Browsing by Author "Miller, Steven D., committee member"
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Item Open Access Environmental controls and aerosol impacts on tropical sea breeze convection(Colorado State University. Libraries, 2020) Park, Jungmin, author; van den Heever, Susan C., advisor; Cooley, Daniel S., committee member; Kreidenweis, Sonia M., committee member; Miller, Steven D., committee member; Rasmussen, Kristen L., committee memberNearly half of the world's human population resides within 150 km of the ocean, and this coastal population is expected to continue increasing over the next several decades. The accurate prediction of convection and its impacts on precipitation and air quality in coastal zones, both of which impact all life's health and safety in coastal regions, is becoming increasingly critical. Thermally driven sea breeze circulations are ubiquitous and serve to initiate and support the development of convection. Despite their importance, forecasting sea breeze convection remains very challenging due to the coexistence, covariance, and interactions of the thermodynamic, microphysical, aerosol, and surface properties of the littoral zone. Therefore, the overarching goal of this dissertation research is to enhance our understanding of the sensitivity of sea breeze circulation and associated convection to various environmental parameters and aerosol loading. More specifically, the objectives are the following: (1) to assess the relative importance of ten different environmental parameters previously identified as playing critical roles in tropical sea breeze convection; and (2) to examine how enhanced aerosol loading affects sea breeze convection through both microphysical and aerosol-radiation interactions, and how the environment modulates these effects. In the first study, the relative roles of five thermodynamic, one wind, and four land/ocean-surface properties in determining the structure and intensity of sea breeze convection are evaluated using ensemble cloud-resolving simulations combined with statistical emulation. The results demonstrate that the initial zonal wind speed and soil saturation fraction are the primary controls on the inland sea breeze propagation. Two distinct regimes of sea breeze-initiated convection, a shallow and a deep convective mode, are also identified. The convective intensity of the shallow mode is negatively correlated by the inversion strength, whereas the boundary layer potential temperature is the dominant control of the deep mode. The processes associated with these predominant controls are analyzed, and the results of this study underscore possible avenues for future improvements in numerical weather prediction of sea breeze convection. The sea breeze circulation and associated convection play an important role in the transport and processing of aerosol particles. However, the extent and magnitude of both direct and indirect aerosol effects on sea breeze convection are not well known. In the second part of this dissertation, the impacts of enhanced aerosol concentrations on sea breeze convection are examined. The results demonstrate that aerosol-radiation-land surface interactions produce less favorable environments for sea breeze convection through direct aerosol forcing. When aerosol-radiation interactions are eliminated, enhanced aerosol loading leads to stronger over-land updrafts in the warm-phase region of the clouds through increased condensational growth and latent heating. This process occurs irrespective of the sea breeze environment. While condensational invigoration of convective updrafts is therefore robust in the absence of aerosol direct effects, the cold-phase convective responses are found to be environmentally modulated, and updrafts may be stronger, weaker, or unchanged in the presence of enhanced aerosol loading. Surface precipitation responses to aerosol loading also appear to be modulated by aerosol-radiation interactions and the environment. In the absence of the aerosol direct effect, the impacts of enhanced aerosol loading may produce increased, decreased, or unchanged accumulated surface precipitation, depending on the environment in which the convection develops. However, when aerosols are allowed to interact with the radiation, a consistent reduction in surface precipitation with increasing aerosol loading is observed, although the environment once again modulated the magnitude of this aerosol-induced reduction.Item Open Access Fast 3D radiative transfer of shortwave reflectance for synergistic remote sensing applications(Colorado State University. Libraries, 2023) Kelly, Joe, author; Chiu, Christine, advisor; Miller, Steven D., committee member; Venkatachalam, Chandra, committee memberMarine stratocumulus clouds are a critical component of Earth's radiation budget and remain a key source of uncertainty in climate projections. Better representing these clouds and their interactions with radiation, precipitation and aerosols in models necessitates observations of three-dimensional (3D) cloud fields. While passive satellite observations provide critical information on cloud properties globally, their retrievals lack information on vertical structure. Most retrieval methods also assume one-dimensional (1D), plane parallel clouds, leading to significant retrieval errors for both stratocumulus and cumulus regimes. In contrast, observations from active sensors allow for the probing of cloud vertical structure. However, active sensor data are limited in coverage. Combining active and passive satellite observations provides an excellent opportunity to reconstruct the 3D cloud fields. To provide 3D cloud property fields that do not suffer from errors introduced by the plane-parallel assumption, 3D radiative effects must be incorporated during the retrieval process. In this thesis, the impact of 3D radiative effects on 1D retrievals of cloud optical and microphysical properties is quantified, focusing on contrasting illuminated and shadowed pixels. When evaluating 1D retrieval on a synthetic cloud field, it is found that shadowed pixels had a larger magnitude of mean optical depth bias (–12) than illuminated pixels (3) at small solar zenith angles, while shadowed pixels had a lower magnitude of mean optical depth bias (–5) than illuminated pixels (12) at large solar zenith angles. For effective radius, the mean biases in shadowed and illuminated pixels are respectively 3.9 μm and –4.9 μm at large solar zenith angles. At small solar zenith angles, shadowed pixels had a smaller mean effective radius bias (0.8 μm) than illuminated pixels (–3.8 μm). By incorporating 3D radiative effects into the retrieval of the synthetic cloud field, the range of retrieved optical depth errors is greatly reduced from [–50, 100] to [–30, 40]. In addition to the synthetic dataset, we highlight a real-world case from the Variability of the American Monsoon System (VAMOS) Ocean-Cloud-Atmosphere-Land Study Regional Experiment (VOCALS-REx), serving as a potential dataset for evaluating 1D and 3D retrievals. Cloud microphysical properties were derived from in-situ cloud probe measurements collected from a profiling flight and three longer and horizontal transects that were within 1 hour of the A-Train overpass. In this particular cloud profile, the cloud droplet number concentrations ranged between 100–150 cm–3 and were relatively constant with height; cloud liquid water content increased approximately linearly with height, following a sub-adiabatic growth rate of 1.4 g m–3 km–1. We have found that properties from three horizontal transects have similar cloud statistics and structures. Applying the retrieval method to real-world data proved challenging due to the limited vertical information available from satellites about clouds near the surface and due to the inherent uncertainties of comparing cloud fields at different times. Lastly, to incorporate 3D radiative effects in the retrieval process, we have developed 3D shortwave radiative transfer emulators for stratocumulus and cumulus cloud fields using a convolutional neural network. The emulators were trained on cloud fields generated from the Large Eddy Simulation (LES) and specific sets of solar and viewing geometry and aerosol conditions. The performance of emulators was evaluated against a testing dataset in which the truth reflectance was computed by a 3D radiative transfer model with a subset of LES output as the input cloud fields. Overall, the predicted reflectance at the top of the atmosphere in the visible and near-infrared spectral regions has mean relative errors smaller than 2%, and the 15th and 85th percentile errors are generally less than ±10% for all setups. This type of emulator can be integrated into remote sensing applications and allow 3D radiative effects to be integrated effectively into advanced retrieval methods.Item Open Access Influence of terrain on the characteristics and life cycle of convection observed in subtropical South America(Colorado State University. Libraries, 2023) Rocque, Marquette N., author; Rasmussen, Kristen L., advisor; Schumacher, Russ S., committee member; Miller, Steven D., committee member; Chandrasekar, V., committee memberSubtropical South America (SSA) is a hotspot for deep, intense convection that often grows upscale into large mesoscale convective systems (MCSs) overnight. The local terrain, including the Andes and a secondary feature known as the Sierras de Córdoba (SDC) are hypothesized to play a major role in the initiation, development, and evolution of convection in the region. Some satellite studies have investigated this role, but storm-scale and life cycle characteristics of these MCSs have not been studied in depth due to the lack of high-resolution, ground-based instruments in the region. However, in 2018-2019, several research-quality platforms were deployed to Córdoba, Argentina as part of the Remote sensing of Electrification, Lightning, And Mesoscale/microscale Processes with Adaptive Ground Observations (RELAMPAGO) and the Cloud, Aerosol, and Complex Terrain Interactions (CACTI) field campaigns. The data collected during these campaigns is used in the studies presented in this dissertation to investigate how the Andes and SDC contribute to convection initiation and rapid upscale growth under varying synoptic conditions. Determining why convection is so unique in SSA may provide insight into characteristics of other storms around the world. The first two studies in the dissertation evaluate how the Andes and SDC modulate the large-scale environment and storm-scale characteristics under strong vs. weak synoptic forcing. High resolution, convection-permitting simulations in which the terrain is modified are designed to investigate synoptic (Chapter 2) and mesoscale (Chapter 3) processes related to the development of two severe mesoscale convective systems (MCSs) observed during RELAMPAGO-CACTI. Results from the simulations are also compared with radar observations to determine how well the model performs. Under strong synoptic forcing, when the Andes are reduced by 50%, the lee cyclone that develops is weaker, the South American Low-level Jet (SALLJ) is weaker and shallower, and the MCS that develops is weaker and moves quickly off the terrain. When the SDC are removed, there are no substantial changes to the large-scale environment. However, there is no back-building signature of deep convection, likely because cold pools are no longer blocked by the SDC. Under weak synoptic forcing, there are no significant changes to the large-scale environment, even when the Andes are halved. Similar to the strongly forced case though, when the SDC are removed, there are fewer deep convective cores toward the west. In both cases, the model tends to overestimate convection compared to observations. These studies show that the terrain plays varying roles in the evolution of convection in SSA. The third and fourth studies use ground-based lightning observations from RELAMPAGO-CACTI to better understand the electrical and microphysical characteristics of these intense storms. Three-dimensional storm structures are identified in the radar data and lightning flashes are matched with these storm modes to evaluate how lightning varies throughout the convective life cycle (Chapter 4). Results show that lightning flashes associated with deep convective cores are most common along the higher terrain of the SDC and occur in the afternoon hours. They also tend to be the smallest in size. Flashes associated with wide convective cores occur more frequently along the eastern edge of the SDC and are observed around midnight local time. Stratiform flashes are found most frequently in the early morning hours about 50-100 km east of the SDC, and they tend to be the largest in area and occur lower within the cloud. These distributions highlight the life cycle of systems, which initiate along the SDC and grow upscale as they move towards the plains overnight. Flash rates are then related to microphysical properties such as graupel mass and ice water path (Chapter 5). The first lightning flash rate parameterizations are developed for storms in SSA. We find these storms have considerably more graupel associated with them compared to storms in the U.S. These new parameterizations are tested on the simulated strongly forced MCS, and results agree well with observed flash rates. If parameterizations based on U.S. storms had been used instead, the flash rates would have been overestimated by up to a factor of 8. This work, in conjunction with other studies in this dissertation, highlights just how different storms in SSA are compared to the U.S.Item Open Access Near-cloud aerosol retrieval and three-dimensional radiative transfer using machine learning(Colorado State University. Libraries, 2021) Yang, Chen-Kuang, author; Chiu, Christine, advisor; Kummerow, Christian D., committee member; Miller, Steven D., committee member; Ebert-Uphoff, Imme, committee memberAccording to the most recent report of the Intergovernmental Panel on Climate Change, aerosols remain one of the largest sources of uncertainty in estimating and interpreting the Earth's changing energy budget. To reduce the uncertainty, an advanced understanding of aerosol optical properties and aerosol-cloud interaction is needed, which has largely relied on (but is not limited to) passive satellite observations. Current aerosol retrieval methods require a separation between cloud-free and cloudy regions, but this separation is often ambiguous. Three-dimensional (3D) cloud radiative effects can extend beyond the physical boundaries and enhance the reflectance in adjacent cloud-free regions as far as 10 km from clouds. Aerosol optical properties cannot be accurately retrieved without considering the 3D cloud radiative effect in this so-called "twilight" or "transition" zone, which denotes the area between cloud-free and cloudy regions. Indeed, most contemporary retrievals discard these regions, making it impossible to estimate the aerosol radiative effects in this zone. To help break the deadlock, 3D cloud radiative effects must be incorporated into the retrieval methods, and two approaches are proposed in this work, both leveraging machine learning techniques. The first approach accounts for 3D cloud radiative effects by building a 3D shortwave radiative transfer emulator as the forward model for the retrieval methods. Our emulator was trained by cumulus scenes generated from large eddy simulations and radiation fields calculated from 3D radiative transfer, to predict downward and upward flux profiles at a 500 m horizontal resolution and 30 m vertical resolution. From a case drawn from the testing dataset, our emulator captures the spatial pattern of the surface downwelling flux (e.g., shadows and illuminations), and the associated PDF has a remarkable similarity to the synthetic truth. In addition, compared to 1D calculation, our 3D emulator improves the root-mean-square-error by a factor of 6. For the flux and heating rate profiles, our emulator is much superior to the 1D calculation for the cloudy column. The application of this 3D radiative transfer emulator to numerical weather modeling or large-eddy simulations type of model is beyond the scope of the current work to develop an aerosol retrieval algorithm, but the possibility exists to do so. While the promising results from the emulator make it possible to conduct 3D RT retrieval methods, this approach still faces ambiguity in separating cloud-free and cloudy pixels. Here, we present a new retrieval algorithm for aerosol optical depth (AOD) in the vicinity of clouds which contains two unique features. First, it does not require pre-separation of aerosols and clouds. Second, it incorporates 3D radiative effects, allowing us to provide accurate aerosol retrievals near clouds. The AOD retrieval uncertainty of this method in the cloud-free region is (0.0004 ± 4% AOD), which is much better than the (0.03 ± 5% AOD) retrieval uncertainty in NASA Aerosol Robotic Network (AERONET). This method shows skill of predicting AOD over the near-cloud regions, and its validity was confirmed by using one of the explainable artificial intelligence methods to demonstrate that the model's decisions are supported by radiative transfer theory. Finally, a case study using MODIS observations shed light on how this new method can be applied to real world observation, possibly leading to new scientific insight on aerosol structure and aerosol-cloud interaction.