Theses and Dissertations

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Open Access
Physical mechanisms of extra area effects from weather modification
(Colorado State University. Libraries, 1977) Mulvey, G. (Gerald), author; Grant, Lewis O., advisor; Karaki, Susumu, committee member; Corrin, Myron L., committee member; Mielke, Paul W., committee member; Cotton, William R., committee member
One of the complexities of weather modification, namely extra area effects have long posed an opportunity for the long-term control of the earth's weather. This study investigates the physical mechanisms by which cloud seeding projects may cause extra area effects. The investigations center on one of the simplest of precipitating systems, namely the cold wintertime orographic clouds of the central Rocky Mountains. Three lines of investigation are followed: (1) field studies of seeding material movement in the atmosphere and receiver cloud characteristics, (2) numerical simulation, and (3) historical studies of the affected cloud system. The field observations consist of case studies of the movement and dispersion of silver iodide from ground based generators. These studies, during the winters of 1974-75 and 1975-76, used nuclei counters aboard two aircraft. Aerosol silver concentration measurements were also made during the last experimental year. The surface observations made as part of the field studies included snow collection for silver analysis, radar observation and ice nuclei measurements. The aircraft studies established the fact that regions of above background ice nuclei concentrations extend from the target cloud systems as far as 240 km downwind while exhibiting concentrations from 10 to over 700 ice nuclei per liter active at -20°C. The analysis of silver concentrations in snow confirmed above background silver concentrations exist in snow samples on days during which cloud seeding occurred in the mountains. The numerical cloud models were used to investigate the mode of seeding and the seeding requirements of the downwind cloud systems. Case study r n s using a cumulus model suggested that seeding the upslope cloud would cause little dynamic intensification. It was therefore inferred that the seeding mode was static. The second cloud model, a rapid glaciation model, estimated the seeding requirements in terms of active ice nuclei or ice crystals for precipitation augmentation to be between 1.0 and 5 No1-1. An ice crystal transport model was used to predict 0 the survival time for a spectrum of crystal sizes under a variety of conditions. The results indicate that under certain meteorological conditions crystals typically observed in orographic conditions can survive long enough to reach the downwind upslope cloud in concentrations between 0.5 and 50 No1-1. The historical studies established characteristics of the typical upslope clouds as well as the surf ace features controlling their formation. The radar observations showed convective-like echoes migrating within the upslope cloud over the eastern plains of Colorado downwind of Climax. These studies show that at least two feasible mechanisms through which mountain orographic clouds can affect the precipitation on the eastern plains exist, and, under certain conditions, are operative.
Open Access
A simple ice phase parameterization
(Colorado State University. Libraries, 1979) Stephens, Mark Argyle, author; Cotton, William R., advisor; Keefe, Thomas J., committee member; Orville, Harold D., committee member
A two variable ice parameterization was developed for use in three dimensional models of cumulonimbus clouds and mesoscale squall lines. Bulk water techniques were employed to simulate the growth and decay of snow crystals and of graupel in order to keep the use of computer resources to a minimum. An externally specified concentration of ice crystals was used to initiate snow. Graupel was assumed to follow the Marshall-Palmer distribution with a constant total concentration. Microphysical growth processes for snow included initiation from the vapor at liquid water saturation, riming, melting, vapor deposition and conversion of rimed crystals into graupel. The graupel microphysical processes that were modeled included raindrop freezing by contact with snow crystals, accretion of raindrops, vapor deposition, riming of cloud droplets and melting. Both types of ice were allowed to precipitate. Sensitivity tests and internal consistency checks on the parameterization were done using a one-dimensional, time-dependent cloud model. Results suggested that the parameterization should simulate adequately the ice phase evolution in higher dimensional models. The parameterization is most suitable for modeling studies in which the major emphasis is on exploring the dynamic consequences of the ice phase rather than exploratory studies in cloud microphysics. Several deficiencies of the parameterization were commented on, specifically: the use of an externally specified snow concentration and its influence on the conversion of snow into graupel. Comments were also made on how local changes in the snow concentration brought about by seeding, ice multiplication and aggregation could be handled in' higher dimensional models.
Open Access
Breakup of temperature inversions in Colorado mountain valleys
(Colorado State University. Libraries, 1980) Whiteman, C. D. (Charles David), 1948-, author; McKee, Thomas B., author; Department of Atmospheric Science, Colorado State University, publisher
Open Access
Spectral methods for limited area models
(Colorado State University. Libraries, 1984) Fulton, Scott R., author; Schubert, Wayne H., advisor; Taylor, Gerald D., committee member; Krueger, David A., committee member; Stevens, Duane E., committee member; Johnson, Richard H. (Richard Harlan), committee member
This study investigates the usefulness of Chebyshev spectral methods in limited area atmospheric modeling. Basic concepts of spectral methods and properties of Chebyshev polynomials are reviewed. Chebyshev spectral methods are illustrated by applying them to the linear advection equation in one dimension. Numerical results demonstrate the high accuracy obtained compared to finite difference methods. The nonlinear shallow water equations on a bounded domain in two dimensions are then considered as a more realistic prototype model. Characteristic boundary conditions based on Reimann invariants are developed, and contrasted with wall conditions and boundary conditions based on the assumption of balanced flow. Chebyshev tau and collocation methods are developed for this model. Results from one-dimensional tests show the superiority of the characteristic conditions in most situations. Results from two-dimensional tests are also presented. Comparison of the tau and collocation methods shows that each has its own advantages and both are practical. Time differencing schemes for Chebyshev spectral methods are studied. The stability condition obtained with explicit time differencing, often thought to be "severe", is shown to be less severe than the corresponding condition for finite difference methods. Numerical results and asymptotic estimates show that time steps may in fact be limited by accuracy rather than stability, in which case simple explicit time differencing is practical and efficient. Two modified explicit schemes are reviewed, and implicit time differencing is also discussed. A Chebyshev spectral method is also used to solve the vertical structure problem associated with vertical normal mode transforms in a hydrostatic atmosphere. Numerical results demonstrate the accuracy of the method, and illustrate the aliasing which can occur unless the vertical levels at which data is supplied are carefully chosen. Vertical transforms of observed forcings of tropical wind and mass fields are presented. The results of this study indicate that Chebyshev spectral methods are a practical alternative to finite difference methods for limited area modeling, especially when high accuracy is desired. Spectral methods require less storage than finite difference methods, are more efficient when high enough accuracy is desired, and are at least as easy to program.
Open Access
Precipitating convective cloud downdraft structure: a synthesis of observations and modeling
(Colorado State University. Libraries, 1985) Knupp, Kevin Robert, author; Cotton, William R., advisor; Brown, John M., committee member; Stevens, Duane E., committee member; Sinclair, Peter C., committee member; Bienkiewicz, Bogusz, committee member
This study represents a comprehensive investigation in which observations are integrated with three-dimensional cloud model results to examine the kinematic, dynamic and thermodynamic structure of downdrafts associated with precipitating convection. One particular downdraft type, the low-level precipitation-associated downdraft, is investigated in considerable detail. It is shown that this downdraft exhibits significant structural, dynamic and thermodynamic properties which differ appreciably from other independent downdrafts within precipitating convective clouds. General airflow and trajectory patterns within low-level downdrafts are typically convergent from ~0.8 km upwards to downdraft top, typically less than 5 km AGL. Observed mass flux profiles often increase rapidly with decreasing height as a result of strong buoyancy forcing below the melting level. Such patterns indicate that strong cooling by melting and evaporation within statically unstable low levels generates low perturbation pressure by virtue of buoyantly-induced pressure perturbations. Cloud model results verify this process and indicate that pressure perturbations are strongest during downdraft developing stages. Maximum modeled pressure reductions up to 2 mb are located within downdrafts and precipitation about 0.6 km below the 273 K level approximately 10 min after heavy precipitation (˃ 2 g kg¯¹) enters low levels. The magnitude of this buoyantly-produced pressure reduction is influenced by temperature, static stability, relative humidity and precipitation characteristics. Model results and related calculations indicate that cooling provides the impetus for downdraft formation. Melting, in particular is generally found to make significant contribution to total cooling in cases having relatively shallow (˂ 2 km) PBL. Cooling by evaporation becomes increasingly important as PBL depth increases. Inflow to the low-level downdraft, although vertically continuous, can be separated into two branches. The up-down branch originating within the PBL initially rises up to 4 km and then descends within the main precipitation downdraft. The midlevel branch, most pronounced during early downdraft stages, originates from above the PBL and transports low-valued ϴₑ to low levels. Pressure forces important along both branches act to lift stable air along the up-down branch, and provide downward forcing of positively-buoyant air in the upper regions of both branches. Two primary conclusions are drawn from the results of this study: (1) Downdrafts are driven at low levels within regions of strong static instability by strong cooling provided by melting and evaporation. Cloud level entrainment effects make secondary contributions. (2) Precipitation size and phase (e.g. melting) are probably the most important controlling parameters for downdraft strength.
Open Access
Economic impacts and analysis methods of extreme precipitation estimates for eastern Colorado
(Colorado State University. Libraries, 1986) Changnon, David, author
Dams are designed to store water and to insure human safety and as such they must withstand, in their lifetimes, any extreme precipitation event in their drainage basin. Correct estimation of this event is critical because on one hand it must provide an adequate level of safety to not occur, but it must not be any greater than needed since the high costs of dam construction and modifications are directly related to the magnitude of the estimated extreme event. Most frequently the extreme precipitation event is labeled as the Probable Maximum Precipitation, or PMP. National and state concerns over the adequacy of existing dams in the United States as well as increased development of the Front Range led to a federal reassessment and redefinition of new PMP values issued for Colorado in 1984. The study area included the region from the Continental Divide to the 103rd Meridian. Study of these new PMP values and their potential economic impacts in Colorado reveals that an enormous cost will result in Colorado. Techniques for estimating cost of modifications for spillways were developed. Among 162 high risk dams, the estimated total cost for modification was approximately $184 million. The economic value of this precipitation estimate is $9.45 million per inch change of rainfall in this limited study area. In one elevation region, 7000 to 9000 feet, the cost is approximately $15.76 million per inch change of rainfall. Regional cost analyses revealed the South Platte River Division had the greatest costs. Inherent limitations in the PMP procedure and the cost of spillway modifications have made evaluating other alternatives necessary. Special aspects of estimates for extreme precipitation, such as snowmelt runoff versus extreme precipitation events and climate variations were examined. Four methods for estimating extreme precipitation events were evaluated; the traditional PMP, the paleogeological, the cloud/mesoscale dynamic model, and the statistical approaches. A collection of approaches were recommended for Colorado dam design in three elevation regions: the plains, the foothills, and the mountains.
Open Access
Investigation of enhanced-reflectivity features embedded within a wintertime orographic cloud on 28-29 November 1984
(Colorado State University. Libraries, 1994) Baker, Ian T., author; Grant, Lewis O., advisor; Mielke, Paul W., committee member; Cotton, William R., committee member
A combination of aircraft, sounding, surface, vertically-pointing ku-Band radar and dual-channel radiometer data was used to investigate the microphysical characteristics of enhanced-reflectivity areas embedded within an orographic cloud in northwestern Colorado on 28-29 November 1984. The orographic cloud was associated with the passage of an open wave and upper-level front over the region, and embedded within the cloud were regularly-spaced areas of increased reflectivity as seen by the vertically-pointing radar. The radiometer observed a cyclical component on both the liquid and vapor channels when oriented in the vertical. Aircraft data reveal that there was supercooled liquid water in the cloud at levels as high as 41 kPa and as far as 55 km upwind of the barrier. 2D-C and 2D-P probe data indicated two crystal regimes, one where concentrations in individual size bins were larger and spectra were broader, indicating crystal growth. In the other, concentrations were smaller and size spectra were narrower. Radar data indicate that the enhanced-reflectivity regions were between 10-20 km apart, with a length dimension on the order of 5 km wide. It is believed that the presence of the enhanced-reflectivity areas is closely linked to the presence of a decoupled layer on the windward side of the barrier, and preliminary evidence points to a gravity-wave mechanism as a physical cause.
Open Access
Understanding and forecasting tropical cyclone intensity change
(Colorado State University. Libraries, 1995) Fitzpatrick, Patrick J., author; Gray, William M., advisor; Schubert, Wayne, committee member; Montgomery, Michael, committee member; Mielke, Paul, committee member
This research investigates several issues pertaining to tropical cyclone intensity change. Previous research on tropical cyclone intensity change is reviewed in great detail. The applicability of upper-level forcing theories is questioned. Inner-core processes related to intensity change are studied, with particular attention on the relationship between the vertical profile of the tangential wind (Vt) field in the eyewall region and future pressure changes. It is hypothesized that a vertically conserved wind profile is conducive to fast intensification. Observations support this theory. By stratifying inner-core data into fast and slow developers, it is shown that fast developing tropical cyclones contain a more vertically stacked inner-core vortex than slow developers. It is also shown that a direct correlation exists between inner-core upper-level winds and tropical cyclone intensification, with the rate of intensification proportional to the magnitude and symmetry of upper-level Vt. An alternative air-sea interaction theory is presented which incorporates boundary layer cooling. The buoyancy calculations include partial water-loading and ice microphysics, and their relevance to CAPE calculations in the tropics is discussed. It is shown that the lateral extension of the eye, above a sloping eyewall, is the crucial component in maintaining the air-sea interaction despite boundary layer cooling. Implications on the maximum intensity a storm may achieve are discussed. A multiple regression scheme with intensity change as the dependent variable has been developed. The new scheme is titled the Typhoon Intensity Prediction Scheme (TIPS), and is similar to one used operationally at the National Hurricane Center. However, TIPS contains two major differences: it is developed for the western North Pacific Ocean, and utilizes digitized satellite data. It is shown that the satellite data can distinguish between fast and slow developing tropical cyclones. The importance of other statistical predictors (such as SSTs, wind shear, persistence, and climatology) to intensity change are also clarified. The statistics reveal threshold values useful to forecasters. It is shown that TIPS is competitive with the Joint Typhoon Warning Center.
Open Access
Simulation of alpine snow distributions in the northern Colorado Rocky Mountains using a numerical snow-transport model
(Colorado State University. Libraries, 1999) Greene, Ethan M., author
Two methodologies for simulating winter snow distributions I alpine terrain are presented. First, a numerical snow-transport model (SnowTran-3D) is driven from direct meteorological observations, and second, SnowTran-3D is driven from a regional atmospheric model (ClimRAMS). In each case the simulated snow distributions are compared to observed snow depth transects within two alpine sites in the Northern Colorado Rocky Mountains, Rocky Mountain National Park, and Medicine Bow Mountains. The atmospheric conditions at these sites are characterized by persistent westerly winds with average speeds of 13 m/s, which is significantly greater than the threshold for snow transport (approximately 5 m/s). Consequently, snow redistribution by wind is the dominate component in this environment Drift features in these areas form around rocks, alpine vegetation, and small and large topographic variations. The model successfully simulated the large-scale snow drifts, but due to the relatively coarse resolution of the vegetation and topographic data inputs (30 m), the model was unable to reproduce some of the smaller scale snow drift features. The model built large drifts in the upper regions of the east facing cirques in Rocky Mountain National Park, in regions where large perennial snow fields are observed. The model result support the theory that snow transport by wind is an important factor in sustaining these snow fields.
Open Access
Vertical distribution of water vapor using satellite sounding methods with new aircraft data validation
(Colorado State University. Libraries, 2001) McNoldy, Brian D., author; Vonder Haar, Thomas, advisor; Stephens, Graeme, committee member; She, Chiao-Yao, committee member
The importance of water vapor in Earth's climate system is undisputedly immense. Its meteorological impacts range from radiative transfer to the hydrologic cycle, on scales ranging from local to planetary. It affects military operations, commercial flights, and private industry. However, global measurements of water vapor can currently only be obtained from satellites, since ground stations are sparse and ocean stations are nearly non-existent. Unfortunately, the satellites cannot directly measure water vapor; instead, they detect radiation at discrete frequencies. The signals originate from different altitudes in the atmosphere, providing vertical resolution. A complex mathematical inversion is necessary to retrieve the desired quantity (water vapor) from the measured quantity (brightness temperatures). Both the satellite calibration and the retrieval algorithm contribute to errors in the retrieved parameters. The focus here is on the validation of a satellite-based retrieval using in-situ measurements of water vapor made by commercial aircraft. The Aircraft Communications Addressing and Reporting System (ACARS) routinely records a plethora of meteorological parameters, including temperature, pressure, wind velocities, and turbulence. The new Water Vapor Sensing System (WVSS) added water vapor mixing ratio and dewpoint to the array of parameters. These measurements will be compared to the humidity measurements retrieved from the satellite-based TIROS Operational Vertical Sounder (TOVS) High-resolution Infrared Radiation Sounder (HIRS) radiances over the continental United States. This study shows that the water vapor retrieval algorithm is approximately 20% too dry through most of the atmosphere when compared to aircraft measurements of the same parameter.
Open Access
Forecasting of Atlantic tropical cyclones using a kilo-member ensemble
(Colorado State University. Libraries, 2004) Vigh, Jonathan L., author; Schubert, Wayne H., advisor; DeMaria, Mark, committee member; Gray, William M., committee member; Taylor, Gerald D., committee member
The past 30 years have witnessed steady improvements in the skill of tropical cyclone track forecasts. These increases have been largely driven by improved numerical weather prediction models and increased surveillance of the storm environment through aircraft reconnaissance and satellite remote sensing. The skill of deterministic track forecasts from full-physics models is gradually approaching the theoretical limit of predictability that arises due to the atmosphere's chaotic nature and limitations in determining the initial state. To make further progress, it is necessary to treat the uncertainty of the initial condition. One practical approach is to sample this uncertainty by perturbing the initial state. The resulting suite of forecasts that result from integrating such perturbations is known as an ensemble. This thesis describes the design, implementation, and evaluation of a semi-operational ensemble forecasting system using an efficient multigrid barotropic vorticity equation model (MBAR). Five perturbation classes are used to simulate uncertainties in the storm environment and vortex structure. Uncertainties in the storm environment are simulated by using the background environmental flow evolutions provided by the NCEP Global Forecasting System (GFS) ensemble forecasts. Several deep layer-mean wind averages account for uncertainty in the depth of the storm steering layer. Uncertainties in the decomposition of the tropical atmosphere's vertical modes are simulated by varying the model equivalent phase speed. Finally, uncertainties in the vortex structure are simulated by varying the vortex size and storm motion vector. Each perturbation in a given class is cross-multiplied with all other perturbations of other classes to obtain an ensemble with 1980 members. One of the fundamental questions addressed by this research is whether such cross-multiplication increases the degrees of freedom in the ensemble. The ensemble is run for 294 cases from the 2001-2003 Atlantic hurricane seasons. Theory dictates that a properly-perturbed ensemble should, on average, be more accurate than any single ensemble member, but it was found that the kilo-ensemble mean forecast did not demonstrate substantial improvement over the control forecast. However, the ensemble mean did show substantial skill relative to the five-day climatology and persistence model (CLP5) throughout the 120-h forecast period. The ensemble mean spread (the mean distance of the individual members from the ensemble mean), x-bias, and y-bias statistics are also evaluated. Probabilistic interpretations are valid with an ensemble of this size, so cumulative strike probabilities are calculated explicitly from the kilo-ensemble output. In a related possibilistic interpretation, the ensemble can be looked upon as mapping out the subspace of all possible storm tracks, so the reliability of this ensemble envelope is examined. Finally, if the ensemble can accurately simulate the uncertainties in the dynamical system, then there should be a positive relationship between ensemble mean spread and the error of the ensemble mean forecast. A strong relationship allows useful forecasts of forecast skill to be made at the time of the forecast. The kilo-member ensemble was found to have a weak spread-error relationship that peaks at 60 h.
Open Access
The birth and death of the MJO: an observation study
(Colorado State University. Libraries, 2005) Benedict, James J., author; Randall, David A. (David Allan), 1948-, advisor; Kirkpatrick, Allan Thompson, committee member; Madden, Roland A., committee member; Thompson, David W. J., committee member
The Madden-Julian Oscillation (MJO), an eastward-propagating equatorial wave most active during the boreal winter, dominates atmospheric intraseasonal (10-100 day) variability in the tropical Indian and West Pacific Ocean areas. This phenomenon is characterized by cyclic periods of suppressed convection (dry phase) and intense rainfall (wet phase). In this study, we examine important physical mechanisms observed during the "birth" (wet phase approach) and "death" (wet phase departure) of the MJO. Analyses of single events and event composites based on TRMM precipitation highlight cogent features of the MJO. Unlike previous studies, we base MJO events on hydrological activity due to its strong ties to latent heating, the primary driver of tropical circulations. Dynamical fields of mesoscale resolution are diagnosed from ECMWF reanalysis datasets (ERA40). Prior to the onset of intense rainfall, a slow increase in low-level temperature and moisture leads to greater instability. An enhancement of shallow cumulus activity, as inferred from the reanalysis data, is associated with increased moisture detrainment and an erosion of a mid-tropospheric dry layer. In this stage, vertical moisture advection is dominant over the horizontal component. The "death" of the MJO involves immediate and delayed drying processes. Within five days after maximum rainfall, we observe anomalous low-level drying by horizontal advection during a time of weak moistening by vertical motions. This immediate drying has not been analyzed explicitly in previous composite studies. Subsidence drying is delayed, beginning and then peaking one and two weeks after intense precipitation, respectively. Physical attributes of the composite results are compared to current wave instability theories. Our findings lend support to the discharge-recharge mechanism which involves a gradual, local build-up of instability. Currently, no widely-accepted theory exists that can fully explain the MJO. Accurately diagnosing and modeling this phenomenon is of critical importance for weather and climate studies. It is our hope that this study contributes toward an improved understanding of the MJO and its depiction in atmospheric models.
Open Access
Investigation of the summer climate of North America: a regional atmospheric modeling study
(Colorado State University. Libraries, 2005) Castro, Christopher Lawrence, author
The Regional Atmospheric Modeling System (RAMS) is used to investigate model sensitivity and the summer climate of North America. The value restored and added by dynamical downscaling is first evaluated. At large scales, RAMS underestimates atmospheric variability and this worsens as the grid spacing increases or domain size increases. The model simulated evolution of kinetic energy relative to the driving reanalysis kinetic energy exhibits a decrease with time which is more pronounced with larger grid spacing. The surface boundary forcing is the dominant factor in generating atmospheric variability and exerts greater control on the model as the influence of lateral boundary conditions diminish. The sensitivity to surface forcing is also influenced by the model parameterizations. Dynamical downscaling with RAMS does not retain value of the large scale of that which exists in the driving global reanalysis. The value added is to resolve smaller-scale features which have a greater dependence on the surface boundary. The NCEP Reanalysis is then dynamically downscaled with RAMS to generate a regional model climatology of the contiguous U.S. and Mexico (1950-2002). The simulations capture climatic features and seasonal transitions associated with the North American monsoon system. The diurnal cycle is the dominant time-varying mode of convective activity and is modulated by the large-scale circulation. Lower frequency modes account for the variability of convection at a remote distance from elevated terrain. The climatology is evaluated with respected to the dominant modes of global sea surface temperature. An additional series of simulations dynamically downscales data from a general circulation model executed with idealized sea surface temperature corresponding to the modes with greatest variability in the Pacific, to establish a casual link to remote sea surface temperature forcing. Time-evolving teleconnections related to tropical Pacific sea surface temperature modulate the evolution of North American summer climate, in particular the low-level moisture transport into the continental interior and convective activity. The most significant response occurs in early summer and affects the distribution of rainfall at that time. A global increase in tropical sea surface temperature over the period has also significantly affected North American summer climate.
Open Access
A parametric optimal estimation retrieval of the non-precipitating parameters over the global oceans
(Colorado State University. Libraries, 2006) Elsaesser, Gregory S., author; Kummerow, Chris, advisor; Reising, Steven C., committee member; Randall, David, committee member
There are a multitude of spacebome microwave sensors in orbit, including the TRMM Microwave Imager (TMI), the Special Sensor Microwave/lmager (SSM/I) onboard the DMSP satellites, the Advanced Microwave Scanning Radiometer - Earth Observing System (AMSR-E), SSMIS, WINDSAT, and others. Future missions, such as the planned Global Precipitation Measurement (GPM) Mission, will incorporate additional spacebome microwave sensors. The need for consistent geophysical parameter retrievals among an ever-increasing number of microwave sensors requires the development of a physical retrieval scheme independent of any particular sensor and flexible enough so that future microwave sensors can be added with relative ease. To this end, we attempt to develop a parametric retrieval algorithm currently applicable to the non-precipitating atmosphere with the goal of having consistent non-precipitating geophysical parameter products. An algorithm of this nature makes is easier to merge separate products, which, when combined, would allow for additional global sampling or longer time series of the retrieved global geophysical parameters for climate purposes. This algorithm is currently applied to TMI, SSM/I and AMSR-E with results that are comparable to other independent microwave retrievals of the non-precipitating parameters designed for specific sensors. The physical retrieval is developed within the optimal estimation framework. The development of the retrieval within this framework ensures that the simulated radiances corresponding to the retrieved geophysical parameters will always agree with observed radiances regardless of the sensor being used. Furthermore, a framework of this nature allows one to easily add additional physics to describe radiation propagation through raining scenes, thus allowing for the merger of cloud and precipitation retrievals, if so desired. Additionally, optimal estimation provides error estimates on the retrieval, a product often not available in other algorithms, information on potential forward model/sensor biases, and a number of useful diagnostics providing information on the validity and significance of the retrieval (such as Chi-Square, indicative of the general "fit" between the model and observations and the A-Matrix, indicating the sensitivity of the model to a change in the geophysical parameters). There is an expected global response of these diagnostics based on the scene being observed, such as in the case of a raining scene. Fortunately, since TRMM has a precipitation radar (TRMM PR) in addition to a radiometer (TMI) flying on-board, the expected response of the retrieval diagnostics to rainfall can be evaluated. It is shown that a potentially powerful rainfall screen can then be developed for use in passive microwave rainfall and cloud property retrieval algorithms with the possibility of discriminating between precipitating and nonprecipitating scenes, and further indicating the possible contamination of rainfall in cloud liquid water path microwave retrievals.
Open Access
Cloud-to-ground lightning polarity and environmental conditions over the central United States
(Colorado State University. Libraries, 2007) Kalb, Christina P., author; Rutledge, Steven A., advisor; Cotton, William R., committee member; Robinson, Steven R., committee member
The majority of cloud-to-ground (CG) lightning across the United States lowers negative charge to the ground. However, recent studies have documented storms that produce an abundance of positive CG lightning. These positive storms have been shown to occur in different mesoscale regions on the same days, and in different thermodynamic environments. This study uses radar data, and CG lightning data, to identify positive and negative storms that occurred in the region between the Rocky Mountains and the Mississippi River. The thermodynamic conditions in the environment of these storms are derived from the Rapid Update Cycle model analysis, where the point nearest to the storm, in the direction of storm motion was used. Considerable scatter was present in the final results that limited the extent of the trends seen. Out of all the variables used, cloud base height, dew point, 850-500 mb lapse rate, and warm cloud depth showed the most difference between the positive and negative storms. Positive storms tended to occur with lower cloud base heights, higher dew points, smaller 850-500 mb lapse rates, and lower warm cloud depths. Little trend was seen for CAPE, CIN, freezing level, lifted index, mean relative humidity, mid-level relative humidity, precipitable water 0-3 km wind shear, 0-6 km wind shear, storm relative helicity, and Se. The strength of the differences seen between the positive and negative storms varies with the choice of percent positive used. Differences between the positive and negative storms tended to decrease when 10% was chosen (as compared to 30%), but they increased when 50% was chosen.
Open Access
A study of the relationship between thunderstorm processes and cloud-top ice crystal size
(Colorado State University. Libraries, 2008) Lindsey, Daniel T., author; Johnson, Richard H., advisor
Satellite observations and numerical models are used to understand the physical mechanisms responsible for thunderstorms with varying cloud-top ice crystal sizes. Geostationary Operational Environmental Satellite (GOES) data are used to create a three-year climatology of cloud-top 3.9 µm reflectivity, a quantity which is closely correlated with particle size. Maximum mean values are found over the High Plains and Rocky Mountain regions of the U.S., suggesting that convection over that region tends to generate smaller anvil ice crystals than areas throughout much of the eastern U.S. To correct for preferred forward scattering by the cloud-top ice crystals, an effective radius retrieval using GOES is developed. Forward radiative transfer simulations are run for a wide range of cloud-top ice crystal sizes and sun-cloud-satellite scattering angles. The output is used to generate a lookup table, so that GOES-measured radiances may be used along with sun-satellite geometry to obtain an estimate for particle size. Validation of the retrieval shows that the assumed scattering properties perform quite well. To help explain the geographical variation in cloud-top ice crystal size, a composite analysis is performed in the High Plains region by averaging environmental conditions for days which produced both small and large ice crystal storms. Small ice is found to occur with relatively high based storms and steep mid-level lapse rates. Additionally, observational evidence from a pyrocumulonimbus event is presented to show the effect of low-level cloud condensation nuclei (CCN) on cloud-top ice crystal size. Model simulations using the Colorado State University Regional Atmospheric Modeling System (RAMS) are performed to help understand the physical mechanisms responsible for cloud-top ice crystal size. Through a series of sensitivity tests, it is found that larger low-level CCN concentrations lead to smaller anvil ice. In addition, as cloud-base temperature decreases (and cloud-base height increases), storm-top ice crystals get smaller. A weaker updraft strength is found to have very little effect on ice crystal size.
Open Access
Model evaluation using space-borne lidar observations
(Colorado State University. Libraries, 2008) Ahlgrimm, Maike, author; Randall, David A., advisor
In this study, the use of space-borne lidar observations for the comparison with, and evaluation of modeled clouds is explored. Four version of the ECMWF Integrated Forecast System and two versions of the Goddard Earth Observing System (GEOS-5) model are assessed for their ability to produce marine boundary layer clouds. The cause of some of the model deficiencies is investigated, and specific suggestions for improvements are made and tested. In order to do so, two cloud types are defined: a stratocumulus type (Sc), and a trade cumulus or transitional cumulus type (TCu). Samples in four oceanic regions are classified into those categories, and the frequency of occurrence, location, and properties of the samples compared between models and observations.
Open Access
Atmospheric nitrogen and sulfur deposition in Rocky Mountain National Park
(Colorado State University. Libraries, 2008) Beem, Katherine B., author; Collett, Jeffrey L., advisor; Davis, Jessica G., committee member; Kreidenweis, Sonia M., committee member
Rocky Mountain National Park (RMNP) is experiencing a number of adverse effects due to atmospheric nitrogen (N) and sulfur (S) compounds. Airborne nitrate and sulfate particles contribute to visibility degradation in the park while nitrogen deposition is producing changes in ecosystem function and surface water chemistry. Both sulfur and nitrogen compounds are essential nutrients for life; however, some environments have naturally limited supplies of sulfur and nitrogen which restrict biological activity. Increasing the amounts of these compounds can be toxic, even life threatening, to the ecosystem. Concerns about increasing deposition are especially important in national parks where excess nitrogen and sulfur can upset the delicate balance between species of flora and fauna in prized natural ecosystems. Measurements were made during the Rocky Mountain Airborne Nitrogen and Sulfur (RoMANS) study to quantify both N and S wet and dry deposition and to determine the most important species and pathways contributing to N deposition. Gas and particle concentrations were measured and precipitation samples were collected to gain a better understanding of nitrogen and sulfur transport to and deposition in RMNP. Samples were collected at 12 sites across the state of Colorado in March and April 2006 and at 13 sites in north central Colorado in July and August 2006. Historical data suggest that these are the seasons when N deposition in RMNP is greatest. The majority of wet deposition in the spring was from a single, large upslope snowstorm, while in the summer wet deposition inputs were spread across many more events. Total wet deposition of N in the summer was larger than during spring. Ammonium was the largest contributor to both spring and summer wet deposition in the park, followed by nitrate. Organic nitrogen, which is not routinely measured, contributed an average of 616.39 μg N/m2/event in the spring and 847.2 μg N/m2/event in the summer at the core sampling site. These deposition amounts were 22% and 16%, respectively, of total wet nitrogen deposition at this site. Dry deposition in RMNP was dominated by gaseous species which feature higher deposition velocities than accumulation mode aerosol particles. Ammonia, which is not routinely measured, was the largest contributor to dry N deposition followed by nitric acid. Dry deposition of fine particle nitrate and ammonium made only small contributions to total N deposition. Total N inputs were dominated by wet processes during both spring and summer. Wet deposition of organic nitrogen and dry deposition of gaseous ammonia comprised the 3rd and 4th largest contributions to the total N deposition budget. Together these pathways contributed nearly one-third of total measured N deposition, suggesting they should be examined more closely in assessing nitrogen impacts on national park ecosystems.
Open Access
Estimating contributions of primary biomass combustion to fine particulate matter at sites in the western United States
(Colorado State University. Libraries, 2008) Holden, Amanda S., author; Collett, Jeffrey L., advisor; Henry, Charles S., committee member; Kreidenweis, Sonia M., committee member
Biomass combustion occurs throughout the world and has many implications for human health, air quality and visibility, and climate change. To better understand the impacts of biomass combustion in the western United States, six-day integrated fine particle samples were collected during the winter and summer seasons of 2004-2006 at seven IMPROVE sampling sites using Hi-Vol samplers. These sites included both urban and rural locations. Filter samples were analyzed for organic and elemental carbon, levoglucosan, and a suite of particulate ions. Levoglucosan, a thermal degradation product of cellulose, is a widely used tracer for primary biomass combustion. Measurements of levoglucosan and other carbohydrates were made using a new approach involving aqueous filter extraction followed by direct analysis using High Performance Anion Exchange Chromatography. In this method carbohydrates are separated on a Dionex Carbopac PA-10 column and detected using pulsed amperometry. Source profiles for primary biomass combustion were applied to each of these samples to estimate the contributions of carbon from both residential wood burning (during the winter seasons) and wildland fires (during the summer seasons). Wildland fire source profiles were determined from FLAME (Fire Lab at Missoula Experiment) campaigns at the USFS/USDA Fire Science Lab in Missoula, MT, during which fine particle samples were collected from source burns of approximately 30 fuel types. Residential wood combustion source profiles were collected from the literature. Primary biomass combustion contributions to contemporary PM2.5 carbon, determined separately from carbon isotope measurements at Lawrence Livermore National Laboratory, ranged from 0.4% to more than 100%. Contributions of primary biomass combustion were higher at rural sites, while urban sites showed greater contributions of fossil carbon. Primary biomass combustion contributed a larger fraction of total carbon in the summer at southern sites, while northern sites had larger contributions during the colder winter months.
Open Access
The optical, chemical, and physical properties of aerosols and gases emitted by the laboratory combustion of wildland fuels
(Colorado State University. Libraries, 2008) McMeeking, Gavin R., author; Kreidenweis, Sonia M., advisor
Biomass burning is a major source of trace gases and particles that have a profound impact on the atmosphere. Trace gases emitted by fires include the greenhouse gases CO2 and CH4, as well as CO and volatile organic compounds that affect air quality. Particle emissions affect climate, visibility, the hydrologic cycle, and human health. This work presents measurements of trace gas and aerosol emissions from a series of controlled laboratory burns for various plant species common to North America. Over 30 fuels were tested through ~250 individual burns during the Fire Laboratory at Missoula Experiment. Emission factors are presented as a function of modified combustion efficiency (MCE), a measure of the fire combustion conditions. The emissions of many trace gas and aerosol species depended strongly on MCE: smoldering-phase combustion dominated fires (low MCE) emitted roughly four times as much gas-phase hydrocarbon species and organic aerosols than flaming-phase dominated fires (high MCE). Inorganic aerosol emissions depended more strongly on plant species and components than on MCE. Flaming-phase dominated fires tended to produce aerosol with high mass fractions of strongly light-absorbing elemental carbon. Smoldering-phase fires produced aerosol with large mass fractions of more weakly light absorbing organic carbon, but this material was found to have a strong wavelength dependence of absorption, greater than the inverse wavelength relationship typically assumed for light absorbing aerosol. A two component model-featuring elemental carbon with a weak wavelength dependence but high mass-normalized absorption efficiency and organic carbon with a strong wavelength dependence but low mass-normalized absorption efficiency-is shown to represent the bulk absorption spectra of biomass burning aerosol. The results show that at wavelengths below ~450 nm, organic carbon light absorption could rival that of elemental carbon for aerosol dominated by organic carbon. If ignored, the light absorption by organic carbon can cause errors in predicted surface ultraviolet and visible radiation fluxes and photochemical photolysis rates in regions affected by biomass burning emissions. The dependence of spectral aerosol optical properties on combustion conditions means that fire behavior must be accurately assessed and predicted to ensure accurate emissions inventories and estimates of biomass burning atmospheric impacts.