UMD Theses and Dissertations
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Item THE DIURNAL AND SEASONAL RADIATIVE EFFECTS OF CIRRUS CLOUDS UTILIZING LARGE AIRBORNE AND SPACE-BORNE LIDAR DATASETS(2019) Ozog, Scott; Dickerson, Russell R; Yorks, John E; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Cirrus clouds are globally the most common cloud type, however, their radiative impact on the Earth remains a large source of uncertainty in global climate models. Cirrus are unique in that they are absorptive to terrestrial outgoing longwave radiation, while also relatively transmissive to incoming solar radiation. The interactions of this greenhouse and albedo effect determine the sign and magnitude of cirrus radiative effects. Cirrus are microphysically complex, and can exhibit a variety of different ice crystal shapes and sizes depending on the thermodynamic environment in which they form, and their dynamic formation mechanism. Our ability to reliably model cirrus radiative effects is dependent upon accurate observations and parameterizations incorporated into radiative transfer simulations. Laser lidar instruments provide valuable measurements of cirrus clouds unavailable by other radar systems, passive remote sensors, or in-situ instruments alone. In this dissertation I developed and tested an improved calibration technique for the ACATS lidar instrument, and its impact on the direct retrieval of cirrus HSRL optical properties. HSRL retrievals theoretically have reduced uncertainty over those from a standard backscatter lidar. ACATS flew on two field campaigns in 2012 and 2015 where it was unable to consistently calibrate its etalon. It has been operating from the lab in NASA GSFC collecting zenith pointing data of cirrus layers where the improved calibration has resulted in consistent and reliable separation of the particulate and Rayleigh signal components. The diurnal trend of cirrus influence on the global scale has primarily been limited to data provided by satellites in sun-synchronous orbit, which provide only a snapshot of conditions at two times a day. Utilizing data from the CATS lidar aboard the ISS I investigated cirrus at four periods throughout the day in morning, afternoon, evening, and night across all seasons. Cirrus radiative effects were found to have a large latitudinal dependence, and have a greater potential to cool than many studies suggest with their primary warming contributions skewed towards the nighttime hours. Constrained lidar retrievals reduce the assumptions made in retrieving cirrus optical properties. Utilizing the expansive airborne CPL dataset from six flight campaigns I model the radiative effects of over twenty thousand constrained cirrus observations. Mid-latitude cirrus were found to have a mean positive daytime forcing equivalent to that of the CO2 greenhouse effect. However, synoptic cirrus were found to have a greater warming effect than convective cirrus, which were more likely to have a cooling effect.Item Investigating Aerosol Effects on Clouds, Precipitation and Regional Climate in US and China by Means of Ground-based and Satellite Observations and a Global Climate Model(2011) Niu, Feng; Li, Zhanqing; Atmospheric and Oceanic Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Aerosols affect climate by scattering/absorbing radiation and by acting as cloud condensation nuclei (CCN) or ice nuclei (IN). One of the least understood but most significant aspects of climate change is the aerosol effect on cloud and precipitation. A hypothesis has recently been proposed that, in addition to reducing cloud effective radius and suppressing precipitation, aerosols may also modify the thermodynamic structure of deep convective clouds and lead to enhanced precipitation when complex thermodynamic processes are involved. Taking advantage of the long-term and extensive ground-based observations at the US Department of Energy's Atmospheric Radiation Measurement (ARM) Southern Great Plains (SGP) site, we thoroughly tested such a hypothesis and provide direct evidence of it. Moreover, the hypothesis is also supported by analysis of satellite-based observations over tropical regions from multiple sensors in the A-Train satellites constellation. Extensive analyses of the long-term ground-based and large-scale data reveal significant increases in rain rate or frequency and cloud top heights with increasing aerosol loading for mix-phase deep convective clouds, decreases rain frequency for low liquid clouds, but little impact on cloud height for liquid clouds. Rigorous tests are conducted to investigate any potential artifacts and influences of meteorological conditions. Large-scale circulation patterns and monsoon systems can be changed by scattering and absorption of solar radiation by aerosols. By means of model simulations with the National Center for Atmospheric Research Community Climate Model (NCAR/CCM3), we found that the increase of aerosol loading in China contributes to circulation changes, leading to more frequent occurrence of fog events in winter as observed from meteorological records. The increase in atmospheric aerosols over China heats the atmosphere and generates a cyclonic circulation anomaly over eastern-central China. This circulation anomaly leads to a reduction in the influx of dry and cold air over that area during winter. Weakening of the East Asian winter monsoon system may also contribute to these changes. All these changes favor the formation and maintenance of fog over this region. The MODerate resolution Imaging Spectroradiometer (MODIS) aerosol products used in the above studies are validated using ground-based measurements from the Chinese Sun Hazemeter Network (CSHNET). Overall, substantial improvement was found in the current version of aerosol products relative to the previous one. At individual sites, the improvement varies with surface and atmospheric conditions.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.