Atmospheric & Oceanic Science
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Formerly known as the Department of Meteorology.
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Item Variability 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.Item RETRIEVAL 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.Item Classification 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.Item An 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.Item Regional 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.Item Analyses 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.Item GLOBAL 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.Item BRED 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.Item VARIATIONAL 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.Item The 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.