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REVIEWS OF GEOPHYSICS AND SPACE PHYSICS, VOL. 21, NO. 5, PAGES 1027-1042, JUNE 1983 U.S. NATIONAL REPORT TO INTERNATIONAL UNION OF GEODESY AND GEOPHYSICS 1979-1982 MESOSCALE METEOROLOGY Kerry Emanuel and Frederick Sanders Department of Meteorology and Physical Oceanography Massachusetts Institute of Technology Cambridge, Massachusetts 02139 Introduction by analysis of special data and of routine surface observations. Nonlinear dynamics, The last four years have seen a considerable often combined with nontrivial cloud micro- expansion in research on mesoscale atmospheric physics, poses formidable theoretical problems. phenomena. The motivation is three-fold: the Hence, the research and operational communities prospect of greater forecast accuracy, an have recently formulated initial plans for emerging ability to observe the mesoscale, a national project on mesoscale meteorology and the challenge of understanding extremely (UCAR, 1982). complex physical mechanisms. The current level The definition of mesoscale phenomena is of accuracy of storm warnings and other local not universally accepted; the simplest one is forecasts can be substantially advanced only morphological: "mesoscale" refers to those by an improved ability to deal with structures systems which are too large to be observed smaller than the traditional synoptic scale. completely from a single point (lacking a These structures are beginning to be described capability for remote sensing) and too small to be observed unambiguously by the routine Copyright 1983 by the American Geophysical Union. upper-level sounding network over the conti- nental areas, with station spacing of a few Paper number 3R0576. hundred kilometers (Ligda, 1951). A physical 0034-6853/83/003R-0576515.00 definition, on the other hand, seems better 1028 Emanuel and Sanders: Mesoscale Meteorology as a guide to research. It has been suggested Experiment (GATE), and a number of others. Much that mesoscale circulations are dynamically other work was based on the close study of data characterized by the importance of the Coriolis available routinely in the United States, typi- force, but not by the dominance that assures cally for cases of spectacular phenomenology. quasi-geostrophic flow. This definition From these observations, a picture has emerged appears to exclude higher-frequency gravity which seems to be valid throughout a large range waves and all equatorial disturbances except of latitudes, confirming suggestions and infer- those of low frequency. It would include ences from earlier studies by Newton (1950) and intense extratropical cyclones, which have Fujita and Brown (1958) for middle latitudes, been regarded as synoptic scale events. Some and by Zipser (1969) for the tropics. Initially, other physical characteristic, such as the there is sporadic convection (e.g. Leary and relative importance of cloud microphysics, Houze, 1979b; Maddox, 1980a); then the convection might also be used as a definition, but many consolidates and a thick layer of stratiform mesoscale phenomena do not involve condensation. cloud appears in the middle and upper troposphere, Although an entirely adequate physical definition producing long-lasting and significant amounts may not be necessary, a dynamically self- of rain and having in some respects a life of consistent definition of "mesoscale" seems its own. This layer represents the accumulation desirable. For the present, however, we shall of debris from the convection, which remains err on the side of insufficient, rather than active at its upshear edge, and may be aided by additional condensation in an internal mesoscale excessive, exclusivity.