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The “Mesoscale”

The “Mesoscale”. How do we define it? What weather phenomena does it encompass? What are its primary forcing mechanisms? Why should we care about it?. Squall Lines. Supercells. Mountain Waves. …and many more. The Origin of the “Mesoscale”. History Early weather observations were

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The “Mesoscale”

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  1. The “Mesoscale” How do we define it? What weather phenomena does it encompass? What are its primary forcing mechanisms? Why should we care about it? Squall Lines Supercells Mountain Waves …and many more M. D. Eastin

  2. The Origin of the “Mesoscale” • History • Early weather observations were • limited in number and data transmission • capabilities, but enough data was available • to see the large-scale (synoptic) weather • every 3-12 hours • These observations often showed • strange isolated values of pressure, • winds, etc. • Some were errors • Some were mesoscale disturbances • “Mesoscale” first coined by Lidga (1951) • He was a radar meteorologist at MIT • Used the term to classify weather • phenomena observed by radars, but too • small to be observed by conventional • (synoptic) observations Example Radar Reflectivity 07 May 2006 1215Z Synoptic Observations (in the 1950s) M. D. Eastin

  3. Definition of Mesoscale • Numerous classification schemes • for the scale of atmospheric • phenomena have been proposed • Schemes are based on: • Observations: Spatial scales • Time scales • Theory: Scales and types of instability • (i.e. static, inertial, baroclinic) • Applicability of balance conditions • (i.e. geostrophic, hydrostatic) • Mesoscale flow • → Ageostrophic • → Nonhydrostatic M. D. Eastin

  4. Definition of Mesoscale This class:Weather phenomena occurring on spatial scales of 2 – 2000 km and time scales of 3 minutes to 2 days M. D. Eastin

  5. Mesoscale Phenomena Surface Phenomena Middle and Upper-Level Phenomena M. D. Eastin

  6. Forcing of Mesoscale Weather • Synoptic-scale weather forcing • Cyclones and anticyclones • Warm / cold fronts • Warm / cold air advection • Large-scale CAPE • Large scale vertical shear • Geography / Topography forcing • Flow over elevation gradients (hills) • Land-sea boundaries • Gradients of surface vegetation • Other organized mesoscale weather • Sea-breeze convergence • Thunderstorm outflow boundaries • Low-level jet streaks • Mesoscale convective vortices (MCVs) M. D. Eastin

  7. Why should we care about Mesoscale weather? • The vast majority of severe weather occurs in relation to organized mesoscale phenomena • Supercells / Squall Lines → Tornadoes • Squall lines / Bow echoes → Derecheos • Mesoscale Convective Complexes → Flash floods • Quasi-stationary convective events → Flash floods • Mountain waves → Downslope wind storms • Current observation networks do not adequately resolve the mesoscale with regular observations of both winds and thermodynamics in space and time • Surface observations (ASOS, AWOS, mesonets) • NOAA rawindsonde network • NEXRAD WSR88-D Doppler radar network • NOAA Geostationary and polar-orbiting satellites • Therefore the greatest challenge for a weather forecaster is on the mesoscale • Must be able to anticipate mesoscale weather based on the synoptic observations • Must be able to adapt quickly to any evolving mesoscale observations • Must be able to effectively disseminate information to the public in a timely manner • A considerable number of lives and property are often at stake M. D. Eastin

  8. Mesoscale Forecasting = Detailed Analysis Expectations in a Mesoscale Analysis Expectations in a Synoptic Analysis M. D. Eastin

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