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Article

Forecasting severe convective weather remains one of the most challenging tasks facing operational meteorology today, especially in the mid-latitudes, where severe convective storms occur most frequently and with the greatest impact. The forecast difficulties reflect, in part, the many different atmospheric processes of which severe thunderstorms are a by-product. These processes occur over a wide range of spatial and temporal scales, some of which are poorly understood and/or are inadequately sampled by observational networks. Therefore, anticipating the development and evolution of severe thunderstorms will likely remain an integral part of national and local forecasting efforts well into the future. Modern severe weather forecasting began in the 1940s, primarily employing the pattern recognition approach throughout the 1950s and 1960s. Substantial changes in forecast approaches did not come until much later, however, beginning in the 1980s. By the start of the new millennium, significant advances in the understanding of the physical mechanisms responsible for severe weather enabled forecasts of greater spatial and temporal detail. At the same time, technological advances made available model thermodynamic and wind profiles that supported probabilistic forecasts of severe weather threats. This article provides an updated overview of operational severe local storm forecasting, with emphasis on present-day understanding of the mesoscale processes responsible for severe convective storms, and the application of recent technological developments that have revolutionized some aspects of severe weather forecasting. The presentation, nevertheless, notes that increased understanding and enhanced computer sophistication are not a substitute for careful diagnosis of the current meteorological environment and an ingredients-based approach to anticipating changes in that environment; these techniques remain foundational to successful forecasts of tornadoes, large hail, damaging wind, and flash flooding.

Article

Since the dawn of the digital computing age in the mid-20th century, computers have been used as virtual laboratories for the study of atmospheric phenomena. The first simulations of thunderstorms captured only their gross features, yet required the most advanced computing hardware of the time. The following decades saw exponential growth in computational power that was, and continues to be, exploited by scientists seeking to answer fundamental questions about the internal workings of thunderstorms, the most devastating of which cause substantial loss of life and property throughout the world every year. By the mid-1970s, the most powerful computers available to scientists contained, for the first time, enough memory and computing power to represent the atmosphere containing a thunderstorm in three dimensions. Prior to this time, thunderstorms were represented primarily in two dimensions, which implicitly assumed an infinitely long cloud in the missing dimension. These earliest state-of-the-art, fully three-dimensional simulations revealed fundamental properties of thunderstorms, such as the structure of updrafts and downdrafts and the evolution of precipitation, while still only roughly approximating the flow of an actual storm due computing limitations. In the decades that followed these pioneering three-dimensional thunderstorm simulations, new modeling approaches were developed that included more accurate ways of representing winds, temperature, pressure, friction, and the complex microphysical processes involving solid, liquid, and gaseous forms of water within the storm. Further, these models also were able to be run at a resolution higher than that of previous studies due to the steady growth of available computational resources described by Moore’s law, which observed that computing power doubled roughly every two years. The resolution of thunderstorm models was able to be increased to the point where features on the order of a couple hundred meters could be resolved, allowing small but intense features such as downbursts and tornadoes to be simulated within the parent thunderstorm. As model resolution increased further, so did the amount of data produced by the models, which presented a significant challenge to scientists trying to compare their simulated thunderstorms to observed thunderstorms. Visualization and analysis software was developed and refined in tandem with improved modeling and computing hardware, allowing the simulated data to be brought to life and allowing direct comparison to observed storms. In 2019, the highest resolution simulations of violent thunderstorms are able to capture processes such as tornado formation and evolution which are found to include the aggregation of many small, weak vortices with diameters of dozens of meters, features which simply cannot not be simulated at lower resolution.

Article

Annick Terpstra and Shun-ichi Watanabe

Polar lows are intense maritime mesoscale cyclones developing in both hemispheres poleward of the main polar front. These rapidly developing severe storms are accompanied by strong winds, heavy precipitation (hail and snow), and rough sea states. Polar lows can have significant socio-economic impact by disrupting human activities in the maritime polar regions, such as tourism, fisheries, transportation, research activities, and exploration of natural resources. Upon landfall, they quickly decay, but their blustery winds and substantial snowfall affect the local communities in coastal regions, resulting in airport-closure, transportation breakdown and increased avalanche risk. Polar lows are primarily a winter phenomenon and tend to develop during excursions of polar air masses, originating from ice-covered areas, over the adjacent open ocean. These so-called cold-air outbreaks are driven by the synoptic scale atmospheric configuration, and polar lows usually develop along air-mass boundaries associated with these cold-air outbreaks. Local orographic features and the sea-ice configuration also play prominent roles in pre-conditioning the environment for polar low development. Proposed dynamical pathways for polar low development include moist baroclinic instability, symmetric convective instability, and frontal instability, but verification of these mechanisms is limited due to sparse observations and insufficient resolution of reanalysis data. Maritime areas with a frequent polar low presence are climatologically important regions for the global ocean circulation, hence local changes in energy exchange between the atmosphere and ocean in these regions potentially impacts the global climate system. Recent research indicates that the enhanced heat and momentum exchange by mesoscale cyclones likely has a pronounced impact on ocean heat transport by triggering deep water formation in the ocean and by modifying horizontal mixing in the atmosphere. Since the beginning of the satellite-era a steady decline of sea-ice cover in the Northern Hemisphere has expanded the ice-free polar regions, and thus the areas for polar low development, yet the number of polar lows is projected to decline under future climate scenarios.