The Pennsylvania State University The Graduate School A MULTI-FACETED VIEW OF WINTER PRECIPITATION: SOCIETAL IMPACTS, POLARIMETRIC RADAR DETECTION, AND MICROPHYSICAL MODELING OF TRANSITIONAL WINTER PRECIPITATION A Dissertation in Meteorology and Atmospheric Science by Dana Marie Tobin 2020 Dana Marie Tobin Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2020 ii The dissertation of Dana Marie Tobin was reviewed and approved by the following: Matthew R. Kumjian Associate Professor of Meteorology Dissertation Advisor Chair of Committee Eugene E. Clothiaux Professor of Meteorology Jerry Y. Harrington Professor of Meteorology Vikash V. Gayah Associate Professor of Civil and Environmental Engineering David J. Stensrud Professor of Meteorology Head of the Department iii ABSTRACT There remain several unanswered questions related to transitional winter precipitation, ranging from the impacts that it has on society to what microphysical processes are involved with its formation. With an improved understanding of the formation and impacts of transitional winter precipitation types, it is possible to reduce or minimize their adverse societal impacts in the future by improving their detection and forecasting. Precipitation is known to have an adverse effect on motor vehicle transportation, but no study has quantified the effects of ice pellets or freezing precipitation. An investigation of the number of vehicle-related fatalities during each precipitation type reveals a bias in the number of transitional-winter-precipitation type categories such that the fatality data cannot be used as-is to quantify the impacts of precipitation on vehicle fatalities with certainty. Matching traffic crash data to nearby precipitation-type reports provides an avenue to identify periods of precipitation during which a crash occurred. This analysis allows crash risk to be estimated during each precipitation type, resulting in a hierarchy of risk based on precipitation, with transitional winter precipitation having a higher overall crash risk than rain or snow. A polarimetric signature indicative of hydrometeor refreezing was recently documented, yet the underlying microphysical explanation remains unclear. The signature is characterized by a prominent and unexpected enhancement in differential reflectivity (ZDR) within a layer of decreasing radar reflectivity (ZH) towards the ground. These observations were made during prolonged periods of ice pellets where hydrometeors were fully melted prior to refreezing. The most probable explanation from the literature currently for the observed ZDR enhancement is preferential refreezing of small drops prior to the larger liquid drops. In contrast to previously published observations, no ZDR enhancement is found during an ice pellet and rain mixture event using high-resolution polarimetric radar data. Although refreezing did occur from small-to-large iv particles, these particles were not completely melted prior to refreezing, and the smallest fully melted hydrometeors did not refreeze. The presence of liquid drops and/or the freezing of partially melted (not fully melted) hydrometeors are likely the culprits of the lack of a ZDR enhancement, yet the observations alone are not sufficient to elucidate why that is the case. A steady-state, one-dimensional column microphysical model is developed to model snowflake melting and the refreezing of fully or partially melted hydrometeors, and coupled with a polarimetric radar forward operator. Simple tests of sequentially freezing size bins of an assumed drop size distribution indicate that preferential refreezing of drops still produces a ZDR enhancement even if the smallest drops remain liquid. The full-physics model simulation, however, produced no ZDR enhancement. Reducing the axis ratio of the liquid cores of freezing drops, as a crude representation of asymmetric freezing of an ice shell, successfully produced a realistic polarimetric refreezing signature. An even greater ZDR enhancement, more similar to observations, is produced by assuming that particle wobbling does not increase with freezing. It is thus suggested that a combination of asymmetric freezing and minimal increases in particle wobbling are responsible for the observed signature when fully melted particles refreeze. There is no such signature for the refreezing of partially melted particles, suggesting that the presence or absence of a ZDR enhancement in the refreezing layer may be used to distinguish between the refreezing of fully melted and partially melted hydrometeors. v TABLE OF CONTENTS LIST OF FIGURES ................................................................................................................. vii LIST OF TABLES ................................................................................................................... xiv ACKNOWLEDGEMENTS ..................................................................................................... xvi Chapter 1 Motivation .............................................................................................................. 1 Chapter 2 Extracting Precipitation-Type Information from ASOS and AWOS Reports: and Overview and Detailed Methods ............................................................................... 10 2.1 Introduction ................................................................................................................ 10 2.2 Background ................................................................................................................ 13 2.2.1 ASOS Development and Systems ................................................................... 13 2.2.2 AWOS Development and Systems.................................................................. 15 2.2.3 Reporting Formats and System Decoding ....................................................... 17 2.2.4 Precipitation-Type Definitions and Abbreviations .......................................... 21 2.2.5 Automated System Operations and Limitations of Precipitation Type ........... 27 2.3 Precipitation-Type Capture and Decoding Procedures .............................................. 38 2.3.1 Raw Weather Report Archive ......................................................................... 39 2.3.2 Present Weather Observations......................................................................... 40 2.3.3 Precipitation-Type Beginning and Ending Times ........................................... 52 2.4 Decoder Utility and Conclusions ............................................................................... 60 Chapter 3 Characteristics of Recent Vehicle-Related Fatalities during Active Precipitation in the United States ..................................................................................... 63 3.1 Introduction ................................................................................................................ 65 3.2 Methods ...................................................................................................................... 71 3.2.1 Characterization of Precipitation Type from FARS ........................................ 71 3.2.2 Characterization of Precipitation Type from ASOS/AWOS ........................... 72 3.2.3 Matching FARS to ASOS/AWOS Precipitation-Type Reports ...................... 74 3.3 Analysis of FARS Precipitation-Related Fatalities .................................................... 76 3.3.1 Precipitation-Related Fatality Totals ............................................................... 78 3.3.2 Roadway Surface Conditions .......................................................................... 79 3.3.3 Climatology of Precipitation-Related Fatalities .............................................. 81 3.4 Assessment of Nearby Precipitation-Type Reports ................................................... 90 3.4.1 Precipitation- Versus Non-Precipitation-Related Fatalities ............................ 90 3.4.2 Precipitation-Related Fatalities ....................................................................... 92 3.5 Discussion .................................................................................................................. 93 3.6 Summary and Conclusion .......................................................................................... 95 Chapter 4 Effects of Precipitation Type on Crash Relative Risk Estimates in Kansas ........... 99 4.1 Introduction ................................................................................................................ 99 4.2 Data and Methods ...................................................................................................... 103 vi 4.2.1 Crash Data ....................................................................................................... 103 4.2.2 Precipitation-Type Data .................................................................................. 106 4.2.3 Event and Control Periods ............................................................................... 109 4.2.4 Relative Risk Estimates ................................................................................... 111 4.3 Results ........................................................................................................................ 113 4.3.1 Sensitivity Testing ........................................................................................... 120 4.3.2 Time of Day and Day of Week Risk Estimates ..............................................