No description available.

No description available.

No description available.

No description available.

No description available.

The quality of MM5 ensembles is evaluated for short-range probabilistic quantitative precipitation forecasts over Arizona during the Southwest monsoon. The sensitivity of different ensemble constructs is examined with respect to analysis uncertainty, model parameterization uncertainty, and a combination of both. Model uncertainty is addressed through different cumulus and planetary boundary layer parameterizations, and through stochastic forcing representative of a component of subgrid-scale uncertainty. A first-order autoregression model adds a stochastic perturbation to the Kain-Fritsch cumulus scheme and MRF planetary boundary layer scheme. A sensitivity study is also conducted to determine the MM5 planetary boundary layer parameterizations capable of simulating the structure of the pre-convective, monsoon atmospheric boundary layer. The results indicate that ensemble precipitation forecasts are skillful and may assist operational weather forecasters during the monsoon. The most skillful ensembles contain both analysis perturbations and mixed-model physics. The Blackadar or MRF planetary boundary layer schemes are recommended for MM5 simulations or forecasts of the Southwest monsoon.

The goal of this study is to provide an improved, high resolution, regional diagnosis of three important surface variables on the land surface energy and water balance, namely the downward short-wave and downward long-wave surface radiation fluxes, and precipitation. Cloud cover is a key parameter linking and controlling these three terms. An automatic procedure was developed to derive high-resolution (4 km x 4 km) fields of fractional cloud cover from visible band, (GOES series) geostationary satellite data using a novel tracking procedure to determine the clear-sky composite image. In our initial data assimilation studies, the surface short-wave radiation fluxes calculated by RAMS were simply replaced by the equivalent estimated values obtained by applying this high-resolution satellite-derived cloud cover in the UMD GEWEX/SRB model. However, this initial study revealed problems associated with inconsistencies between the revised solar radiation fields and the RAMS-calculated incoming long-wave radiation and precipitation fields, because modeled cloud cover remained unchanged and, consequently, these other surface fields retained their low, clear-sky values. It was recognized that the UMD GEWEX/SRB model provides an important relationship between cloud albedo, cloud optical depth and cloud water/ice. Thus, exploration was made of feasibility of directly assimilating vertically integrated cloud water/ice fields to update modeled cloud cover. This approach will not only enhance the realism of radiation scheme in RAMS, but it may also dramatically increase the model's capability to predict the location of precipitation, thus enhancing the ability of such mesoscale modeling systems to make accurate short-term forecasts of precipitation. This, in turn, would benefit flood forecasting as an associate hydrologic response. In the method adopted, the assimilated image takes the horizontal distribution of cloud from the satellite image but it retains a vertical distribution which is the area-average simulated by RAMS across the modeled domain in the time step immediately prior to cloud assimilation. Cloud assimilation is made every minute, with linear interpolation applied to derive cloud images for each minute between two GOES samples. Comparisons were made between modeled and observed data taken from the AZMET weather station network for model runs with and without cloud assimilation to demonstrate the improvement in RAMS' ability to describe surface radiation and precipitation fields. Cloud assimilation was found to substantially improve the RAMS model's ability to capture both the temporal and spatial variations in surface fields associated with observed cloud cover. The sensitivity of these comparisons to model initiation was explored by making five ensemble runs starting from different initiation. In general, RAMS with cloud assimilation technique is not sensitive to realistic perturbation of initial conditions.

The heat engine framework is examined in terms of large-scale atmospheric convection in order to investigate several theoretical and modeling issues related to the steady-state convecting atmosphere. Applications of the heat engine framework to convective circulations are reviewed. It is shown that this framework provides fundamental insights into the nature of various atmospheric phenomena and estimates of their potential intensity. The framework is shown to be valid for both reversible and irreversible systems; the irreversible processes' sole effect is to reduce the thermodynamic efficiency of the convective heat engine. The heat engine framework is then employed to demonstrate that the two asymptotic limits of quasi-equilibrium theory are consistent. That is, the fractional area covered by convection goes to zero, σ → 0, as the ratio of the convective adjustment to large-scale time scale (e.g. radiative time scale) go to zero, tADJ/tLS →0 , despite recent arguments to the contrary. Furthermore, the heat engine framework is utilized to develop a methodology for assessing the strength of irreversibilities in numerical models. Using the explicit energy budget, we derive thermodynamic efficiencies based on work and the heat budget for both open (e.g., the Hadley circulation) and closed (e.g., the general circulation) thermodynamic systems. In addition, the Carnot efficiency for closed systems is calculated to ascertain the maximum efficiency possible. Comparison of the work-based efficiency with that of the efficiency based on the heat budget provides a gauge for assessing how close to reversible model-generated circulations are. A battery of experiments is carried out with an idealized GCM. The usefulness of this method is demonstrated and it is shown that an essentially reversible GCM is sensitive (i.e., becomes more irreversible) to changes in numerical parameters and horizontal resolution.

This research extends an observation program that recorded the night airglow from a Tucson ground station using an imaging spectrograph known as GLO. GLO was designed at the Lunar and Planetary Laboratory of the University of Arizona to observe auroral and airglow emissions, and recorded midlatitude airglow data near equinox during Space Shuttle mission STS-69 in September 1995. GLO observations from the shuttle recorded the night airglow layer seen edge on at the Earth's limb. These observations from orbit exhibit a fundamentally different picture of the night airglow compared to observations from the ground. GLO data also represent the first simultaneous optical measurements of airglow emissions over the spectral range from 1150 to 9000 A, showing global emission variations in the night sky. Intensity variations are not correlated among emitting species, implying greater dynamism and more complex chemical interactions in the airglow than previously assumed. Although other researchers have described observations of organized waves and tides in the night airglow, these prior observations are sporadic or averaged over long time periods. The night sky intensity variations recorded by GLO do not exhibit any obvious relationship to atmospheric tides. They may instead be the result of a chaotic superposition of upward and downward vertical motions. This upwelling and subsidence may cause the decoupling of airglow emissions in the O2 Atmospheric band, the OH Meinel band, and the atomic oxygen green line at 5577 A. Emission enhancements with maxima-to-minima ratios of 4 to 12 depending on emitting species, have been observed in the GLO data. Emissions in the O2 Atmospheric band system and the OI (5577 A) green line show a greater dynamic range of variation than the OH Meinel band system. The chemistry along a limited line-of-sight can be explained by classical airglow chemistry, but only over a limited altitude range. Dynamic effects in the 80 to 100 km region are sufficiently chaotic to present mixed results when inferring chemical processes as a function of altitude.

The development and investigation of a new technique for measuring tropospheric concentrations of hydroxyl radicals (OH) is presented. The technique is based on the near-infrared fluorescence of IR125 which is quenched upon reaction with OH. IR125, is shown to react with OH, and be sufficiently less reactive with other tropospheric oxidants that when exposed to tropospheric air samples, changes in the dye fluorescence are related to the ambient OH concentration. A near-infrared fluorimeter was constructed to determine IR125 concentrations. Detection of 10-12 M IR125 in solution was obtained. This sensitivity allows observation of changes in IR125 concentrations due to reaction with typical tropospheric OH concentrations. Changes in the fluorescence of IR125 when sampling the ambient air using dye impregnated quartz wool cartridges were shown to follow predicted OH concentrations for the observed environmental conditions. Photodecomposition by sunlight and reaction with other, longer lived, oxidants were accounted for in determining the IR125 response to OH. An OH source of known concentration to calibrate the IR125 response based on the photolysis of HONO or H2 was constructed. A photochemical computer model developed and used to determine the steady-state OH concentrations was validated by the successful prediction of concentrations of some cogenerated compounds. This OH source was not compatible with the sampling technique using dye impregnated quartz wool cartridges, because of the overwhelming interference caused by suspected heterogeneous reactions of the precursors. Absolute calibration remains to be completed.