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Massive wildfires and extreme fire behavior are becoming more frequent across the western United Statescreating a need to better understand how megafire behavior will evolve in our warming world. Here, the fire spread model Prometheus is used to simulate the initial explosive growth of the 2020 August Complex, which occurred in northern California (CA) mixed conifer forests. High temperatures, low relative humidity, and daytime southerly winds were all highly correlated with extreme rates of modeled spread. Fine fuels reached very dry levels, which accelerated simulation growth and heightened fire heat release (HR). Model sensitivity tests indicate that fire growth and HR are most sensitive to aridity and fuel moisture content. Despite the impressive early observed growth of the fire, shifting the simulation ignition to a very dry September 2020 heatwave predicted a >50% increase in growth and HR, as well as increased nighttime fire activity. Detailed model analyses of how extreme fire behavior develops can help fire personnel prepare for problematic ignitions.

The Madden–Julian Oscillation (MJO) is the most prominent mode of tropical intraseasonal variability in the climate system and has worldwide influences on the occurrences and forecasts of heavy precipitation. This paper investigates the sensitivity of precipitation over the contiguous United States (CONUS) in a case study (boreal 2004–05 winter). Several major storms affected the western and eastern CONUS producing substantial economic and social impacts including loss of lives. The Weather Research and Forecasting (WRF) model is used to perform experiments to test the significance of the MJO amplitude. The control simulation uses the MJO amplitude observed by reanalysis, whereas the amplitude is modified in perturbation experiments. WRF realistically simulates the precipitation variability over the CONUS, although large biases occur over the Western and Midwest United States. Daily precipitation is aggregated in western, central and eastern sectors and the frequency distribution is analyzed. Increases in MJO amplitude produce moderate increases in the median and interquartile range and large and robust increases in extreme (90th and 95th percentiles) precipitation. The MJO amplitude clearly affects the transport of moisture from the tropical Pacific and Gulf of Mexico into North America providing moist rich air masses and the dynamical forcing that contributes to heavy precipitation.

A GLOBAL WARMING PRIMER--ANSWERING YOUR QUESTIONS ABOUT THE SCIENCE, THE CONSEQUENCES, AND THE SOLUTIONS Jeffrey Bennett, 2016, 128 pp., $10.50, softbound, Big Kid Science, ISBN 978-1937548780 The Global Warming Primer [...]

A simple, fully automated, and efficient method to determine the structural properties and evolution (tracking) of cloud shields of convective systems (CS) is described. The method, which is based on the maximum spatial correlation tracking technique (MASCOTTE), is a new alternative to the existent techniques available for studies that monitor the evolution of CS using satellite images. MASCOTTE provides as CS structural properties the following parameters: mean and variance of brightness temperature, horizontal area, perimeter, minimum brightness temperature, fractional convective area, center of gravity, and fragmentation. The fragmentation parameter has the potential to monitor the evolution of the CS. A new way of estimating the orientation and eccentricity of CS is proposed and is based on the empirical orthogonal function analysis of CS pixel coordinates. The method includes an accurate detection of splitting and merging of convective systems, which is a critical step in the automated satellite CS life cycle determination. Based on the magnitudes of the spatial correlation between consecutive satellite images and the changes in horizontal areas of CS, MASCOTTE provides a simple and skillful technique to track the evolution of CS life cycles. The MASCOTTE methodology is applied to infrared satellite images during seven consecutive days of the Wet-Season Atmospheric Mesoscale Campaign of the Large-Scale Biosphere--Atmosphere Experiment and ground validation experiment of the Tropical Rainfall Measuring Mission in the Brazilian state of Rondonia in the Amazon basin. The results indicate that MASCOTTE is a valuable approach to understanding the variability of CS.

Tropical intraseasonal convective anomalies (TICA) have a central role in subseasonal changes in the coupled ocean–atmosphere system, but the climatology of TICA events has not been properly documented. This study exploits 24 years of outgoing longwave radiation (OLR) data and a tracking algorithm to develop a climatology of eastward propagating TICA events. Three distinct types of TICA occurrences are documented according to their propagation characteristics. The first type (IND) is characterized by events that propagate in the Indian Ocean without significant influence in the western Pacific Ocean. The second and third types are associated with occurrences of the Madden–Julian oscillation during boreal winters (MJO) and summers (ISO). The frequency of occurrence of TICA events is highest in April–June and October–December and lowest in July–September. An analysis of the spatial and temporal characteristics reveals that MJO events tend to have the longest life cycle, greatest intensity, and largest variability inside the contiguous region of OLR anomaly. Given the data record of 24 years, the analysis of interannual occurrences of TICA events does not show statistically significant differences among events that occur in different phases of the El Niño–Southern Oscillation (ENSO). A procedure is developed to identify major MJO events and estimate their frequency of occurrence in the data record. [ABSTRACT FROM AUTHOR]

This study investigates the influence of planetary boundary layer (PBL) schemes and land surface models (LSMs) on the performance of the Weather Research & Forecasting model in simulating the development of downslope windstorms in the lee of the Santa Ynez Mountains in Santa Barbara, California (known as Sundowner winds). Using surface stations, a vertical wind profiler, and a multi-physics ensemble approach, we found that most of the wind speed biases are controlled by the roughness length z 0 , and so by the choice of LSM. While timing characteristics of Sundowners are insensitive to both LSM and PBL schemes, a clear sensitivity in the horizontal extent of strong surface winds is found for both PBL parameterization and z 0 , which are related to patterns of self-induced wave-breaking near the mountaintop, and the erosion of the marine layer. These results suggest that LSMs with relatively high values of z 0 , and TKE-based or hybrid PBL schemes adequately simulate downslope windstorms in the lee of mountain ranges, specifically in areas where downslope windstorms interact with the marine boundary layer with stably stratified characteristics. [ABSTRACT FROM AUTHOR]