Browsing Atmospheric & Oceanic Science Theses and Dissertations by Issue Date
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- ItemPotential Predictability of 500 mb Geopotential Heights on both Monthly and Seasonal Scales over the Northern Hemisphere(1989) Singh, Ramdas Ram; Shukla, Jagadish; Department of Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, MD)
- ItemDevelopment and use of a Fast Response, Nitric Oxide Detector for Air Quality Monntoring and Eddy Correlation Flux Measurements(1996-06-01) Civerolo, Kevin; Dickerson, Russell
- ItemMEASUREMENTS AND CHARACTERIZATION OF OPTICAL PROPERTIES IN THE CHESAPEAKE BAY'S ESTUARINE WATERS USING IN-SITU MEASUREMENTS, MODIS SATELLITE OBSERVATIONS, AND RADIATIVE TRANSFER MODELING(2004-01-27) Tzortziou, Maria; Hudson, Robert; MeteorologyThe core subject of this thesis is the development of coordinated atmospheric, in-water, and laboratory measurements leading to characterization of in-water optical properties in the estuarine environment of northern Chesapeake Bay, where natural and human-induced processes strongly interact. One of the main objectives is obtaining a sufficiently complete suite of measurements, combined with detailed radiative transfer calculations, so as to produce a closure experiment for the underwater inherent and apparent optical properties. The in-situ results are applied to the interpretation of satellite (MODIS) water leaving radiance data and their validation. The applicability of bio-optical models and parameterizations currently used in satellite algorithms are examined for the case of the optically complex Chesapeake Bay waters. Relationships between remotely sensed water leaving radiances and properties of optically active components in these waters are investigated. The resulting techniques and analysis should be broadly applicable to other coastal areas of the world. The results from this thesis, and other future work, will contribute to our ability to obtain more accurate information from remotely measured optical characteristics of estuarine and coastal regions. The combined use of in-situ measurements and detailed radiative transfer modeling enables the improvement of both the theoretical models and satellite remote sensing algorithms needed to a better understanding of biotic responses to environmental forcing.
- ItemLONGWAVE RADIATIVE TRANSFER THROUGH 3D CLOUD FIELDS: TESTING THE PROBABILITY OF CLEAR LINE OF SIGHT MODELS WITH THE ARM CLOUD OBSERVATIONS(2004-03-16) Ma, Yingtao; Robert, Ellingson G.; MeteorologyClouds play a key role in regulating the Earth's climate. Real cloud fields are non-uniform in both the morphological and microphysical sense. However, most climate models assume the clouds to be Plane-Parallel Horizontal (PPH) plates with homogeneous optical properties. Three characteristics of 3D clouds have been found to be important for longwave radiative transfer. They are: (1) the 3D geometrical structure of the cloud fields, (2) the horizontal variation of cloud optical depth, and (3) the vertical variation of cloud temperature. One way to incorporate the 3D geometrical effect in climate studies is through the use of an effective cloud faction, for which a major component is the Probability of Clear Line Of Sight (PCLOS). The PCLOS also plays an important role in accounting for longwave 3D effects caused by variable cloud optical depth and vertical change of cloud temperature. Aimed at improving the parameterization of longwave radiative transfer through 3D clouds, this study formulated a set of PCLOS models and tested the models with the Atmospheric Radiation Measurement (ARM) cloud observations. In order to investigate the sampling issue that arises from attempting to obtain domain-averaged information from time series of observations, an evaluation technique was developed and tested with Cloud Resolving Model (CRM) and Large Eddy Simulation (LES) model data. Various cloud properties that are necessary for the PCLOS models such as the absolute cloud fraction (N), cloud thickness, cloud spacing, and horizontal size were inferred from the ARM observations. A set of automated inference techniques were developed. The modeled PCLOS was then tested with the PCLOS inferred from time series of total sky images.
- ItemShort-term Variability of Atmospheric Extinction During the Night, Under Clear-Sky Conditions, Investigated by Broadband Stellar Photometry(2004-09-02) Musat, Ileana Cristina; Ellingson, Robert G.; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This is a study about the possibility of determining the aerosol optical depth by using star broadband observations from a whole sky imager. The main difficulty in such measurements consists of accurately separating the star flux value from the non-stellar diffuse light, which is overwhelmingly present in the whole sky imagery. A correction method to solve this problem is found and the monochromatic extinction at the ground due to aerosols is extracted from heterochromatic measurements. A form of closure is achieved by comparison with simultaneous or temporally close measurements with other instruments. The accuracy and precision of the method are assessed: the total error is a combination of random error of measurements and systematic error of calibration and model and is between 2.6 and 3% rms.
- ItemSTRATOSPHERIC OZONE TRENDS AS DETERMINED BY REGIME ANALYSIS: THE SOUTHERN HEMISPHERE(2004-11-17) Andrade, Marcos Froilán; Hudson, Robert D; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Recent studies in the Northern Hemisphere have shown that the total ozone field can be classified by meteorological regimes, whose boundaries are the tropospheric upper-level jets. A similar approach has been followed to classify the Southern Hemisphere. Total ozone data from the TOMS instruments have been used to locate the upper-level jets and therefore, the regimes' boundaries. Here however, I use vertically integrated potential vorticity from the reanalysis data from NCEP/NCAR and ECMWF to estimate the first guess in an iterative procedure that calculates the ozone regime boundary values. The classification of the total ozone field using these boundary values was validated using daily rawinsonde temperature profiles, SAGE II ozone profiles, and TOMS total ozone data. Rawinsondes showed that, on any given day, the tropopause heights within a regime were relatively constant despite the large latitudinal extent of the regime itself. Data from SAGE II also showed that the classification produces distinct ozone profiles for each regime, which is consistent with the fact that total ozone is nearly constant within each regime, as shown by TOMS data. The temporal behavior of total ozone within each regime between 1979 and 2004 was analyzed for the 25º-60ºS latitude band. Total ozone trend analysis showed that statistically significant decadal trends within each regime were smaller than the overall trend in this band with no classification. In addition, the areas of the meteorological regimes within the 25º-60ºS latitude band also showed statistically significant trends. During the period of study, the area of the tropical regime has increased at the expense of the areas of the midlatitude and polar regimes. Both processes, changes in total ozone within each regime as well as changes in their contribution to the 25º-60ºS band, are responsible for the overall trend in that region. The analysis also showed that the QBO and the solar flux have an important influence on the temporal behavior of both the total ozone within each regime and their corresponding areas. The results indicate, as is the case of the Northern Hemisphere, that the total ozone regime boundaries have migrated poleward between 1979 and 2004.
- ItemThe Effect of Deep Convection on Temperatures in the Tropical Tropopause Layer and Its Implications to the Regulation of Tropical Lower Stratospheric Humidity(2005-04-19) Kim, Hyun Cheol; Dessler, Andrew E; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation focuses on the impact of deep convection on the thermal structure in the Tropical Tropopause Layer (TTL). Temperatures in this region play an important role in the regulation of water vapor, which in turn affects radiation, chemistry, and dynamics in the lower stratosphere. This dissertation includes two important conclusions concerning the regulation of temperature in the TTL. First, significant cooling near the tropical tropopause is observed during the time when active convection is occurring. A composite technique is used to relate the local temperature anomalies to the evolution of local convection. Temperature profiles are measured by the Atmospheric Infrared Sounder (AIRS) onboard the Aqua satellite, and the time evolution of local convections are determined by the National Centers for Environmental Protection / Aviation Weather Center (NCEP/AWS) half-hourly infrared global geostationary composite. The observations demonstrate that the TTL is cooled by convection, in agreement with previous observations and model simulations. By using a global data set, the variations in this convective cooling are investigated by season and region. The estimated cooling rate during active convection is - 7 K/day. This exceeds the likely contribution from cloud-top radiative cooling, suggesting turbulent mixing of deep convection plays a role in cooling the TTL. Second, height and thermal structure of the overshooting deep convection in the TTL are investigated using visible and infrared observations from the Visible and Infrared Scanner (VIRS) onboard the Tropical Rainfall Measuring Mission (TRMM) satellite. The heights of overshooting clouds are estimated from the sizes of the visible shadows that these clouds cast. The temperature information is obtained from the mid-infrared channel. From these, the lapse rate in the cloud is estimated. The result shows that the measured lapse rate of these clouds is significantly below adiabatic. Mixing between these clouds and the near-tropopause environment is the most likely explanation. As a result, these clouds will likely settle at a final altitude above the convections' initial level of neutral buoyancy.
- ItemVARIATIONAL DATA ASSIMILATION OF SOIL MOISTURE INFORMATION(2005-04-20) Grunmann, Pablo Javier; Kalnay, Eugenia E; Mitchell, Kenneth E; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This research examines the feasibility of using observations of land surface temperatures (in principle available from satellite observations) to initialize soil moisture (which is not available on a continental scale). This problem is important because it is known that wrong soil moisture initial conditions can negatively affect the skill of numerical weather prediction models. Since this problem requires the availability of a good soil model, considerable effort was devoted to the improvement of several aspects of the NCEP Noah land surface model and its numerical properties (reliability, efficiency, updates and differentiability). When tested against the experimental station data at Champaign, IL collected by Dr. Tilden Meyers of NOAA/ARL, where the surface fluxes, precipitation, and surface temperature were available, the Noah model forced with observed downward radiative surface fluxes and near-surface meteorology, including precipitation, was able to reproduce the observations quite well. A method for data assimilation was developed and tested, in a manner similar to 4-dimensional variational assimilation (4D-Var) in the sense of applying the temporal behavior of the observed variable but with a single spatial dimension (land surface models are typically “column models”, as they do not usually compute horizontal derivatives). The results show that it is indeed possible to assimilate land surface temperature and use it to correct soil moisture initial conditions, which may manifest significant errors if, for example, the precipitation forcing the model is significantly biased. This is true, however, only if the surface forcings besides precipitation are essentially correct. When surface forcing come from the North American Land Data Assimilation System (NLDAS) as they would be available for operational use over the US, the results are not satisfactory. This is because the assimilation changes the soil moisture to correct for problems in the simulated land surface temperature that are at least partially due to other sources of errors, such as the surface radiative fluxes. We suggest that in order to succeed in the soil moisture initialization, more (and more accurate) observations are needed in order to constrain the dependence of the observation part of the cost function solely on soil moisture.
- ItemBRED VECTORS IN THE NASA NSIPP GLOBAL COUPLED MODEL AND THEIR APPLICATION TO COUPLED ENSEMBLE PREDICTIONS AND DATA ASSIMILATION(2005-04-27) Yang, Shu-Chih; Yang, Shu-Chih; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The theme of my thesis research is to perform breeding experiments with NASA/NSIPP coupled general circulation model (CGCM) in order to obtain ENSO-related growing modes for ensemble perturbations. We show for the first time that the breeding method is an effective diagnostic tool for studying the coupled ENSO-related instabilities in a coupled ocean-atmosphere general circulation model that includes physical and dynamical processes of many different time scales. We also show for the first time that it is feasible to utilize the coupled bred vectors (BV) as a way to construct perturbations for ensemble forecasts for ENSO prediction using an operational coupled climate prediction model. The results of the thesis research show that coupled breeding can detect a precursor signal associated with ENSO events. Bred vectors are characterized by air-sea coupled features and they are very sensitive to ENSO phases and background season. This indicates that bred vectors can effectively project on the seasonal-to-interannual instabilities by growing upon the slowly varying coupled instability. These results are robust: bred vectors obtained from both the NASA and NCEP coupled systems exhibit similarities in many fields, even in atmospheric teleconnected regions. We show that bred vectors have a structure similar to the one-month forecast error (analysis increment). The BV growth rate and the one-month forecast error show similar low frequency variations. Both of their subsurface temperatures have large-scale variability near the depth of thermocline. Evidence shows that bred vectors capture the eastern movement of the analysis increment (one-month forecast error) along the equatorial Pacific during 1997-1998 El Niño evolution. The results suggest that one-month forecast error in NSIPP CGCM is dominated by dynamical errors whose shape can be captured by bred vectors, especially when the BV growth rate is large. These results suggest that bred vectors should be effective coupled perturbations for ensemble ENSO predictions, compensating for the lack of coupled ENSO-related perturbations in current operational ensembles. The similarity between the bred vectors and the one month forecast errors suggests that bred vectors can capture "errors of the month" and could also be applied to improve oceanic data assimilation by providing information on the month-to-month background variability.
- ItemENSEMBLE KALMAN FILTER EXPERIMENTS WITH A PRIMITIVE-EQUATION GLOBAL MODEL(2005-06-30) Miyoshi, Takemasa; Kalnay, Eugenia; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The ultimate goal is to develop a path towards an operational ensemble Kalman filtering (EnKF) system. Several approaches to EnKF for atmospheric systems have been proposed but not systematically compared. The sensitivity of EnKF to the imperfections of forecast models is unclear. This research explores two questions: 1. What are the relative advantages and disadvantages of two promising EnKF methods? 2. How large are the effects of model errors on data assimilation, and can they be reduced by model bias correction? Chapter 2 contains a theoretical review, followed by the FORTRAN development and testing of two EnKF methods: a serial ensemble square root filter (serial EnSRF, Whitaker and Hamill 2002) and a local EnKF (LEKF, Ott et al. 2002; 2004). We reproduced the results obtained by Whitaker and Hamill (2002) and Ott et al. (2004) on the Lorenz (1996) model. If we localize the LEKF error covariance, LEKF outperforms serial EnSRF. We also introduce a method to objectively estimate the optimal covariance inflation. In Chapter 3 we apply the two EnKF methods and the three-dimensional variational method (3DVAR) to the SPEEDY primitive-equation global model (Molteni 2003), a fast but relatively realistic model. Perfect model experiments show that EnKF greatly outperforms 3DVAR. The 2-day forecast "errors of the day" are very similar to the analysis errors, but they are not similar among different methods except in low ensemble dimensional regions. Overall, serial EnSRF outperforms LEKF, but their difference is substantially reduced if we localize the LEKF error covariance or increase the ensemble size. Since LEKF is much more efficient than serial EnSRF when using parallel computers and many observations, LEKF would be the only feasible choice in operations. In Chapter 4 we remove the perfect model assumption using the NCEP/NCAR reanalysis as the "nature" run. The advantage of EnKF to 3DVAR is reduced. When we apply the model bias estimation proposed by Dee and da Silva (1998), we find that the full dimensional model bias estimation fails. However, if instead we assume that the bias is low dimensional, we obtain a substantial improvement in the EnKF analysis.
- ItemGLOBAL SCALE AEROSOL PROPERTIES: IMPLICATIONS FOR SURFACE SHORTWAVE RADIATION BUDGET(2005-12-01) Liu, Hongqing; Pinker, Rachel T; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols are known to affect the shortwave radiation budget of the Earth-atmosphere system. Using truncated Empirical Orthogonal Functions (EOF) fitting, we derive monthly mean aerosol optical depth (AOD) at 0.55 μm using information from: the Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model; the MODerate resolution Imaging Spectro-radiometer (MODIS); and the AErosol RObotic NETwork (AERONET). The single scattering albedo, the asymmetry parameter and the normalized extinction coefficient over the solar spectrum are estimated from GOCART data, MODIS Ångström exponent and AERONET almucantar retrievals. The University of Maryland (UMD) Global Energy and Water Cycle Experiment (GEWEX) shortwave Surface Radiation Budget (SRB) model is updated to allow the treatment of complex aerosol properties. The modified model is implemented with the International Satellite Cloud Climatology Project (ISCCP) D1 for a one year period. From the evaluation of the improvements against ground measurements we find that the bias in retrieved AOD at 0.55 μm is reduced from 0.20 to 0.05. The overall bias in the estimated surface SW fluxes is reduced by about 7 Wm-2 for the total irradiance and 11 and 4 Wm-2 for the direct and diffuse parts, respectively. The new version of the UMD SRB model has now the capability to address the issue of aerosol direct radiative effects. Annually averaged global clear-sky direct radiative aerosol forcing is estimated to be -1.31 Wm-2 at the top of atmosphere and -2.71 Wm-2 at the surface. This indicates that the effect of aerosols on the SW energy absorption is comparable with their effect on the reflection at the TOA. At regional scales, aerosol effects can be much larger. In a case study preformed at a sub-Sahel site in Africa, the depletion of the daily surface irradiance can be as large as 120 Wm-2. Compared with other methods used to estimate aerosol direct effects, the advantage of our scheme is that it preserves closure with TOA satellite measurements. With anticipated progresses in aerosol research and satellite observations, the UMD SRB model has the potential to address aerosol radiative effects in a realistic and coherent way.
- ItemAnalyses of multiple global and regional aerosol products: investigation of aerosol effects and artifacts(2005-12-02) Jeong, Myeong Jae; Li, Zhanqing; Meteorology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Multiple aerosol products derived from satellite, ground-based, and air-borne instruments were analyzed with a focus on satellite-based aerosol products. Aerosol measurements based on different techniques were utilized to investigate the effects and the artifacts of aerosols and clouds by taking advantages of respective techniques. The global aerosol products derived from Advanced Very High Resolution Radiometer (AVHRR) and Total Ozone Mapping Spectrometer (TOMS), were analyzed for extracting synergic information. Global distributions of dominant aerosol type(s) were derived and the two products were combined to acquire an extended spatial coverage of aerosol optical thickness (AOT) at a common wavelength (0.55um). It was shown that the derived AOT agreed reasonably with AOT from the state-of-the-art Moderate Resolution Imaging Spectroradiometer (MODIS). In-depth comparison of aerosol products derived from the MODIS and the AVHRR was performed. New insights and understanding were gained for the discrepancies between the two prominent aerosol products, allowing for bridging the current and past products. Several factors causing the discrepancies were investigated. Cloud-screening techniques and aerosol models employed by the retrieval algorithms were found to be the most important factors explaining the observed discrepancies. The column aerosol humidification effect (AHE) was investigated. The column AHE was shown to be sensitive to changes in relative humidity (RH). Six methods to infer the column AHE were introduced. The knowledge of the AHE helps investigate aerosol properties and retrievals near clouds, enabling separation of aerosol real effects from artifacts associated with clouds. Finally, apparent correlations between AOT and cloud amount from ground- and satellite-based measurements were investigated. Several factors including air convergence, cloud contamination and uncertainty in cloud cover estimation, the AHE, cloud-processed/new particle genesis were studied to explain the correlations. We showed that the correlation found in ground-based measurements is mostly due to real effects while satellite-based measurements are significantly influenced by artifacts caused by clouds.
- ItemRegional Aspects of the North American Land Surface-Atmosphere Interactions and Their Contributions to the Variability and Predictability of the Regional Hydrologic Cycle(2006-04-17) Luo, Yan; Berbery, Ernesto Hugo; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this study, we investigate the pathways responsible for soil moisture-precipitation interactions and the mechanisms for soil moisture memory at regional scales through analysis of NCEP's North American Regional Reanalysis dataset, which is derived from a system using the mesoscale Eta model coupled with Noah land surface model. The consideration of the relative availability of water and energy leads to the relative strengths of land-atmosphere interaction and soil moisture memory, which are related to the predictability of the regional hydrologic cycle. The seasonal and geographical variations in estimated interaction and memory may establish the relative predictability among the North American basins. The potential for seasonal predictability of the regional hydrologic cycle is conditioned by the foreknowledge of the land surface soil state, which contributes significantly to summer precipitation: (i) The precipitation variability and predictability by strong land-atmosphere interactions are most important in the monsoon regions of Mexico; (ii) Although strong in interactions, the poor soil moisture memory in the Colorado basin and the western part of the Mississippi basin lowers the predictability; (iii) The Columbia basin and the eastern part of the Mississippi basin also stand out as low predictability basins, in that they have good soil moisture memory, but weak strength in interactions, limiting their predictabilities. Our analysis has revealed a highly physically and statistically consistent picture, providing solid support to studies of predictability based on model simulations.
- ItemAn analysis of convective transport, Lightning NO.sub.x production, and chemistry in midlatitude and subtropical thunderstorms(2006-10-18) Ott, Lesley Elaine; Dickerson, Russell R.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The impact of lightning NO.sub.x production and convective transport on tropospheric chemistry was studied in four thunderstorms observed during field projects using a 3-dimensional (3-D) cloud-scale chemical transport model (CSCTM). The dynamical evolution of each storm was simulated using a cloud-resolving model, and the output used to drive the off-line CSCTM which includes a parameterized source of lightning NO.sub.x based on observed cloud-to-ground (CG) and intracloud (IC) flash rates. Simulated mixing ratios of tracer species were compared to anvil aircraft observations to evaluate convective transport in the model. The production of NO per CG flash (P.sub.CG) was estimated based on mean observed peak current, and production per IC flash (P.sub.IC) was scaled to P.sub.CG. Different values of P.sub.IC/P.sub.CG were assumed and the results compared with in-cloud aircraft measurements to estimate the ratio most appropriate for each storm. The impact of lightning NO.sub.x on ozone and other species was examined during the storm in the CSCTM and following each storm in the convective plume using a chemistry-only version of the model which includes diffusion but without advection, and assumes clear-sky photolysis rates. New lightning parameterizations were implemented in the CSCTM. One parameterization uses flash length data, rather than flash rates, as input, and production per meter of flash channel length is estimated. A second parameterization simulates indivdual lightning flashes rather than distributing lightning NOx uniformly among a large number of gridcells to better reproduce the variability of observations. The results suggest that PIC is likely on the order of PCG and not significantly less as has been assumed in many global modeling studies. Mean values of PCG=500 moles NO and PIC=425 moles NO have been estimated from these simulations of midlatitude and subtropical continental thunderstorms. Based on the estimates of production per flash, and an assumed ratio of the number of IC to CG flashes and global flash rate, a global annual lightning NO source of 8.6 Tg N yr-1 is estimated. Based on these simulations, vertical profiles of lightning NOx mass for subtropical and midlatitude continental regimes have been computed for use in global and regional chemical transport models.
- ItemClassification of Northern Hemisphere Stratospheric Ozone and Water Vapor Profiles by Meteorological Regime: Validation, Climatology, and Trends(2007-01-19) Follette, Melanie Beth; Hudson, Robert D.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The presence of stratospheric ozone is essential for the survival of life on the Earth's surface. The decrease in the column content of ozone over mid-latitudes from 1979-1991 has previously been attributed to destruction by anthropogenic halogens, and changes in the general circulation. The research presented here shows that a poleward movement of the subtropical and polar upper troposphere fronts is responsible for 35% of this observed decrease. In Hudson et al. (2003) we showed that the Northern Hemisphere total ozone field could be separated into meteorological regimes, bounded by the subtropical and polar upper troposphere fronts. These regimes were characterized by relatively constant total ozone, tropopause height, and ozonepause height. Negative trends in total ozone within each regime were found for the time period January 1979-May 1991. These trends corresponded to a statistically significant increase in the relative area of the tropical regime, and decrease in the relative area of the polar regime, indicating a net poleward movement of the subtropical and polar fronts over this time period. This poleward frontal movement was responsible for ~35% of the negative zonal trend in total ozone over this time period and latitude range, the remaining 65% being the result of total ozone changes within the meteorological regimes. Ozone and water vapor profiles from 1997-2004, from the HALOE and SAGE II satellite-based instruments, were classified by regime. Each regime was characterized by a distinct ozonepause and hygropause height, and profile shape below ~25km, over a wide latitude range (25°-60°N). Therefore, previously reported zonal trends in the lower stratosphere and upper troposphere are a combination of both tropospheric and stratospheric air. Trends within each regime were calculated for both ozone and water vapor from 1997-2004 and from October 1984-May 1991. The relationship between the observed zonal vertical trends and the trends within each regime were consistent with the idea of meteorological regimes and reinforce the major conclusion of this work. A true understanding of zonal trends in either the column or in the lower stratosphere involves understanding both changes within each regime and changes in the relative weighting of each regime over time.
- ItemVariability of the Great Plains Low-Level Jet: Large Scale Circulation Context and Hydroclimate Impacts(2007-04-26) Weaver, Scott Jamie; Nigam, Sumant; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Variability of the Great Plains Low-Level Jet (GPLLJ) is analyzed from the perspective of larger-scale, lower-frequency influences and regional hydroclimate impacts; as opposed to the usual analysis of its frequency, diurnal variability and mesoscale structure. The circulation-centric core analysis is conducted with monthly and pentad data from the high spatio-temporal resolution, precipitation-assimilating North American Regional Reanalysis, and ERA-40 global reanalysis (as necessary) to identify the recurrent patterns of GPLLJ variability and their large-scale circulation and regional hydroclimate links. The analysis reveals that GPLLJ variability is, indeed, linked to coherent, large-scale, upper-level height patterns over the Pacific, and NAO variability in the Atlantic. A Rossby Wave Source analysis shows the Pacific height pattern to be potentially linked to tropical diabatic heating anomalies in the west-central basin and in the eastern Pacific sector. EOF analysis of GPLLJ variability shows it to be comprised of three modes that exert profound influence on Great Plains precipitation variability, and together, account for ~75% of the variance. Ocean basin centered EOF analysis on summertime SLP anomalies shows similar GPLLJ and precipitation impacts as those found in the Great Plains centric perspective, supporting the claim for remotely generated influences on Great Plains low-level jet and hydroclimate variability. Pentad analysis of the atmospheric and terrestrial water balances during the 1988 drought and 1993 flood show that, jet variability, while influential over many of the subseasonal anomalous precipitation episodes was not a necessary condition for precipitation anomalies. Great Plains evaporation exhibited a 2-week delay with respect to precipitation suggesting a minor role for precipitation recycling during these events. ENSO and NAO variability are shown to contribute significantly to the large midsummer positive precipitation anomalies during 1993. EEOF analysis of pentad 900 hPa meridional winds during MJJ show three temporally stable modes of variability, each exhibiting similar spatial characteristics to the monthly EOF spatial patterns. Lead/lag regressions show a one pentad delay in moisture flux convergence generated precipitation anomalies, perhaps, suggesting the importance of moisture transports in generating Great Plains precipitation anomalies. Climate models are shown to be challenged in depicting the jet and precipitation variability over the Great Plains.
- ItemRETRIEVAL OF TROPOSPHERIC AEROSOL PROPERTIES OVER LAND FROM INVERSION OF VISIBLE AND NEAR-INFRARED SPECTRAL REFLECTANCE: APPLICATION OVER MARYLAND(2007-04-26) Levy, Robert; Dickerson, Russell R.; Remer, Lorraine A; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols are major components of the Earth's global climate system, affecting the radiation budget and cloud processes of the atmosphere. When located near the surface, high concentrations lead to lowered visibility, increased health problems and generally reduced quality of life for the human population. Over the United States mid-Atlantic region, aerosol pollution is a problem mainly during the summer. Satellites, such as the MODerate Imaging Spectrometer (MODIS), from their vantage point above the atmosphere, provide unprecedented coverage of global and regional aerosols over land. During MODIS' eight-year operation, exhaustive data validation and analyses have shown how the algorithm should be improved. This dissertation describes the development of the 'second-generation' operational algorithm for retrieval of global tropospheric aerosol properties over dark land surfaces, from MODIS -observed spectral reflectance. New understanding about global aerosol properties, land surface reflectance characteristics, and radiative transfer properties were learned in the process. This new operational algorithm performs a simultaneous inversion of reflectance in two visible channels (0.47 and 0.66 μm) and one shortwave infrared channel (2.12 μm), thereby having increased sensitivity to coarse aerosol. Inversion of the three channels retrieves the aerosol optical depth (τ) at 0.55 μm, the percentage of non-dust (fine model) aerosol (η) and the surface reflectance. This algorithm is applied globally, and retrieves τ that is highly correlated (y = 0.02 + 1.0x, R=0.9) with ground-based sunphotometer measurements. The new algorithm estimates the global, over-land, long-term averaged τ ~ 0.21, a 25% reduction from previous MODIS estimates. This leads to reducing estimates of global, non-desert, over-land aerosol direct radiative effect (all aerosols) by 1.7 W·m-2 (0.5 W·m-2 over the entire globe), which significantly impacts assessment of aerosol direct radiative forcing (contribution from anthropogenic aerosols only). Over the U.S. mid-Atlantic region, validated retrievals of τ (an integrated column property) can help to estimate surface PM2.5 concentration, a monitored criteria air quality property. The 3-dimensional aerosol loading in the region is characterized using aircraft measurements and the Community Multi-scale Air Quality Model (CMAQ) model, leading to some convergence of observed quantities and modeled processes.
- ItemLocal ensemble transform Kalman filter with realistic observations(2007-08-03) Li, Hong; Kalnay, Eugenia; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The main goal of my research is to improve the performance of the EnKF in assimilating real observations in order to accelerate the development of EnKF systems towards operational applications. A Local Ensemble Transform Kalman Filter (LETKF, Hunt et al. 2007) is used as an efficient representative of other EnKF systems. This dissertation has addressed several issues relating to the EnKF for assimilating real data. The first issue is model errors. We assimilated observations generated from the NCEP/NCAR reanalysis fields into the SPEEDY model. The performance of the LETKF without accounting for model errors is seriously degraded compared with that in the perfect model scenario. We then investigated several methods to handle model errors including model bias and system-noise. Our results suggest that the pure bias removal methods (DdSM and LDM) are not able to beat the multiplicative or additive inflation schemes that account for the effects of total model errors. By contrast, when the bias removal methods (DdSM+ and LDM+) are supplemented by additive noise for representing the system-noise, they outperform the inflation schemes. Of these augmented methods, the LDM+, where the constant bias, diurnal bias and state-dependent errors are estimated from a large sample of 6-hour forecast errors, gives the best results. The other two issues addressed are the estimation of the inflation factor and of observation error variance. Without the accurate observation error statistics, a scheme for adaptively estimating inflation alone does not work, and vice versa. We propose to estimate simultaneously both the adaptive inflation and observation error variance. Our results for the Lorenz-96 model examples suggest that the simultaneous approach works perfectly in the perfect model scenario and in the presence of random model errors. For the case of systematic model bias, although it underestimates the observation error variance, our algorithm produces analyses that are comparable with the best tuned inflation value. SPEEDY model experiments indicate that our method is able to retrieve the true error variance for different types of instrument separately when applied to a more realistic high-dimension model. Our research in this dissertation suggests the need to develop a more advanced LETKF with both bias correction and adaptive estimation of inflation within the system.
- ItemInferring Radiative Fluxes from a New Generation of Satellites: Model Updates(2007-08-05) Wang, Hengmao; Pinker, Rachel T.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)In this study an inference scheme is developed to derive surface, Top of the Atmosphere (TOA), and atmospheric spectral shortwave (SW) radiative fluxes for implementation with observations from the Moderate Resolution Imaging Spectroradiometer (MODIS) type of sensors. The model takes into account all atmospheric constituents and addresses the characteristics of water and ice clouds and the variation of cloud particle effective radius. The near infrared spectrum is divided into three bands to better represent the spectral variation of cloud optical properties and water vapor absorption. A multi-layered structure allows for the treatment of surface elevation effects and for the representation of the vertical distribution of the radiative fluxes. Spectral fluxes such as Photosynthetically Active Radiation (PAR) and near-infrared radiation (NIR) are also estimated. The new inference scheme is implemented with MODIS one degree products as well as with the 5 km swath products. The derived fluxes are evaluated against the globally distributed Baseline Radiation Network (BSRN) measurements and compared with products from independent satellites. It was demonstrated that the MODIS products are in good agreement with ground observations and provide improved estimates of radiative fluxes than the other evaluated satellite products. In problematic areas for most satellite retrievals, such as the Tibet Plateau and Antarctica, the MODIS results have shown a substantial improvement. Availability of the high resolution swath based estimates of surface radiative fluxes allow, for the first time, to address unique space-time coupling issues.
- ItemRadiative and Cloud Microphysical Effects of Forest Fire Smoke over North America and Siberia(2007-09-28) Vant-Hull, Brian Lee Charles; Li, Zhanqing; Remer, Lorraine A; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosol affects climate both through direct radiative effects and by indirect effects on cloud development. Absorbing aerosols have additional influence on the vertical temperature profile of the atmospheric column. Radiative effects of smoke are studied for the case of a Canadian smoke plume that blanketed the U.S. mid-Atlantic seaboard. Optical properties derived from aircraft in situ measurements demonstrate that the smoke formed a layer with a base 2 km above the surface, and absorptive heating in this layer could have strengthened and maintained the subsidence inversion responsible for the layer structure. An optical model of the smoke formed from a blend of aircraft and AERONET measurements allows retrieval of the smoke aerosol by satellite, so that comparisons are possible to measurements made by any instrument in the region. For this case an optical model based purely on AERONET measurements provides the best satellite retrieval of optical depth, but a model based mainly on aircraft measurements agreed best with spectrum wide-forcing measurements, demonstrating the dangers of a simple optical model for all retrievals. A study done in the Amazonian burning season demonstrates that sun/observation geometry is useful to control bias from shadowed and illuminated portions of clouds. Sub-pixel mixing of cloud and aerosol also produces bias that is minimized for optically thick clouds. Since such biases can never be fully eliminated, the only valid study is a comparison of two data sets with equivalent geometry and so, presumably, equal bias. Canada and Siberia were chosen so that forested areas are compared at the same latitudes. Summertime Canadian aerosol is primarily smoke, while Europe contributes a great deal of sulfate to Siberia aerosol. The average cloud droplet size was significantly smaller in Siberia, as expected from the higher sulfate load with greater activity as cloud condensation nuclei (CCN). The aerosol indirect effect on cloud microphysics increases with aerosol loading in both regions, but much more so in Canada. This is attributed to a large sulfate background in Siberia, so the addition of smoke makes a smaller percentage change to the amount of cloud CCN.