-critical and computationally demanding, since it must include regional- to local-scale weather forecasting together with the capability to accurately simulate both intense gradients across a fireline, and atmosphere/fire/fuel interactions. Los Alamos has recently (January 1997) acquired a number of SGI/Cray Origin 2000 machines, each presently having 32 to 64 processors. These high performance computing systems are part of the Department of Energy`s Accelerated Strategic Computing Initiative (ASCI). While offering impressive performance now, upgrades to the system promise to deliver over 1 Teraflop (10(12) floating point operations per second) at peak performance before the turn of the century

Investigation of Microphysical Parameters within Winter and Summer Type Precipitation Events over Mountainous [Complex] Terrain

In this study we investigate complex terrain effects on precipitation with RAMS for both in winter and summer cases from a microphysical perspective. We consider a two dimensional east-west topographic cross section in New Mexico representative of the Jemez mountains on the west and the Sangre de Cristo mountains on the east. Located between these two ranges is the Rio Grande Valley. In these two dimensional experiments, variations in DSDs are considered to simulate total precipitation that closely duplicate observed precipitation

Simulations of Precipitation Variability over the Upper Rio Grande Basin

In this research, we study Albuquerque`s water and how it may be affected by changes in the regional climate, as manifested by variations in Rio Grande water levels. To do this, we rely on the use of coupled atmospheric, runoff, and ground water models. Preliminary work on the project has focused on uncoupled simulations of the aquifer beneath Albuquerque and winter precipitation simulations of the upper Rio Grande Basin. The latter is discussed in this paper