Atmospheric & Oceanic Science
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Formerly known as the Department of Meteorology.
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Item A STUDY OF REMOTELY SENSED AEROSOL PROPERTIES FROM GROUND-BASED SUN AND SKY SCANNING RADIOMETERS(2012) Giles, David Matthew; Dickerson, Russell R; Thompson, Anne M.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosol particles impact human health by degrading air quality and affect climate by heating or cooling the atmosphere. The Indo-Gangetic Plain (IGP) of Northern India, one of the most populous regions in the world, produces and is impacted by a variety of aerosols including pollution, smoke, dust, and mixtures of them. The NASA Aerosol Robotic Network (AERONET) mesoscale distribution of Sun and sky-pointing instruments in India was established to measure aerosol characteristics at sites across the IGP and around Kanpur, India, a large urban and industrial center in the IGP, during the 2008 pre-monsoon (April-June). This study focused on detecting spatial and temporal variability of aerosols, validating satellite retrievals, and classifying the dominant aerosol mixing states and origins. The Kanpur region typically experiences high aerosol loading due to pollution and smoke during the winter and high aerosol loading due to the addition of dust to the pollution and smoke mixture during the pre-monsoon. Aerosol emissions in Kanpur likely contribute up to 20% of the aerosol loading during the pre-monsoon over the IGP. Aerosol absorption also increases significantly downwind of Kanpur indicating the possibility of the black carbon emissions from aerosol sources such as coal-fired power plants and brick kilns. Aerosol retrievals from satellite show a high bias when compared to the mesoscale distributed instruments around Kanpur during the pre-monsoon with few high quality retrievals due to imperfect aerosol type and land surface characteristic assumptions. Aerosol type classification using the aerosol absorption, size, and shape properties can identify dominant aerosol mixing states of absorbing dust and black carbon particles. Using 19 long-term AERONET sites near various aerosol source regions (Dust, Mixed, Urban/Industrial, and Biomass Burning), aerosol absorption property statistics are expanded upon and show significant differences when compared to previous work. The sensitivity of absorption properties is evaluated and quantified with respect to aerosol retrieval uncertainty. Using clustering analysis, aerosol absorption and size relationships provide a simple method to classify aerosol mixing states and origins and potentially improve aerosol retrievals from ground-based and satellite-based instrumentation.Item SURFACE AND AEROSOL EFFECTS ON THE SOUTH ASIAN MONSOON HYDROCLIMATE(2010) Bollasina, Massimo A.; Nigam, Sumant; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This work targets important couplings in the South Asian monsoon system at interannual or longer time-scales and associated processes and mechanisms: aerosol-hydroclimate, atmosphere-ocean, and land-atmosphere. Anomalous springtime absorbing aerosols loading over the Indo-Gangetic Plain (IGP) leads to large-scale variations of the monsoon: cloudiness reduction associated with increased aerosols is suggested to play an important role in triggering surface heating over India, which strengthens the monsoon. Indeed, a closer analysis with high resolution data depicts a complex interplay between aerosols, dynamics and precipitation. Interestingly, observations do not provide any evidence for the Elevated Heat Pump hypothesis, a mechanism proposed for the aerosol-monsoon link. Current coupled climate models, which have been extensively used to study aerosol-monsoon interactions, are shown to have large, systematic, and coherent biases in precipitation, evaporation, sea-surface temperature (SST) over the Indian Ocean during the monsoon. Models are also found to deficiently portray local and non-local air-sea interactions. For example, they tend to emphasize local oceanic forcing on precipitation or to poorly simulate the relationship between SST and evaporation. The Indian monsoon rainfall-SST link is also spuriously misrepresented, suggesting caution when interpreting model-based findings. Both regional and remote forcings modulate the annual cycle of the heat-low over the desert areas (including the Thar Desert) between Pakistan and northwestern India, source of most of the dust loading over India. Land-surface heating has a limited role in the development of the low. Regional orography and monsoon summertime deep-convection over the Bay of Bengal, with its upstream descent to the west and related northerlies, contribute to the strengthening of the low, indicating a monsoon modulation on desert processes, including dust emission. The Thar Desert is expanding westward and the potential impact of land-cover change (without consideration of the additional aerosol loading) on summer monsoon hydroclimate and circulation is found to be significant. Locally, the atmospheric water cycle weakens, air temperature cools and subsidence prevails. An anomalous northwesterly flow over the IGP weakens the monsoon circulation over eastern India, causing precipitation to decrease. Orographic enhanced precipitation occurs over the Eastern Himalayas and southern China.Item High-Resolution Clouds and Radiative Fluxes from Satellites: Transferability of Methods and Application to Monsoon Regions(2009) Wonsick, Margaret; Pinker, Rachel T.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)High-resolution information on clouds and radiative fluxes is produced for the Indian and African monsoon regions of interest to the GEWEX Project as articulated under the Coordinated Energy and Water Cycle Observations Project (CEOP). Such data are needed to provide forcing parameters for regional climate models, to evaluate them, and to facilitate their transferability to various climatic regions. Emphasis is placed on capturing the small-scale spatial variability and the diurnal cycle of cloud systems and on improving flux retrievals under the challenging conditions of high elevation and abundant aerosol loads that are characteristic of the various monsoon regions. Once developed, the data are applied to several issues investigated under CEOP and related to hydro-climate and aerosols. Documentation of the diurnal cycle of clouds and convection throughout the progression of the Indian monsoon has been limited due to lack of hourly satellite data over the region prior to 1998. This study adds to the base of knowledge by contrasting the diurnal cycle of clouds and convection in six diverse sectors of the Indian monsoon region and compositing the data for the pre-, peak-, and post-monsoon seasons to better understand the evolution of the monsoon. Comparison of satellite-observed clouds to model-predicted values points out model deficiencies in simulating clouds during the peak-monsoon season and at locations with elevated terrain. The high-resolution cloud information and cloud optical depth data derived with the radiative flux inference scheme are used to re-evaluate the "Elevated Heat Pump" (EHP) hypothesis. The hypothesis predicts early initiation and enhancement of monsoon precipitation in northern India and the Bay of Bengal due to anomalous warming caused by high aerosol loads in the Indo-Gangetic Basin. Newly derived information on convection is used to study the contrast in precipitation patterns during years with high and low aerosol loads. Evidence of the EHP effect is not found. This may be attributed to aerosol indirect effects or air-sea interactions which are not accounted for in the model simulations that were used to develop the hypothesis. Experiments are conducted with different aerosol treatments in the radiative flux inference scheme over Africa with the goal of determining whether using observed aerosol inputs can improve on fluxes retrieved with climatological aerosol values. This question is pertinent to the African Monsoon Multidisciplinary Analysis (AMMA) program, a subprogram of CEOP, which seeks to improve prediction of the West African Monsoon. The radiation component of the surface forcing database used for all AMMA land surface models overestimates clear-sky radiation under high aerosol loads due to poor representation of aerosols. The experiments show that flux retrievals improve when observed aerosol values are used, but biases are reduced even more significantly when aerosol absorbing properties are incorporated into the inference scheme as well. The improved scheme is then used to study the spatial and seasonal variations in downwelling surface shortwave flux and surface albedo over the African continent.Item Radiative 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.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.