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1. The 2018 Camp Fire: Meteorological Analysis Using In Situ Observations and Numerical Simulations

   https://doi.org/10.3390/atmos11010047  

   Brewer, Matthew J. ; Clements, Craig B. ( January 2020 , Atmosphere)

   null (Ed.)

   The November 2018 Camp Fire quickly became the deadliest and most destructive wildfire in California history. In this case study, we investigate the contribution of meteorological conditions and, in particular, a downslope windstorm that occurred during the 2018 Camp Fire. Dry seasonal conditions prior to ignition led to 100-h fuel moisture contents in the region to reach record low levels. Meteorological observations were primarily made from a number of remote automatic weather stations and a mobile scanning Doppler lidar deployed to the fire on 8 November 2018. Additionally, gridded operational forecast models and high-resolution meteorological simulations were synthesized in the analysis to provide context for the meteorological observations and structure of the downslope windstorm. Results show that this event was associated with mid-level anti-cyclonic Rossby wave breaking likely caused by cold air advection aloft. An inverted surface trough over central California created a pressure gradient which likely enhanced the downslope winds. Sustained surface winds between 3–6 m s−1 were observed with gusts of over 25 m s−1 while winds above the surface were associated with an intermittent low-level jet. The meteorological conditions of the event were well forecasted, and the severity of the fire was not surprising given the fire danger potential for that day. However, use of surface networks alone do not provide adequate observations for understanding downslope windstorm events and their impact on fire spread. Fire management operations may benefit from the use of operational wind profilers to better understand the evolution of downslope windstorms and other fire weather phenomena that are poorly understood and observed. 

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2. Formation Mechanisms of the Mesoscale Environment Conducive to a Downslope Windstorm over the Cuyamaca Mountains Associated with Santa Ana Wind during the Cedar Fire (2003)

   https://doi.org/10.1175/JAMC-D-22-0025.1  

   Karim, S. M. ; Lin, Yuh-Lang ; Kaplan, Michael L. ( November 2022 , Journal of Applied Meteorology and Climatology)

   Abstract Numerical simulations were conducted to investigate the upstream environment’s impacts on the airflow over the lee slope of the Cuyamaca Mountains (CM) near San Diego, California, during the Cedar Fire that occurred from 25 to 29 October 2003. The upstream environment was largely controlled by a southwest–northeast-oriented upper-tropospheric jet streak that rotated around a positively tilted ridge within the polar jet stream. Three sequential dynamical processes were found to be responsible for modifying the mesoscale environment conducive to low-level momentum and dry air that sustained the Cedar Fire. First, the sinking motion associated with the indirect circulation of the jet streak’s exit region strengthened the midtropospheric flow over the southern Rockies and the lee slope of the Sawatch and San Juan Ranges, thus modestly affecting the airflow by enhancing the downslope wind over the CM. Second, consistent with the coupling process between the upper-level sinking motion, downward momentum transfer, and developing lower-layer mountain waves, a wave-induced critical level over the mountain produced wave breaking, which was characterized by a strong turbulent mixed region with a wind reversal on top of it. This critical level helped to produce severe downslope winds leading to the third stage: a hydraulic jump that subsequently enhanced the downstream extent of the strong winds conducive to the favorable lower-tropospheric environment for rapid fire spread. Consistent with these findings was the deep-layer resonance between the mountain surface and tropopause, which had a strong impact on strengthening the severe downslope winds over the lee slope of the CM accompanying the elevated strong easterly jet at low levels. 

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3. The December 2021 Marshall Fire: Predictability and Gust Forecasts from Operational Models

   https://doi.org/10.3390/atmos13050765  

   Fovell, Robert G. ; Brewer, Matthew J. ; Garmong, Richard J. ( May 2022 , Atmosphere)

   We analyzed meteorological conditions that occurred during the December 2021 Boulder, Colorado, downslope windstorm. This event is of particular interest due to the ignition and spread of the Marshall Fire, which quickly became the most destructive wildfire in Colorado history. Observations indicated a rapid onset of fast winds with gusts as high as 51 m/s that generally remained confined to the east-facing slopes and foothills of the Rockies, similar to previous Boulder windstorms. After about 12 h, the windstorm shifted into a second, less intense phase. Midtropospheric winds above northwestern Colorado weakened prior to the onset of strong surface winds and the event strength started waning as stronger winds moved back into the area. Forecasts from NOAA high-resolution operational models initialized more than a few hours prior to windstorm onset did not simulate the start time, development rate and/or maximum strength of the windstorm correctly, and day-ahead runs even failed to develop strong downslope windstorms at all. Idealized modeling confirmed that predictability was limited by errors on the synoptic scale affecting the midtropospheric wind conditions representing the Boulder windstorm’s inflow environment. Gust forecasts for this event were critically evaluated. 

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4. Near-Surface Frontogenesis and Atmospheric Instability along the U.S. East Coast during the Extratropical Transition of Hurricane Matthew (2016)

   https://doi.org/10.1175/MWR-D-18-0094.1  

   Powell, Scott W. ; Bell, Michael M. ( February 2019 , Monthly Weather Review)

   Hurricane Matthew locally generated more than 400 mm of rainfall on 8–9 October 2016 over the eastern Carolinas and Virginia as it transitioned into an extratropical cyclone. The heaviest precipitation occurred along a swath situated up to 100–200 km inland from the coast and collocated with enhanced low-tropospheric frontogenesis. Analyses from version 3 of the Rapid Refresh (RAPv3) model indicate that rapid frontogenesis occurred over eastern North and South Carolina and Virginia on 8 October, largely over a 12-h time period between 1200 UTC 8 October and 0000 UTC 9 October. The heaviest rainfall in Matthew occurred when and where spiral rainbands intersected the near-surface front, which promoted the lift of conditionally unstable, moist air. Parallel to the spiral rainbands, conditionally unstable low-tropospheric warm, moist oceanic air was advected inland, and the instability was apparently released as the warm air mass rose over the front. Precipitation in the spiral rainbands intensified on 9 October as the temperature gradient along the near-surface front rapidly increased. Unlike in Hurricane Floyd over the mid-Atlantic states, rainfall totals within the spiral rainbands of Matthew as they approached the near-surface front evidently were not enhanced by release of conditional symmetric instability. However, conditional symmetric instability release in the midtroposphere may have enhanced rainfall 200 km northwest of the near-surface front. Finally, although weak cold-air damming occurred prior to heavy rainfall, damming dissipated prior to frontogenesis and did not impact rainfall totals.

    

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5. Analysis of Snow Multibands and Their Environments with High-Resolution Idealized Simulations

   https://doi.org/10.1175/MWR-D-23-0211.1  

   Leonardo, Nicholas M. ; Colle, Brian A. ( April 2024 , Monthly Weather Review)

   Nested idealized baroclinic wave simulations at 4-km and 800-m grid spacing are used to analyze the precipitation structures and their evolution in the comma head of a developing extratropical cyclone. After the cyclone spins up by hour 120, snow multibands develop within a wedge-shaped region east of the near-surface low center within a region of 700–500-hPa potential and conditional instability. The cells deepen and elongate northeastward as they propagate north. There is also an increase in 600–500-hPa southwesterly vertical wind shear prior to band development. The system stops producing bands 12 h later as the differential moisture advection weakens, and the instability is depleted by the convection. Sensitivity experiments are run in which the initial stability and horizontal temperature gradient of the baroclinic wave are adjusted by 5%–10%. A 10% decrease in initial instability results in less than half the control run potential instability by 120 h and the cyclone fails to produce multibands. Meanwhile, a 5% decrease in instability delays the development of multibands by 18 h. Meanwhile, decreasing the initial horizontal temperature gradient by 10% delays the growth of vertical shear and instability, corresponding to multibands developing 12–18 h later. Conversely, increasing the horizontal temperature gradient by 10% corresponds to greater vertical shear, resulting in more prolific multiband activity developing ∼12 h earlier. Overall, the relatively large changes in band characteristics over a ∼12-h period (120–133 h) and band evolutions for the sensitivity experiments highlight the potential predictability challenges.

   Multiple-banded precipitation structures are difficult to predict and can greatly impact snowfall forecasts. This study investigates the precipitation bands in the comma head of a low pressure system in a numerical model to systematically isolate the roles of different ambient conditions. The results emphasize that environments with instability (e.g., air free to rise after small upward displacement) and increasing winds with height favor the development of banded structures. The forecast challenge for these bands is illustrated by starting the model with relatively small changes in the temperature field. Decreasing the instability by 10% suppresses band development, while increasing (decreasing) the horizontal temperature change across the system by 10% corresponds to the bands developing 12 h earlier (later).

    

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