Doctor of Philosophy

dissertationThis dissertation attempts to detail the necessary and sufficient conditions for tropical cyclogenesis; specifically those environmental, convective, and thermodynamic properties that may determine the fate of disturbances with apparent genesis potential. Unlike previous observational case studies which evaluate a few cases with limited spatial and temporal resolution in-situ and satellite data, this study examines 1 2 developing and four nondeveloping cases from recent (since 2005) tropical cyclone field campaigns using dropsonde data from multiple agency aircraft, as well as data from infrared and multiple passive microwave satellite platforms. Results, composited for all developing cases, indicate that the inner core of developing disturbances prior to genesis exhibits a midlevel moisture that is greater than the surrounding environment, high relative humidity, a warm temperature anomaly at upper levels that progressively lowers through genesis, and a cool, dry anomaly at low levels. Likewise, the vertical alignment of the low- and midlevel vorticity centers is necessary for formation. The midlevel moisture content only shows a slight "progressive moistening" during the pregenesis stage, while the total precipitable water does not apparently increase among the cases studied. Consistent with conclusions from previous observational and modeling studies, the cool, dry anomaly and increased static stability at low levels in the composite, perhaps as a result of persistent convective precipitation near the center within 1-3 days of genesis, appears to be a necessary condition for formation; this genesis pathway suggests that an initially stronger midlevel vortex precedes primarily low-level spin-up within a day of formation. Among the convective properties examined using the satellite datasets (raining area, convective intensity, area of intense convection, duration, and proximity), the results suggest that the proximity and duration of precipitation within three degrees of the center are the most important properties for formation. However, the developing cases studied do not exhibit any common distinguishing convective characteristics during the pregenesis stage; not all developing cases exhibit widespread, intense convective episodes, and although some of the cases exhibit their most "favorable" convective episodes (in terms of intensity, area, and proximity to the center) around 30-36 hours prior to formation, in a few cases that episode occurs as many as 3 days before formation

Studies of satellite support to weather modification in the western US region

The applications of meteorological satellite data to both summer and winter weather modification programs are addressed. Appraisals of the capability of satellites to assess seedability, to provide real-time operational support, and to assist in the post-experiment analysis of a seeding experiment led to the incorporation of satellite observing systems as a major component in the Bureau of Reclamations weather modification activities. Satellite observations are an integral part of the South Park Area cumulus experiment (SPACE) which aims to formulate a quantitative hypothesis for enhancing precipitation from orographically induced summertime mesoscale convective systems (orogenic mesoscale systems). Progress is reported in using satellite observations to assist in classifying the important mesoscale systems, and in defining their frequency and coverage, and potential area of effect. Satellite studies of severe storms are also covered

Importance of resolution and model configuration when downscaling extreme precipitation

Dynamical downscaling is frequently used to investigate the dynamical variables of extra-tropical cyclones, for example, precipitation, using very high-resolution models nested within coarser resolution models to understand the processes that lead to intense precipitation. It is also used in climate change studies, using long timeseries to investigate trends in precipitation, or to look at the small-scale dynamical processes for specific case studies. This study investigates some of the problems associated with dynamical downscaling and looks at the optimum configuration to obtain the distribution and intensity of a precipitation field to match observations. This study uses the Met Office Unified Model run in limited area mode with grid spacings of 12, 4 and 1.5 km, driven by boundary conditions provided by the ECMWF Operational Analysis to produce high-resolution simulations for the Summer of 2007 UK flooding events. The numerical weather prediction model is initiated at varying times before the peak precipitation is observed to test the importance of the initialisation and boundary conditions, and how long the simulation can be run for. The results are compared to raingauge data as verification and show that the model intensities are most similar to observations when the model is initialised 12 hours before the peak precipitation is observed. It was also shown that using non-gridded datasets makes verification more difficult, with the density of observations also affecting the intensities observed. It is concluded that the simulations are able to produce realistic precipitation intensities when driven by the coarser resolution data

Inter-Decadal Shifts in Intense Extratropical Cyclones in the Northern Hemisphere

Cyclones, both tropical and extratropical, have a large socioeconomic impact during any given year. Understanding the formation and evolution of these cyclones in the current climate therefore becomes imperative to minimize loss to property and life. Previous work by Kossin et al (2014) showed a significant poleward migration for the most intense tropical cyclones from 1982 to 2009. This sparks the interest in whether extratropical cyclones exhibit a similar trend within a changing climate. Data used stems from the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA-Interim Analysis for an analogous time period from 1980-2015. Tracking and identification of cyclones is performed using the 850-mb level relative vorticity field with procedures similar to that used by Hodges (1995, 1996, 1999) and then limited to 30 degrees North latitude and higher. A statistically significant shift in the most intense cyclones, defined separately for minimal central pressure and vorticity maxima, from the Pacific Oceanic basin to the Atlantic Oceanic basin is found

Modeling Aerosol-Cloud-Precipitation Interactions in Mountainous Regions: Challenges in the Representation of Indirect Microphysical Effects with Impacts at Subregional Scales

In mountainous regions, the nonlinear thermodynamics of orographic land-atmosphere interactions (LATMI) in organizing and maintaining moisture convergence patterns on the one hand, and aerosol-cloud-precipitation interactions (ACPI) in modulating the vertical structure of precipitation and space-time variability of surface precipitation on the other, are difficult to separate unambiguously because the physiochemical characteristics of aerosols themselves exhibit large sub-regional scale variability. In this chapter, ACPI in the Central Himalayas are examined in detail using aerosol observations during JAMEX09 (Joint Aerosol Monsoon Campaign 2009) to specify CCN activation properties for simulations of a premonsoon convective storm using the Weather Research and Forecasting (WRF) version 3.8.1. The focus is on contrasting AIE during episodes of remote pollution run-up from the Indo-Gangetic Plains and when only local aerosols are present in Central Nepal. This study suggests strong coupling between the vertical structure of convection in complex terrain that governs the time-scales and spatial organization of cloud development, CCN activation rates, and cold microphysics (e.g. graupel production is favored by slower activation spectra) that result in large shifts in the spatial distribution of precipitation, precipitation intensity and storm arrival time

A Statistical Analysis of the Influence of Deep Convection on Water Vapor Variability in the Tropical Upper Troposphere

The factors that control the influence of deep convective detrainment on water vapor in the tropical upper troposphere are examined using observations from multiple satellites in conjunction with a trajectory model. Deep convection is confirmed to act primarily as a moisture source to the upper troposphere, modulated by the ambient relative humidity (RH). Convective detrainment provides strong moistening at low RH and offsets drying due to subsidence across a wide range of RH. Strong day-to-day moistening and drying takes place most frequently in relatively dry transition zones, where between 0.01% and 0.1% of Tropical Rainfall Measuring Mission Precipitation Radar observations indicate active convection. Many of these strong moistening events in the tropics can be directly attributed to detrainment from recent tropical convection, while others in the subtropics appear to be related to stratosphere-troposphere exchange. The temporal and spatial limits of the convective source are estimated to be about 36-48 h and 600-1500 km, respectively, consistent with the lifetimes of detrainment cirrus clouds. Larger amounts of detrained ice are associated with enhanced upper tropospheric moistening in both absolute and relative terms. In particular, an increase in ice water content of approximately 400% corresponds to a 10-90% increase in the likelihood of moistening and a 30-50% increase in the magnitude of moistening.NASA Global Energy and Water Cycle programNASA Earth System Science researchTerraACRIMSAT NNG04GK90GGeological Science

North American extreme precipitation events and related large-scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

This paper surveys the current state of knowledge regarding large-scale meteorological patterns (LSMPs) associated with short-duration (less than 1 week) extreme precipitation events over North America. In contrast to teleconnections, which are typically defined based on the characteristic spatial variations of a meteorological field or on the remote circulation response to a known forcing, LSMPs are defined relative to the occurrence of a specific phenomenon-here, extreme precipitation-and with an emphasis on the synoptic scales that have a primary influence in individual events, have medium-range weather predictability, and are well-resolved in both weather and climate models. For the LSMP relationship with extreme precipitation, we consider the previous literature with respect to definitions and data, dynamical mechanisms, model representation, and climate change trends. There is considerable uncertainty in identifying extremes based on existing observational precipitation data and some limitations in analyzing the associated LSMPs in reanalysis data. Many different definitions of "extreme" are in use, making it difficult to directly compare different studies. Dynamically, several types of meteorological systems-extratropical cyclones, tropical cyclones, mesoscale convective systems, and mesohighs-and several mechanisms-fronts, atmospheric rivers, and orographic ascent-have been shown to be important aspects of extreme precipitation LSMPs. The extreme precipitation is often realized through mesoscale processes organized, enhanced, or triggered by the LSMP. Understanding of model representation, trends, and projections for LSMPs is at an early stage, although some promising analysis techniques have been identified and the LSMP perspective is useful for evaluating the model dynamics associated with extremes.11Ysciescopu

North American extreme precipitation events and related large-scale meteorological patterns: a review of statistical methods, dynamics, modeling, and trends

This paper surveys the current state of knowledge regarding large-scale meteorological patterns (LSMPs) associated with short-duration (less than 1 week) extreme precipitation events over North America. In contrast to teleconnections, which are typically defined based on the characteristic spatial variations of a meteorological field or on the remote circulation response to a known forcing, LSMPs are defined relative to the occurrence of a specific phenomenon-here, extreme precipitation-and with an emphasis on the synoptic scales that have a primary influence in individual events, have medium-range weather predictability, and are well-resolved in both weather and climate models. For the LSMP relationship with extreme precipitation, we consider the previous literature with respect to definitions and data, dynamical mechanisms, model representation, and climate change trends. There is considerable uncertainty in identifying extremes based on existing observational precipitation data and some limitations in analyzing the associated LSMPs in reanalysis data. Many different definitions of "extreme" are in use, making it difficult to directly compare different studies. Dynamically, several types of meteorological systems-extratropical cyclones, tropical cyclones, mesoscale convective systems, and mesohighs-and several mechanisms-fronts, atmospheric rivers, and orographic ascent-have been shown to be important aspects of extreme precipitation LSMPs. The extreme precipitation is often realized through mesoscale processes organized, enhanced, or triggered by the LSMP. Understanding of model representation, trends, and projections for LSMPs is at an early stage, although some promising analysis techniques have been identified and the LSMP perspective is useful for evaluating the model dynamics associated with extremes.11Ysciescopu