Megan Schiede

Icing associated with the accretion of supercooled large droplets (SLD) serves as a significant hazard to aviation activities. Forecasting SLD events is complex due in part to the limitations of numerical weather prediction models. The Winter Precipitation Type Research Multiscale Experiment (WINTRE-MIX) aimed to comprehend such limitations within numerical forecasts of precipitation type (p-type) through the use of extensive observations during events with multiple p-types. To investigate precipitation processes aloft in winter storms with near-freezing surface conditions, research flights were orchestrated with the National Research Council of Canada Convair-580 aircraft. This study examines a WINTRE-MIX intensive observing period that took place on 07–08 March 2022 and coincided with a warm frontal passage through northern New York and into southern Quebec. As the Convair ascended in altitude, widespread freezing drizzle (FZDZ) connected with SLD icing was observed with abnormally cool cloud-top temperatures as low as -15°C. This study evaluates the High-Resolution Rapid Refresh (HRRR) model forecasts of FZDZ against Convair-580 observations. The HRRR is favored within the aviation meteorology community for its 3-km grid spacing, its frequent hourly data assimilation, and its use of complex microphysics schemes with the ability to diagnose multiple hydrometeor categories. Icing conditions are examined using in-situ aircraft observations by analyzing ice detector frequency, liquid water content, cloud number concentration, and rain number concentration, and diagnosed hydrometeor types which are compared to their model-simulated equivalent. Additionally, Weather Research and Forecasting (WRF) model simulations explore sources of inconsistencies between observations and model runs. Airborne W-band radar profiles collected by the Convair-580 are compared to simulated radar reflectivity. HRRR and control (CTRL) WRF predominately simulate snow and ice hydrometeors at flight level which contrasts observations of FZDZ and SLD icing. The presence of snow hydrometeors throughout the vertical column suggests that seeding from the upper-level cloud inhibits collision-coalescence. Frozen hydrometeors will serve as ice nuclei or lead to the evaporation of liquid through the Bergeron-Wegener-Findeisen process. This leads to the removal of available liquid in the atmosphere. Two sensitivity experiments are performed by removing moisture above an average altitude of 5092 m and between average altitudes of 4076 m and 5836 m to explore the influence of seeding from the upper-layer cloud. The reduction of snow mixing ratios and improvements to rain mixing ratios at locations of observed icing demonstrate the biases introduced by the seeding mechanisms.

Throughout 2020, ambient air pollution was reduced as a result from limiting anthropogenic activities to mitigate the spread of COVID-19. Not all air pollution is created the same as measurements of particulate matter 2.5 (PM2.5) were generally unaffected by this reduction due to PM2.5's source from wildfires. Despite influences from COVID-19 lockdowns, rises in PM2.5 concentrations can be attributed to the anomalously active wildfire season of 2020. As climate change progresses, these extraordinarily active seasons can be classified as the "new normal"; thus, comprehension of such events are vital. Given the ability of lightning to naturally cause wildfires, there exists a positive feedback loop that exists as convection can be influenced by aerosols. To understand this feedback loop, a proper analysis of the interactions between lightning and wildfires is necessary. Data from the Environmental Protection Agency (EPA) Air Quality System, National Lightning Detection Network (NLDN), and the ERA5 Reanalysis was utilized for a statistical analysis. Airports were carefully chosen to result in 132 correlation coefficients for either PM2.5 concentration or lightning strikes to meteorological variables and month. Planetary boundary layer height (PBLH) had the strongest negative correlation with all but one calculated correlation being negative on the East Coast. Numerical modeling methods were implemented to understand aerosol behavior beyond a statistical scope. Two Weather Research and Forecasting (WRF) model simulations were conducted surrounding the Detroit Metropolitan Wayne County Airport (KDTW) using a 3-km High Resolution Rapid Refresh (HRRR) initialization at 0000 UTC 7 September 2020 to compare model runs with and without the inclusion of NASA's Goddard Earth Observing System (GEOS) model aerosol data. The difference between the simulations including aerosols to no aerosols showed that PBLH mimicked the frontal structure associated with convection for the case. Positive values indicated that WRF simulations that included aerosols had higher PBLH that were observed behind the cold front. Additional cases must be examined to draw adequate conclusions regarding numerical model output to further confirm the existence of a frontal-based relationship surrounding PBLH.

Methyl chloroform (CH3CCl3) was a commonly used solvent and degreasing agent prior to its classification as an ozone depleting substance, and subsequent prohibition by the Montreal Protocol in 1987. With an atmospheric lifetime of five to six years, temporal measurements of methyl chloroform illustrate the success of these restrictions as background values fell from approximately 130 ppt in the early 1990s to 1.5 ppt in 2020. The COVID-19 lockdowns brought about an ease in EPA restrictions, as well as a new opportunity for the NASA Student Airborne Research Program to extend whole air sampling across the United States. The results from this widespread sampling discovered enhanced concentrations of methyl chloroform near various highly populated coastal areas with the highest observed in Ithaca, NY at 16.4 ppt. Given the status of methyl chloroform as an ozone depleting substance, its use requires permission from the EPA implying its use should be easy to track. An investigation into past uses of methyl chloroform was conducted to determine if any plausible sources for these enhanced concentrations still currently exist. To confirm the validity of these sources, multiple NOAA HYSPLIT trajectories were run on the dates of enhanced observations. There are promising associations between the location of potential emission sources and enhanced methyl chloroform measurements. In order to further support these results additional whole air sampling must be conducted near sites of interest.