The size of a tropical cyclone (TC) is a critical structure parameter that is associated with the greatest extent of societal impacts, and it can be estimated by several different metrics depending on the variable used. In this study, a revised method of quantifying the size of a TC is introduced. This method expands upon the work of Merrill (1984) to present an alternative tropical cyclone size parameter that uses the surface pressure field and the area enclosed by it. This new approach is made possible by higher resolution and more accurate gridded meteorological data. The revised method measures the relative area of each closed isobar around a tropical cyclone to compute the area ratio of adjacent isobars. These calculations are then compared with an analytical area ratio derived from Holland (1980). The outermost closed isobar (OCI) is generally determined to be the most outward isobar whose area ratio does not significantly depart from the analytical ratio derived from Holland (1980). The algorithm is applied to a 36-year Western North Pacific (WNP) tropical cyclone data set (1979 – 2014), and the results of this are analyzed statistically and physically. This derived climatology utilized three reanalysis data sets: NASA's Modern-Era Retrospective Reanalysis data set (MERRA), ECMWF's ERA-Interim reanalysis (ERA-I), and NCEP's Climate Forecast System Reanalysis database (CFSR). On average, the algorithm was able to successfully determine an OCI for 75 – 80% of the 6-h best-track storm fixes. The primary reason for the inability of the algorithm to determine an OCI was poor representation of the TC in the gridded reanalysis, especially at and soon after formation. The statistical analysis reveals that TC size measurements using the revised metric is generally in agreement with existing climatologies. These results include a maximum mean TC size in October, a positive relationship between size and age of a TC, interannual variability of size, and an apparent maximum size near 25 degrees North. When the small-sized TCs at formation were compared to the large-sized TCs at formation, it was found that there was a highly statistically significant difference in the geographic distribution of these two groups. The size and position of the monsoon trough, in articular, generally determines where and how a TC forms in the WNP. The expected growth of a TC throughout later in its life cycle, especially during extratropical transition, was well-represented by this analysis. The study concludes with an examination of several case studies representative of the analysis presented above. / 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. / Spring Semester 2017. / March 23, 2017. / Cyclone, Pacific, Size, Tropical, Typhoon / Includes bibliographical references. / Robert E. Hart, Professor Directing Thesis; Vasubandhu Misra, Committee Member; Jeffrey M. Chagnon, Committee Member.

Interactions of Atlantic tropical cyclones (TCs) with upper tropospheric flow are identified in 37 years of ERA-Interim reanalysis data and analyzed from multiple perspectives. Upper tropospheric troughs are identified in a more comprehensive way than past methodologies, targeting features on the dynamic tropopause to reduce exclusivity of feature selection and sensitivity to the background environment. To overcome some limitations of the trough perspective, a new approach to analyzing TC-environment interactions is developed through the identification of upper tropospheric jets near TCs. Jet axes are identified in 200-hPa wind fields within 3000 km of TCs using a robust, objective algorithm, forming a novel dataset that provides a unique way of characterizing and subsetting environmental flow. The climatology of these jets is explored through various means, including an objective clustering technique, which yielded seven statistically distinct groups of jets associated with recognizable flow patterns near TCs. The dynamical coupling between TCs and nearby jets is also quantified, with acceleration of jets downstream of the TC found to be a nearly ubiquitous feature, and entrance regions of jet streaks are observed to significantly enhance the strength of the TC secondary circulation. The influence of nearby upper tropospheric troughs and jets on TC intensity is also assessed through a variety of approaches. In order to minimize systematic sampling biases when quantifying this impact, a spatially varying climatology of TC intensification rate is developed using a second-order, generalized least squares regression model, allowing TC intensity responses to external forcing to be evaluated as departures from their expected value. Both troughs and jets are found to be net negative influences on TC intensity, on average, primarily due to increasing vertical shear with proximity to the vortex. Differences between rapidly intensifying (RI) and rapidly weakening (RW) cases during TC-trough-jet interactions depend not only on shear, but on dynamic forcing imposed by baroclinic processes and eddy momentum fluxes that can counter the influence of shear. Intensifying cases are primarily associated with jets that approach the poleward side of the TC and possess jet streaks that amplify over time, increasing dynamic forcing for ascent near the TC core while maintaining enough distance to prevent shear from overwhelming those effects. This study expands the set of tools for analyzing TC interactions with upper tropospheric flow by improving trough identification and introducing a new perspective through the use of jets. Jets afford greater specificity in describing environmental flow, and allow unique methods of quantifying its impact on TCs. Close links are found between jet proximity and vertical shear, as well as jet acceleration and dynamically-forced ascent, both relationships that have been physically understood, but until now unidentified in bulk observational datasets. Some measures of jet entrance region orientation are found to correlate with the relative magnitude of shear and baroclinic forcing, exposing the subtlety in how flow geometries differ between intensifying and weakening TC cases. Prior research has tended to evaluate upper tropospheric influences on TCs individually or relied on case studies to elucidate their collective impact on a single storm. This body of work seeks to illuminate relationships between TCs and upper tropospheric flow that are robust across large samples of TCs and storm environments, utilizing novel approaches such as the jet perspective to extract previously unquantified information. / 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. / 2019 / November 1, 2019. / intensity, jet interaction, tropical cyclone, trough interaction, upper troposphere / Includes bibliographical references. / Robert Hart, Professor Directing Dissertation; Kristine Harper, University Representative; Jeffrey Chagnon, Committee Member; Philip Sura, Committee Member; Vasubandhu Misra, Committee Member.

The background state of the Arctic atmosphere under prolonged warming is analyzed by evaluating blended CFSR-CFSv2 reanalysis data from an isentropic lens, where potential temperature is used as the vertical coordinate and the principle dynamical framework is potential vorticity (PV). Since the quantity is materially conserved along adiabatic, friction-less flow, PV can be used to describe the background vorticity and static stability as well as to give insight on how diabatic processes might alter this balance. Of particular interest is how the background state of the lower troposphere has evolved in recent years to become preferential to inducing anticyclonic wind stress at the Arctic ice-ocean surface. We compare the trends in Arctic PV to the Arctic Ocean Oscillation (AOO), which entered a phase of a dominantly anticyclonic circulation of the Beaufort Gyre (BG) in the mid-to-late 1990's and has persisted since. We find that PV has a relationship with sea surface height (SSH), which in turn can be used to quantify the index of the AOO, and that the strongest relationship occurs during the ice melt season. The reduced meridional thermal gradient during the ice melt season, here designated as the summer (JJA) and autumn (SON) seasons, allows for increased meridional transport of mid-latitude air masses. This is quantified by correlating melt season PV flux and the surface heat budget as well as wintertime geopotential height and zonal wind to connect the background environment preceding the peak Arctic Oscillation (AO) signal, which can enhance or suppress the typical sea level pressure (SLP) maximum that exists over the Beaufort Sea during this time. We find that positive meridional fluxes of PV during summer correlate with increased net shortwave fluxes at the surface and increased outgoing longwave fluxes in autumn. The relationship between the influx of low-latitude PV and the surface heat budget is consistent with previous works that demonstrate the role Arctic cyclones have on sea ice transport, which has major implications on the absorption and release of heat to and from the Arctic Ocean. The transport of lower-PV air associated with latent heat release via saturation of air parcels and higher specific humidity modulates the environmental PV, stabilizing the boundary layer and reducing the cyclonic tendency over the Beaufort Sea region. This may have further impacts in reducing the baroclinicity of the Arctic and suppress cyclonic development during fall, which in return may promote conditions favorable to downwelling earlier that promote an anticyclonic ocean current to develop and persist for much of the year. / 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. / 2019 / October 31, 2019. / AOO, Arctic, potential vorticity, sea ice / Includes bibliographical references. / Eric P. Chassignet, Professor Directing Thesis; Mark A. Bourassa, Committee Member; Dmitry S. Dukhovskoy, Committee Member; Philip G. Sura, Committee Member.

While advances in tropical cyclone (TC) track forecasting have been substantial over the past few decades, and modest advances in intensity forecasting have occurred more recently, the quality of TC rainfall forecasts has not undergone the same rigorous verification. This is despite the 27% of total TC-related deaths being due to rainfall-induced flooding and that rainfall-related deaths occur more frequently than those due to any another weather-related hazard. A continual effort is needed to understand and better-forecast TC rainfall. This dissertation research seeks to contribute to this endeavor. A climatological dataset is created using 6-h Stage IV rainfall accumulations combined with Best Track 6-h locations for all TCs within 300 km of the U.S. Gulf and Atlantic coastlines during years 2004 - 2013. Stage IV data are used due to their higher spatiotemporal resolution, their extension to high latitudes, and because they have been found to be the superior option when compared to other TC rainfall data sources. The 6-h Stage IV rainfall accumulations are composited by shear magnitude and storm intensity in earth-, motion-, and shear-relative reference frames. Additionally, a full composite comprised of all storms is created. This compositing is done for TCs impacting the U.S. Gulf and Atlantic coastlines. Seven geographical regions are created within this domain to further composite the rainfall. The geographical regions are determined based on 2004 - 2013 Best Track (HURDAT2) landfall locations. Results show that some Stage IV rain rate characteristics, especially those in specific regions, are different when compared to prior findings based on satellite-derived rain rates. Results from the Stage IV-derived climatological datasets then are used together with track forecasts from the Global Forecast System (GFS) during years 2014 - 2016 to create 72-h TC rainfall forecasts. Separate forecasts are created for each 6-h TC position forecast based on shear magnitude, storm intensity, and the all-storms composites in earth-, motion-, and shear-relative reference frames. This yielded 1,290 verifiable forecasts during the 3-yr period. These statistical rainfall forecasts along with forecasts from the GFS and an R-CLIPER created from Stage IV data are verified using the Fractions Skill Score (FSS) metric. Results show that the statistical method based on shear magnitude in a shear-relative reference frame that used regional rainfall composites is the best performing of the methods. Additionally, FSSs from the statistical model are shown to be larger than those from R-CLIPER. The preliminary results from the statistical model show that this method is a viable candidate to supplement R-CLIPER as a statistical baseline TC rainfall forecast method. GFS analysis and forecast environmental parameters are composited based on the skill (FSS) of each forecast. Three categories are created: Top (FSS > 0.6), Bottom (FSS < 0.3), and Middle (0.3 < FSS < 0.6). This methodology is based on the desire to provide "guidance on guidance," i.e., suggesting to a forecaster whether the TC's environment is conducive to a skillful or not-skillful GFS rainfall forecast, and to help determine possible factors to increase the FSS of the statistical model. Results show that some aspects of the mean sea level pressure, 1000 - 500 hPa thickness anomalies, eddy flux convergence, and upper-level winds and divergence differ between skillful and non-skillful TC rainfall forecasts. / 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. / 2019 / July 9, 2019. / forecast, precipitation, rainfall, tropical cyclone / Includes bibliographical references.

After decades of focused research into tropical cyclone (TC) dynamics and evolution, operational centers are now able to predict TC track out to a lead time of five days with a high degree of accuracy. However, during this time, forecast skill for TC intensity has not kept the same pace. There are likely many reasons for this slowing improvement in TC intensity forecasts, but the one that is cited often in the community is a lack of frequent and accurate observations of winds in the inner core of TCs. Specifically, current satellite observing systems are unable to penetrate through heavy rainfall, and in situ measurements by aircraft and dropsondes are limited in space and time. The paucity of observations of surface wind speeds in the most dynamically active portion of a TC leads to (1) inaccuracies in the initial conditions used in subsequent model forecasts and (2) insufficient information for evaluating parameterizations of convection and surface fluxes. The NASA Cyclone Global Navigation Satellite System (CYGNSS) mission is designed to address these shortcomings by providing more accurate and timely observations of surface winds in all precipitation conditions. Eight micro-satellites launched in December 2016 (CYGNSS), providing an unprecedented opportunity to obtain ocean surface wind at increased revisit frequency compared to polar-orbiting satellites. Release 2.1 of the CYGNSS data contain improved wind speed quality and can be used to run data impact studies for the cases where the operational center had a weak intensity forecast. This study explores the expected benefits of this retrieved data to numerical simulations of tropical cyclones using two different data assimilation methods within the experimental framework of Observing System Simulation Experiments (OSSE) and Observing System Experiments (OSE). The goals of this study are three-fold: first, investigate the potential for CYGNSS to improve analyses and forecasts of tropical cyclones in an OSSE framework (pre-Launch); second, application of the variational analysis method (VAM) method on the CYGNSS data; third, evaluate the actual influence of assimilating CYGNSS data into NOAA’s operational hurricane model (Post-Launch). From a highly detailed and realistic hurricane nature run (NR), CYGNSS winds were simulated with error characteristics that are expected to occur in reality, and directional information is added using a two dimensional VAM for near-surface vector winds that blends simulated CYGNSS wind speeds with an a priori background vector wind field at 6-h analysis times. The OSSE system makes use of NOAA’s Hurricane Weather and Research Forecast (HWRF) model and Gridpoint Statistical Interpolation (GSI) data assimilation system in a configuration that was operational in 2012. CYGNSS winds were assimilated as scalar wind speeds and as wind vectors determined by a variational analysis method. Both forms of wind information had positive impacts on the short-term HWRF forecasts, as shown by key storm and domain metrics. Data assimilation cycle intervals of 1, 3, and 6 hours were tested, and the 3-h impacts were consistently best. The OSE quantifies the impact of assimilating both CYGNSS retrieved wind speed and derived CYGNSS wind vectors in tropical cyclone Michael (2018) on 6-hourly analyses and 5-day forecasts, using the 2019 version of the operational HWRF model. It is found that the assimilation of CYGNSS data results in improved track, intensity, and structure forecasts for both retrieved and derived CYGNSS data, implying the potential benefits of using such data for future research and operational applications. / 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. / 2019 / October 30, 2019. / Includes bibliographical references. / Guosheng Liu, Professor Co-Directing Dissertation; Ruby Krishnamurti, Professor Co-Directing Dissertation; An-I Andy Wang, University Representative; Vasubandhu Misra, Committee Member; Mark Bourassa, Committee Member.

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Meteorology, September 1972. / "August, 1972." / Includes bibliographical references (leaf 53). / by Douglas William Dockery. / M.S.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Meteorology, February 1974. / "December, 1973." Vita. / Includes bibliographical references (leaves 125-128). / The linearized Boussinesq equations with rotation, viscosity, conduction, and a mean stratification are used to model the sea breeze in two dimensions. the motion is forced by a prescribed surface temperature function. The linear model produces a sea breeze with realistic velocities and spatial dimensions. Hydrostatic solutions are found to differ very little from the nonhydrostatic solutions. The phase of the solution depends on the Coriolis parameter f; the only distinguishable feature of the solution at the inertial latitude is a slight amplitude maximum far from the coastline. Both the phase and the amplitude depend on the stability parameter N². An inversion, simulated by a discontinuity in N², reduces the intensity of the circulation. The land-sea temperature difference required by the model to create a net onshore flow in opposition to a basic current agrees well with the empirical criterion defined by Lyons (1972). The computed vertical heat fluxes, when summed along the coastlines of the principal land masses, indicate that the sea breeze effect can account for several per cent of the globally averaged vertical flux of sensible heat at a height of several hundred meters. The nonlinear advection process is studied with a finite difference model based on a series of overlapping grids. The principal effect of the nonlinear terms is a landward advection of the sea breeze circulation. / by John E. Walsh. / Ph.D.

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1963. / Includes bibliographical references (leaves 115-117). / by William Blumen. / Ph.D.

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Meteorology, 1974. / "July, 1974." Vita. / Includes bibliographical references (leaves 137-139). / by Lloyd L. Schulman. / Ph.D.

Thesis (Sc. D.)--Massachusetts Institute of Technology, Dept. of Meteorology, January 1967. / "October 1966." Vita. / Includes bibliographical references (leaves 232-235). / A laboratory study was made of the errors of miniature bead thermistors of 5, 10, and 15 mils nominal diameter when used for the measurement of atmospheric temperature. Although the study was primarily concerned with the errors of the thermistors when used in Meteorological rocket soundings between about 70 km and 30 km altitude the results are also valid for other applications of these thermistors at all altitudes down to sea level. Several distinct sources of error are present, and these have each been discussed and estimates of their magnitude made from laboratory tests. In general, all errors increase rapidly above about 50 km. Certain items which had not been fully considered in previous discussions of this problem hale been shown to be highly significant. In particular it is found that the lead wires play an important part in determining thermistor response, particularly at high altitude, and that the temperature rise of the thermistor due to solar radiation is strongly dependent on the radiation absorbed by the lead wires as well as by the bead proper. / by Donald C. Thompson. / Sc.D.