Atmospheric & Oceanic Science Theses and Dissertations

Permanent URI for this collectionhttp://hdl.handle.net/1903/2747

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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.
Interaction Between the Aerosol Direct Effect in the Lower Troposphere and the Planetary Boundary Layer
(2015) Sawyer, Virginia Ruth; Li, Zhanqing; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The planetary boundary layer (PBL) limits the vertical mixing of aerosol emitted to the lower troposphere. The PBL depth and its change over time affect weather, surface air quality and radiative forcing. While model simulations have suggested that the column optical properties of aerosol are associated with changes in the PBL depth in turn, there are few long-term measurements of PBL depth with which to validate the theory. Of the existing methods to detect the PBL depth from atmospheric profiles, many require supporting information from multiple instruments or cannot adapt to changing atmospheric conditions. This study combines two common methods for PBL depth detection (wavelet covariance and iterative curve-fitting) in order to produce more reliable PBL depths for micropulse lidar backscatter (MPL). The combined algorithm is also flexible enough to use with radiosonde and atmospheric emitted radiance interferometer (AERI) data. PBL depth retrievals from these three instruments collected at the Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site are compared to one another to show the robustness of the algorithm. The comparisons were made for different times of day, four seasons, and variable sky conditions. While considerable uncertainties exist in PBL detection using all three types of measurements, the agreement among the PBL products is promising, and the different measurements have complementary advantages. The best agreement in the seasonal cycle occurs in winter, and the best agreement in the diurnal cycle when the boundary-layer regime is mature and changes slowly. PBL depths from instruments with higher temporal resolution (MPL and AERI) are of comparable accuracy to radiosonde-derived PBL depths. The new PBL depth measurements for SGP are compared to MPL-derived PBL depths from a multiyear lidar deployment at the Hefei Radiation Observatory (HeRO), and the column aerosol optical depth (AOD) for each site is considered. A one-month period at SGP is also modeled to relate AOD to PBL depth. These comparisons show a weak inverse relationship between AOD and daytime PBL depth. This is consistent with predictions that aerosol suppresses surface convection and causes shallower PBLs.
Satellite Remote Sensing of Smoke Particle Optical Properties, Their Evolution and Controlling Factors
(2021) Junghenn, Katherine Teresa; Li, Zhanqing; Kahn, Ralph A.; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The optical and chemical properties of biomass burning (BB) smoke particles greatly affect the impact wildfires have on climate and air quality. Previous work has demonstrated some links between smoke properties and factors such as fuel type and meteorology. However, the factors controlling BB particle speciation at emission are not adequately understood, nor are those driving particle aging during atmospheric transport. As such, modeling wildfire smoke impacts on climate and air quality remains challenging. The potential to provide robust, statistical characterizations of BB particles based on ecosystem and ambient conditions with remote sensing data is investigated here. Space-based Multi-angle Imaging Spectrometer (MISR) observations, combined with the MISR Research Aerosol (RA) algorithm and the MISR Interactive Explorer (MINX) tool, are used to retrieve smoke plume aerosol optical depth (AOD), and to provide constraints on plume vertical extent, smoke age, and particle size, shape, light-absorption, and absorption spectral dependence. These capabilities are evaluated using near-coincident in situ data from two aircraft field campaigns. Results indicate that the satellite retrievals successfully map particle-type distributions, and that the observed trends in retrieved particle size and light-absorption can be reliably attributed to aging processes such as gravitational settling, oxidation, secondary particle formation, and condensational growth. The remote-sensing methods are then applied to numerous wildfire plumes in Canada and Alaska that are not constrained by field observations. For these plumes, satellite measurements of fire radiative power and land cover characteristics are also collected, as well as short-term meteorological data and drought index. We find statistically significant differences in the retrieved smoke properties based on land cover type, with fires in forests producing the tallest and thickest plumes containing the largest, brightest particles, and fires in savannas and grasslands exhibiting the opposite. Additionally, the inferred dominant aging mechanisms and the timescales over which they occur vary between land types. This work demonstrates the potential of remote sensing to constrain BB particle properties and the mechanisms governing their evolution, over entire ecosystems. It also begins to realize this potential, as a means of improving regional and global climate and air quality modeling in a rapidly changing world.