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The southern-facing coastal plain of Santa Barbara County (CA) is bounded by the cool waters of the Pacific Ocean to the south and the west-east oriented Santa Ynez Mountains (SYM) to the north. Due to its proximity to these complex features, this region commonly experiences a gusty, northerly, downslope flow known locally as Sundowner Winds or simply Sundowners. Named for their typical time of onset (i.e. sunset), Sundowners have been involved in many of the wildfires which have affected communities living along the Santa Barbara coastal plain. Thus, improved forecasts of the intensity and spatiotemporal variability of these winds are necessary to prevent and prepare for wildfire influenced disasters. In order to study the mesoscale mechanisms responsible for Sundowners, a multi-institution field campaign, termed the Sundowner Wind Experiment (SWEX), was completed over April-May of 2022 across Santa Barbara County. This study examines observations from various meteorological instruments, with a particular focus on conditions over the eastern SYM, during the final intensive observation period (IOP) of the SWEX campaign. This IOP (IOP #10) occurred over a 24-hour period beginning on May 12, 2022 at 10:00PDT (17:00UTC). Synoptic scale analysis during IOP #10 shows zonal flow at upper levels of the atmosphere (i.e. 500hPa) occurring alongside a steep, west-east oriented mean sea level pressure gradient that developed as a result of a high pressure system offshore of California and comparatively lower pressure over the US mainland, supporting a primarily surface pressure driven Sundowner event. Results from three ground-based light detection and ranging (LiDAR) wind profilers, situated within the slopes along and foothills below the eastern SYM, reveal strong differences in horizontal wind speeds between stations. The highest wind speeds and associated turbulence, as revealed by a series of radiosonde launches, occur near the easternmost portion of the SYM, in the city of Montecito. Common surface meteorological variables (e.g. air temperature, dewpoint, and winds) from previously established local weather station networks, as well as from specific surface stations deployed during the SWEX campaign, are used to add additional context to the evolution of Sundowner conditions during IOP #10 across the study domain. The results presented here aim to contribute pertinent information to the operational forecasting community regarding the connection between surface and upper-level atmospheric conditions during a particular Sundowner event over a region that has experienced numerous instances of destructive Sundowner blown wildfires.

The residents of Santa Barbara County know Sundowners as the extreme northerlydownslope windstorms that enhance after sunset on the southern slopes of the Santa Ynez Mountains (SYM). Sundowners are notoriously known to dramatically and unpredictably influence wildfire spread in the region. The SYM reside north of the east-west coastline and reach over 1200 meters, creating a barrier between the cool ocean and the warm inland environments. Further north, the V-shaped Santa Ynez Valley (SYV) separates the SYM from the San Rafael Mountains (SRM), which exceed 2000 meters and influences the regions circulations.The goal of this work is to examine the importance of mountain waves in relation toSundowner spatial variability and the influence of regional topography on mountain wave and Sundowner behavior. This work investigates “Eastern Sundowners”, or Sundowners with NE direction that are particularly strong over eastern SYM (Montecito region). For this purpose, we use the existing simulated 30-year climatology at 1 km grid resolution performed with the Weather Research and Forecasting Model (WRF). The present work developed a mountain wave intensity metric based on the maximum downward vertical velocity observed on the southern lee slope of the SYM and SRM. Using the mountain wave metric, we demonstrate that there is a linear and positive correlation between modeled surface wind speed and modeled mountain wave intensity in both the SYM and SRM mountain ranges. Additional correlation analysis with the mountain wave metric confirms that mountain wave activity in the region is linearly and positively correlated with conditions known to be important to mountain wave activity such as the Froude Number, mountain top stability, cross mountain wind speeds, and stable layer height.This work uses the mountain wave metric to investigate the regions mountain wavediurnal cycles and the upstream influence of the SRM on Sundowners in the SYM. The findings of the latter suggest that the relative elevation of both mountain ranges, combined with the valley width, are two important mechanisms explaining the great spatial variability in high intensity Sundowner winds. This study also provides important elements to explain factors associated with Eastern Sundowners. These results help in explaining the great spatial variability seen in Sundowner high intensity winds and to why Eastern Sundowner events have a hot spot of activity in the Montecito region. These findings are relevant to improve understanding and predictability of Sundowner winds, contributing to better fire weather forecasts and red-flag warnings, thus increasing resiliency of the Santa Barbara County to wildfire disasters.

A review of existing solution methodology for non-hydrostatic atmospheric models identified the split-explicit time stepping approach as the preferred solution method for a new non-hydrostatic atmospheric model. The linear advection equation was used for stability analysis of different combinations of time integration and advection schemes before the third order Runge-Kutta (RK3) time integration scheme was chosen. A number of one- and two-dimensional test problems were then applied to the linear advection equation to study the robustness of the different finite difference advection schemes. It was found that the third and fifth order upwind advection schemes have good phase properties and are generally robust even in coarse grid resolutions. The RK3 time integration scheme with third and fifth order upwind advection schemes were then applied to the split explicit formulation for the new non-hydrostatic atmospheric model. The model was verified with well known test cases of increasing complexity. The performance of the new model was compared with other established non-hydrostatic models. Due to the highly non-linear ow in the test case, there was some variability in simulation results from the other models. Our new model simulation results were all within the average of the majority of the models and was not an outlier. A two-dimensional version of the new model was used to study mountain waves in the middle atmosphere over the southern Andes. Using realistic temperatures, winds and topography, the model simulations generate large amplitude long wavelength breaking mountain waves in the middle atmosphere that compare favourably with satellite measure- ments. Modelled waves have preferred horizontal wavelengths. Spectral analysis reveals correspondences between wavelengths and peaks in the spectrum of Andean topographic elevations. The shorter waves reach the stratosphere well before the longer ones, consistent with group velocity arguments, with longer wavelengths ultimately dominating. At later times we find evidence of downward propagating secondary waves produced by upper level breaking of the primary waves.

The McMurdo Dry Valleys (“MDVs”) are the largest ice-free region in Antarctica, featuring perennially ice-covered lakes that are fed by ephemeral melt streams in the summer. The MDVs have been an NSF-funded Long-Term Ecological Research (LTER) site since 1993, and LTER research has shown that the hydrology and biology of the MDVs are extremely sensitive to small climatic fluctuations, especially during summer when temperatures episodically rise above freezing. However, the atmospheric processes that control MDVs summer climate, namely the westerly foehn and easterly sea-breeze regimes, are not well understood. The goals of this study are to (i) produce a coherent physical mechanism for the development and spatial extent of foehn winds in the MDVs, and (ii) determine aspects of large-scale climate variability responsible for intraseasonal and interannual differences in MDVs temperature. Polar WRF simulations are run for a prominent foehn case study at 500 m horizontal grid spacing to study the mesoscale components of foehn events, and 15 summers at 2 km horizontal grid spacing to analyze event and temporal variability. The Polar WRF simulations have been tailored for use in the MDVs through modifications to the input soil conditions, snow cover, land use, and sea ice. An objective foehn identification method is used to identify and categorize events, as well as validate the model against LTER AWS observations. The MDVs foehn mechanism consists of a gap wind through a topographic constriction south of the MDVs, forced by pressure differences on each side of the gap and typically set up by cyclonic flow over the Ross and Amundsen Seas. Significant mountain wave activity over the gap modulates the flow response over the MDVs themselves, and pressure-driven channeling drives foehn flow down-valley. During strongly forced events, mass accumulation east of the MDVs from flow around Ross Island is responsible for easterly intrusions, and not a thermally forced sea breeze as previously thought. A variety of ambient flow directions and associated synoptic-scale patterns can result in MDVs foehn, but adequate forcing is necessary to activate the foehn mechanism. The warmest foehn events are associated with amplified circulation patterns that are not associated with particular interannual modes of variability, but instead related to intraseasonal variability forced by the extratropical response to a stagnant MJO. Implications of the findings upon current MDVs paleoclimate theories on the existence of huge melt lakes at the LGM are also presented.