An objective index of the onset and demise of the Indian summer monsoon (ISM) is introduced. This index has the advantage of simplicity by using only one readily available variable, All-India rainfall (AIR), which has been reliably observed for more than a century. The proposed All-India rainfall onset and demise (AIROD) is shown to be insensitive to all recorded false onsets. By definition, the seasonal ISM rainfall anomalies become a function of the variations of onset and demise dates, with early onset and late demise resulting in greater season length and total seasonal rainfall. Seasonal rainfall itself is a strong predictor of the following ENSO phase and provides a more accurate depiction of the ISM than does the commonly-used June-September (JJAS) All-India monsoon rainfall (AIMR) index. This new index provides an accurate and comprehensive representation of the seasonal evolution of the ISM by capturing dramatic changes in large-scale dynamic (i.e. wind- and current-based) and thermodynamic (temperature- and moisture-based) variables, which is found to make the onset an especially important feature to monitor to understand the evolution of the ensuing monsoon season. In particular, the zonal (meridional) progression of 300hPa meridional temperature gradient (meridional ocean heat transport) reversal may be monitored about twenty days before onset to help determine the timing of its arrival. Interannual variability of ISM features and their associated large-scale phenomena are also analyzed. An early (late) onset corresponds to an increase (decrease) in anomalies of kinetic energy of 850hPa wind over the Arabian Sea and central Indian rainfall up to fifteen and ten days before onset, respectively. Conversely, an early (late) demise corresponds to a decrease (increase) in the aforementioned anomalies up to ten days after demise. Additionally, the preceding December-February ENSO phase is associated with the onset of the ISM, as an early (late) onset is preceded by La Niña (El Niño). / A Thesis submitted to the Department of Earth, Ocean and Atmospheric Sciences in partial fulfillment of the Master of Science. / Spring Semester 2016. / March 24, 2016. / Circulations, Index, India, Monsoon, Onset, Rainfall / Includes bibliographical references. / Vasubandhu Misra, Professor Directing Thesis; Robert Hart, Committee Member; Mark Bourassa, Committee Member.

ABSTRACT The bias adjustment of visually estimated ship winds in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS) is addressed through the comparison to the QuickSCAT scatterometer equivalent neutral winds. We assume that visually estimated winds and satellite scatterometer winds share similar characteristics, which are a function of stress rather than wind speed, and treat the estimated ship winds as equivalent neutral winds. Under such an assumption, we use statistical analyses to calculate the bias correction for estimated ship winds. Because observation practices vary by country and data provider, ICOADS identifies datasets by "deck" which is a number that allows for differentiating the source of the records (different deck numbers indicate different data collections provided to ICOADS, each which may contain one or more sources/countries). Three ICOADS decks 792, 926, and 992 contain the vast majority (~90%) of collocated visually estimated ship winds covering the time period November 1999-October 2009. The Root-Mean-Square difference between these visually estimated ship winds and scatterometer winds are 3.0ms-1, 2.8ms-1 and 2.9ms-1 for each major deck respectively. Following the methodology of Freilich (1997) and Freilich and Dunbar (1999), we numerically show that for lower wind speeds (0ms-1-5ms-1 in this case) that the random error in the component of the visually estimated ship winds causes an artificial appearance of an overestimation relative to satellite scatterometer winds. We also extend this statistical artifact test to test higher wind speeds (12ms-1-18ms-1 in this case) through a Monte Carlo approach. An apparent slight drop of the conditional sample means relative to reference line is shown to be a statistical artifact. These artificial biases are properly accounted in this study. A new bias correction, LMS correction, is calculated and also compared to prior corrections such as Lindau (1995). This new bias correction is available for wind speeds ranging from 0ms-1 to 17ms-1, because there are too few spatial and temporal collocated matches at wind speed greater than 17ms-1. We are limited in our ability to perform the adjustments required for intercallibration because when comparing visual winds to scatterometer winds the necessary wind speed observations are rare and small in magnitude. / A Thesis submitted to the Department of Earth Oceanic and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Spring Semester 2016. / March 29, 2016. / Beaufort winds, bias adjustment, ICOADS, Lindau's (1995) correction, LMS correction / Includes bibliographical references. / Mark A. Bourassa, Professor Directing Thesis; Shawn R. Smith, Committee Member; Guosheng Liu, Committee Member; Jeffrey Chagnon, Committee Member.

Mesoscale convective systems (MCSs) provide much of the annual rainfall over central equatorial Africa (CEA) during the March-April-May (MAM) and September-October-November (SON) rainy seasons. The characteristics and propagation of these systems are essential components to rainfall variability in this region. This has economic implications related to agriculture, livestock, and drought monitoring. Understanding MCSs will lead to better regional and global climate models that help predict the effects of the changing hydrologic cycle and heat budget as they relate to MCS activity. This study identifies and tracks MCSs for the 33-year period 1983-2015 using GridSat-B1 cloud top temperature (CTT) data. Characteristics of the MCSs (displacement, duration, speed, heading, minimum CTT, and maximum size) are determined for the MAM and SON rainy seasons. Statistical significance testing is performed to determine if there are differences between the seasons as they relate to the variables and MCS counts. Long-term trends are also examined. Differences and trends are analyzed using the National Oceanic and Atmospheric Administration’s (NOAA) National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR). This study finds statistically significant differences between the rainy seasons domain-wide, but these differences are variable- and latitude-dependent. There is high interannual variability and weak-to-absent trends for nearly all variables in both seasons. The exceptions are the average minimum CTTs, which show less interannual variability and cooling trends. Differences between the seasons are largely due to changes in low-level equivalent potential temperature and large scale circulations. The primary factor for initiation is thought to be thermally-driven gravity waves in the lee of the Great Rift Valley. Low-level vertical wind shear is believed to contribute to the maintenance of MCSs as they propagate, but do not seem to be a major factor for initiation. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the Master of Science. / Fall Semester 2016. / November 21, 2016. / Africa, Congo, convective, equatorial, mesoscale, rainfall / Includes bibliographical references. / Sharon E. Nicholson, Professor Directing Thesis; Jeffrey Chagnon, Committee Member; Vasubandhu Misra, Committee Member.

This study develops nudging methods of data assimilation that will expand the utility of the forthcoming GOES-R satellite series by including data from the Geostationary Lightning Mapper (GLM) and Advanced Baseline Imager (ABI). We develop a method to help trigger storms where lightning is observed, but no storm in simulated. Another method suppresses spurious simulated storms based on estimates of satellite-derived cloud top height (CTH) that will be improved due to the increased number of infrared channels on the ABI. In lieu of GOES-R data, Earth Networks Total Lightning Network (ENTLN) observations and GOES-13 10.7-micron observations are used in the current research. The assimilation methods are developed and evaluated using the Weather Research and Forecasting (WRF) model at convection-permitting scales (3-km horizontal grid spacing). We verify precipitation simulations against NCEP Stage IV hourly precipitation observations by computing fraction skill score (FSS; a neighborhood approach) and frequency bias for three case days. Simulated temperature, winds, humidity, and surface pressure are also verified against METAR surface observations. Our new method of assimilating lightning observations nudges low-level vertical velocity to trigger storms. This method is compared to a previous method that nudges low-level temperature (MU). Both lightning assimilation methods then are combined with the assimilation of CTH. CTH assimilation removes hydrometeors above the CTH estimate and applies an amount of cooling that is proportional to the latent heat of the removed hydrometeors to suppress spurious convection. Without applying CTH assimilation, the MU method produces better precipitation forecasts in terms of FSS than our optimal configuration of the vertical velocity nudging (WNO) method. However, if WNO is applied together with CTH assimilation, WNO produces FSS during the forecast period similar to MU applied with CTH assimilation. MU generally produces stronger storms than WNO that cause more mesoscale subsidence and indirect suppression of spurious storms. Direct suppression during CTH assimilation diminishes the impact of the indirect suppression. CTH assimilation also provides greater convective available potential energy that supports the weaker WNO storms. Regardless of whether CTH assimilation is applied, WNO generally produces superior forecasts of surface fields relative to MU and a control that employs no assimilation. This improvement is 1-6% of root mean square error during a 12-h forecast period subsequent to assimilation. Lightning assimilation (WNO or MU) combined with CTH assimilation typically provides the best precipitation forecasts. These are better than the control during the first 6-12 h of the forecast period for 1-mm and 10-mm precipitation thresholds. This combined assimilation method with the operation of GOES-R enables assimilation of clouds and storms over areas devoid of quality radar, including mountainous terrain, ocean basins, and Central and South America. / A Dissertation submitted to the Department of Earth, Atmospheric, and Ocean Sciences in partial fulfillment of the Doctor of Philosophy. / Fall Semester 2016. / October 13, 2016. / Includes bibliographical references. / Henry E. Fuelberg, Professor Directing Dissertation; James B. Elsner, University Representative; Robert E. Hart, Committee Member; Guosheng Liu, Committee Member; Vasubandhu Misra, Committee Member.

The movement of precipitation around the Earth has an integral impact on fresh water availability, vegetation, and the occurrence of natural disasters, and therefore human society at large. In the African Sahel, the rainfall is the limiting factor in agriculture and the most variable characteristic of climate both spatially and temporally. In light of recent famine concerns, research on rainfall variability in the Sahel is critical as well as timely. A review of climate and environmental literature provided a perspective and a set of methodologies upon which the research could build. This research emphasizes the role of the regional and global atmospheric circulation in governing the variability of Sahel rainfall. It examines the changing spatiotemporal characteristics of the rainfall regime in the context of the Tropical Easterly Jet (TEJ), the El Nino/Southern Oscillation (ENSO), the Indian Ocean Dipole (IOD), and the Tibetan High. Historical data was obtained from the African rain gauge network, NCEP/NCAR Reanalysis 1, and HadISST version 1.1. The results of this study show that the relationship between the TEJ and Sahel rainfall is a casual one, with a stronger TEJ enhancing upper-level divergence to promote broader, more intense vertical ascent. It also showed that diagnostic variables relevant to the general circulation, such as the maximum ascent within the rainbelt, the northward displacement of the AEJ, and the advection of precipitable water over the Sahel have a robust positive relationship with the Sahel rainfall anomaly due to the influence of the TEJ. The results also determined that a significant relationship exists in terms of the high-frequency interannual variability between negative ENSO events and the strengthening of the TEJ, which would similarly lead to anomalously wet years in the Sahel during a negative ENSO anomaly. This study posits a new framework for understanding rainfall variability in the Sahel due to its focus on the impacts of the atmospheric circulation. Furthermore, it can have important applications to understanding and forecasting droughts and floods in the Sahel, improve the modeling of Sahel climate, and provide a basis for further study of how West Africa and the TEJ fit into the global picture. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester 2016. / November 22, 2016. / Africa, Climate, Jet, Rainfall, Sahel, Tropical / Includes bibliographical references. / Sharon Nicholson, Professor Directing Thesis; Jeffrey Chagnon, Committee Member; Vasubandhu Misra, Committee Member.

Wave and wind direction effects on remote sensing measurements of ocean surface emissivity are investigated using a microwave radiometer in high wind conditions with a focus on tropical cyclones. Surface wind speed, which drives many atmospheric and oceanic phenomena, can be inferred from the ocean surface emissivity measurements through the use of a radiative transfer model and inversion algorithm. The accuracy of the ocean surface emissivity to wind speed calibration relies on accurate knowledge of the surface variables that are influencing the ocean surface emissivity. This study will identify an asymmetry in ocean surface emissivity measurements at off-nadir incidence angles that is related to the surface wind direction modifying the distribution of whitewater coverage, which is composed of active whitecaps and residual foam that persists after wave breaking, on the ocean surface in high wind conditions viewed by the radiometer. It will also be shown that asymmetries are present in ocean surface emissivity measurements from a nadir pointing instrument in hurricanes. This asymmetry can be related to swell and wind wave propagation directions with respect to the wind direction modifying the stress on the ocean surface, which presumably impacts the wave breaking and thus the whitewater coverage characteristics on the ocean surface. These results help achieve the study goals: 1) improving the understanding of how wave and wind direction modify ocean surface emissivity in high wind conditions and 2) identifying conditions, particularly in tropical cyclones, where wind direction and sea state modify the ocean surface emissivity and should be considered in order to further improve algorithms for the remote sensing of the surface wind. / A Dissertation submitted to the Department of Earth, Ocean and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2016. / August 2, 2016. / Brightness Temperature, Emissivity, High Wind, Microwave Radiometer, Remote Sensing, Tropical Cyclones / Includes bibliographical references. / Mark A. Bourassa, Professor Directing Dissertation; Dan McGee, University Representative; Robert Hart, Committee Member; Guosheng Liu, Committee Member; Allan Clarke, Committee Member; Eric Uhlhorn, Committee Member; Mark Powell, Committee Member.

Rossby waves are waves in potential vorticity that propagate along the extratropical tropopause and can be impacted by the advection of low-PV air originating from the subtropics. In this study, the subtropical precipitation influence on the extratropical Rossby wave activity during the Atlantic winter season is investigated for a ten year period. Using both TRMM and TIGGE 12-Hr forecasted precipitation data, heavy precipitation events were identified near the footprints regions of warm conveyor belts in the northern Atlantic, specifically in the Gulf of Mexico and Bermuda region. The extratropical Rossby waves were then analyzed using PV on a 320K surface. By use of wavelet transforms, the amplitude of the Rossby waves were analyzed as a function of wavelength and longitude. The interaction between a single heavy precipitation event and the extratropical Rossby waves was examined for the days preceding and the week following the event. A climatological analysis of heavy precipitation events was conducted on the winter seasons from 2006 - 2015. Case study and climatological analysis identified the following: A ridge in the Northern Atlantic undergoes amplification downstream of the heavy precipitation event in the days following the event. A southerly flow, likely associated with a warm conveyor belt, connects the region of the heavy precipitation event and the extratropical tropopause. The interaction was most prominent during the late winter season and during the heaviest of precipitation events. The teleconnection identified in this study highlights a mechanism by which cloud-scale subtropical precipitation is connected to synoptic scale extratropical dynamics in the Atlantic. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Fall Semester 2016. / November 17, 2016. / Potential Vorticity, Rossby Waves, Subtropical Precipitation, Teleconnection / Includes bibliographical references. / Jeffrey Chagnon, Professor Directing Thesis; Robert Hart, Committee Member; Philip Sura, Committee Member.

Diabatic processes including latent heating and radiation are parameterized in numerical weather prediction (NWP) models and constitute a major source of error. Isolating the extent of diabatic modification in an extratropical cyclone in observational atmospheric data is difficult. This study analyzes four idealized Advanced Weather Research and Forecasting (WRF-ARW) model sensitivity experiments in which radiation or moisture are either included or withheld. The experiments consist of the following: no radiation and no moisture (NRNM), radiation and no moisture (RNM), moisture and no radiation (NRM), and radiation and moisture (RM). Simulations are of an idealized baroclinic wave in the midlatitudes. Because baroclinic waves are associated with synoptic scale weather events, these WRF-ARW simulations are the ideal platform to better understand the interactions of diabatic and dynamic processes. Inclusion of moisture and radiation has a significant impact on baroclinic wave structure and evolution. On the synoptic scale, the RM experiment demonstrates greatest amplification/depth, earliest growth, and was the only experiment with cyclonic breaking in the troposphere and high amplitude anticyclonic breaking in the lower stratosphere. On the sub-synoptic scale, vertical dipoles in isentropic potential vorticity (IPV) and cloud-scale anomalies developed in the middle to upper troposphere as the wave matured. While radiation alone yields no anomaly, and moisture alone yields a small anomaly, the inclusion of radiation and moisture together yields a significantly larger IPV anomaly. Anomalies may be due to constructive interference or nonlinear feedbacks between radiative cooling and latent heating. Results of this study have important implications for NWP, especially in the representation of physical processes via parameterization schemes. Knowledge gained about physical processes and their feedbacks on resolved flow might help to better understand error and bias in NWP models. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester 2016. / June 28, 2016. / Includes bibliographical references. / Jeffrey M. Chagnon, Professor Directing Thesis; Ming Cai, Outside Committee Member; Guosheng Liu, Committee Member.

This study investigates the response of the extratropical jet to recurving North Atlantic tropical cyclones (TCs). A joint climatology of north Atlantic recurving TCs from 2007-2013 and Rossby waves on the extratropical jet is presented. Using a potential vorticity (PV) framework, Rossby wave breaking (RWB) and trough/ridge amplitude were quantified in order to evaluate jet characteristics downstream from recurving TCs. Additionally, the meridional PV gradient was evaluated to attempt to isolate a possible interaction mechanism. The extratropical jet is disturbed downstream of recurving TCs in the week following recurvature. RWB is made more likely in the week following recurvature, while suppression of RWB occurs at recurvature. Both ridge and trough amplitude increase downstream from recurving TCs in the week following recurvature, also supported by ridge amplification in composite PV anomalies. PV gradient modification remains unclear. / A Thesis submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Master of Science. / Summer Semester 2016. / June 30, 2016. / Atlantic, Climatology, Cyclones, Extratropical, Recurving, Tropical / Includes bibliographical references. / Jeffrey M. Chagnon, Professor Directing Thesis; Robert Hart, Committee Member; Henry Fuelberg, Committee Member.

An atmospheric model is coupled to a sea surface temperature algorithm that calculates the diurnal variability in order to understand the responses to near surface winds and boundary layer temperature, pressure, and moisture in the north Central Atlantic. 7-day case study simulations with diurnally varying sea surface temperatures and daily-constant sea surface temperatures are compared. With the inclusion of diurnal heating during the day, the local heat fluxes are increased and the surface pressure decreases. The extent of the surface-induced heating to the atmosphere is not necessarily restricted to the boundary layer depending on the atmospheric stability and the interaction with synoptic scale weather. The diurnal variations in sea surface temperature also induce positive and negative mean wind speed anomalies on the order of 0.5 m/s over the 7-day period. Hourly changes to the wind field, however, can exceed 3 m/s particularly where modifications to the regional weather occur. A comparison to geostationary data show that the sea surface temperature algorithm overestimates the warming on average, but this overestimation could be exaggerated from several factors including smoothing of the geostationary data. We examine the spatial variability and data distribution of the wind field anomalies in response to the diurnal sea surface temperature gradients at hourly and daily time scales. The changes to the wind field on the first day of diurnal warming exhibit a linear, but temporally lagged response to the direction in which flow crosses the diurnal warming gradient. This trend was also observed on day 2, but does not exist for subsequent days after. It is thought that the larger amplitude responses of interaction with the synoptic scale (secondary feedbacks) dominate the distribution for areas in which we expect wind-sea surface temperature coupling. The surface pressure gradient and Coriolis are deemed the dominant forcing processes in the model that generate the initial wind-diurnal sea surface temperature coupling response on the first day of the simulation. In order to understand the importance of interactive feedbacks of the wind and the diurnal cycle of sea surface temperatures, we compared the duration and amplitude of the diurnal warming produced in a one-way coupled simulation. The one-way coupled simulation allows the diurnal variations in sea surface temperature to influence the surface fluxes at the concurrent time step, but uses winds from a non-diurnally modified SST to calculate the diurnal warming of the sea surface temperature. As compared to two-way coupled simulations, the mean amplitude of the diurnal warming is larger and the duration is longer. This is one way to demonstrate that the integrated hourly feedbacks to the diurnal variability of sea surface temperature are important in producing an accurate duration and amplitude of diurnal heating over the day. / A Dissertation submitted to the Department of Earth, Ocean, and Atmospheric Science in partial fulfillment of the requirements for the degree of Doctor of Philosophy. / Summer Semester 2016. / June 30, 2016. / Marine Boundary Layer, Numerical Weather Prediction, Sea Surface Temperature / Includes bibliographical references. / Mark Bourassa, Professor Directing Dissertation; Kristine Harper, University Representative; Vasubandhu Misra, Committee Member; Phillip Sura, Committee Member; Allan Clarke, Committee Member.