KS1.1 MESOSCALE : THE LAST TWENTY YEARS AND THE NEXT

Kerry Emanuel* Massachusetts Institute of Technology, Cambridge, Massachusetts

1. INTRODUCTION 2. TWENTY YEARS OF PROGRESS

In this brief presentation, I will attempt to The first Conference on Mesoscale review the major findings in the field of Meteorology took place twenty years ago, in mesoscale meteorology over the last two Norman, Oklahoma. At that time, semi- decades, and offer some thoughts on where geostrophic theory was only about 10 years the field may go over the next two, with the old, Doppler radar was a novelty confined to requisite allowance made for the very great a few labs, universities and TV stations, the uncertainties inherent in this kind of forecast. mesoscale structure of extratropical was just beginning to be properly The definition of mesoscale has never delineated, and anyone with a background been fully agreed on, but a glance at the in atmospheric dynamics was chronically papers in this volume suggests an operative hydrophobic (one gifted researcher definition that includes moist convective proclaimed that “friction + heating = 0”). The systems (but not individual clouds), Klemp-Wilhelmson model was five years substructure of tropical and extratropical old; the great-great grandfather of the MM5 cyclones, including fronts and cloud and model, the LAMPS-NCAR model, was the precipitation bands, inertia-gravity waves, mesoscale model of the day, and T-bones and topographically and thermally forced were then considered a cut of meat. On the circulations on scales at or smaller than the operational side, the U.S. regional model deformation radius. It is attempting to was the “limited-area, fine-mesh” model, or categorize these myriad phenomena as “LFM”, and we thought that 190 km was a those having Rossby numbers that are “fine-mesh”; it was run on a supercomputer neither large nor small, but such a definition that was less powerful than the desktop does not adequately encompass the computer you might be using to read or print abovementioned processes. For example, this paper. Real live human beings made classical fronts are almost always thought of official observations, and would as mesoscale phenomena, but have have become apoplectic at the gems of Lagrangian Rossby numbers that are small today’s automated observations, such as and are in many respects quasi-balanced, freezing rain at a temperature of 36F. Those obeying, to a good approximation, the semi- of us who were outside the NOAA geostrophic equations. It is also tempting to organization received weather data over offer a completely pragmatic definition: Western Union teletype and Alden facsimile mesoscale phenomena are those that are machines, the chemical odor of whose not properly resolved by today’s global special paper inspired ecstasy in the models but which are not so small in scale weather nut. We scientists wrote each other that their statistical properties are well letters and talked quite a bit on the defined within a single grid box. As tempting telephone. as these proposed definitions may or may not be, I will fall back on the operative A look back at the abstract volume for the definition described at the beginning of this first mesoscale conference shows a paragraph. reassuring similarity between the topics covered then and now. The first two sessions and a poster session were devoted ______to fronts and substructure of cyclonic *Corresponding author address: KERRY , and had papers on frontogenesis, EMANUEL, Rm. 54-1620 MIT, 77 Mass. Ave., coastal fronts and cold air damming, Cambridge, MA 02139; e-mail: observations of precipitation bands, and [email protected] conditional symmetric instability. Sessions on mesoscale aspects of moist convection Satellites have continued to improve, to included papers on tropical and extratropical the point of resolving significant mesoscale lines, MCCs, gravity waves, cumulus phenomena in a variety of channels, parameterization, and the problem of including traditional visible and infrared as initializing mesoscale models. In sessions well as water vapor bands and other on topographically induced circulations, the channels. Only one scatterometer had flown effect of latent heat release on flow over by 1983, and that remained operational for mountains was discussed, as were the only six months; today, scatterometers propagation of density currents in provide routine information at the mountainous terrain, mesoscale aspects of ocean surface. Cloud track provide coastal climate, including sea- and land- almost incredible resolution of mesoscale breezes, and terrain-induced . phenomena, especially near the tropopause. The results of some of the very first three- At the same time, measurements taken dimensional simulations of flow over routinely by commercial aircraft are mountains were presented, and the effects providing much better information over of high mountains on baroclinic systems traditional data voids, such as the oceans. such as cyclones and fronts were just beginning to be explored. Other papers Unfortunately, this picture is not entirely concerned such phenomena as undular rosy. Certain valuable measurement bores and polar lows. The great debate systems, especially rawinsondes, continue about the origins of the mesoscale energy to decline in number, fueled in part by spectrum was just beginning in 1983, and economic factors in such places as Russia, continues to this day. and in the more developed world by the perception that satellites have rendered It would be difficult to overstate the them unnecessary. Despite strong appeals degree of change that has occurred since from such organizations at the National that time. There has been progress on four Academy of Sciences and the U.S. Weather fronts: mesoscale observation tools and Research Program, we have not yet analysis techniques, physical understanding, developed a rational plan for an observing numerical modeling and data assimilation, system that takes into account both costs and archiving and communication of and benefits and which balances the needs meteorological data. of researchers, forecasters and climate monitors. On the observational side, the advent of NEXRAD in the late 1980s provided Along with changes to the observing continuous Doppler radar coverage of the system have come rapid advances in almost all of the continental U.S., and it analysis techniques, especially variational became practical to talk about assimilating analyses which can smoothly incorporate Doppler-derived winds into mesoscale new data into prior estimates (usually short- forecast models. NOAA and NCAR term forecasts), with built-in constraints if purchased airborne Doppler radars, which desired. These techniques provide meant that researchers could go to where dynamically consistent initial conditions for the weather was and fly past it quickly models, and means by which enough to do dual-Doppler type analysis of measurement/sampling errors can be the three-dimensional wind field. This led, detected and dealt with. Among the fruit of among other things, to greatly improved this development is the advent of global re- analyses of the substructure of tropical and analyses, which can be used as boundary extratropical cyclones, especially as and initial conditions for running mesoscale observed during the annual hurricane models in cases of historical interest. programs conducted by NOAA’s Hurricane Research Division, and several large field Progress in physical understanding has experiments, such as the Genesis of Atlantic been perhaps more spotty. Theories and Lows Experiment (GALE) in 1986, and the reduced models of frontogenesis were well Experiment on Rapidly Intensifying Cyclones developed by the mid 1980s and various over the Atlantic (ERICA) in 1989. approximations, such as semi-geostrophy, have been compared with primitive equation simulations, but there remain some curious Large increases in computer power and discrepancies between what these models reductions in price have made mesoscale predict and what we observe; I will show an modeling much more affordable over the last example at the conference. Theories and two decades. As a result, there is a clear simulations of mesoscale precipitation trend toward the increasing application of bands abound, but it has proven difficult to models to mesoscale phenomena. This has ascribe with any confidence individual enabled the detailed simulation of fine scale observed cases to any one theory. The processes, from air flow over complex development of theories for the generation terrain to eddies in the eyewalls of of inertia-gravity waves through geostrophic hurricanes. It has also allowed for running adjustment and other non-topographic ensembles of forecasts, aiding the quest for processes has proven very difficult, and a better understanding of the degree of while beautiful examples of such waves predictability of mesoscale phenomena. have been nicely documented, there is little ability to predict such phenomena. There The advent of the internet and the has been arguably more progress in the enormous increases in the capacity of data understanding of mesoscale convective storage devices have revolutionized access systems. For example, the RKW (Rotunno- to environmental data. Twenty years ago, Klemp-Weisman) theory of squall lines, most research centers and university focusing on the interaction of - departments were storing paper facsimile produced density currents with ambient wind charts and teletype output in large racks and shear, has stood the test of time. But while file cabinets, and missing information was there are intriguing ideas about the origins of often obtained through informal networks of mesoscale convective complexes, they have synopticians, sometimes by frantic midnight not yet been rigorously tested against phone calls in advance of conference observations. deadlines. Adding to the technical barriers to efficient archiving and distribution were In recent years, there has been certain political barriers, as government accelerated progress in understanding weather services sought to fend off real or certain mesoscale characteristics of tropical perceived competition from the private cyclones, after a curious period of sector and/or the weather services of other dormancy. The role of vortex Rossby waves, political entities. Even in the U.S., there considered as early as the 1950s, has been were dark hours in the early 1980s when the revived to help explain spiral and National Weather Service was poised to the axisymmeterization of vortices, and implement a new internal data distribution there is active research on the phenomenon network that would have prevented the flow of concentric eyewall replacement cycles, of real-time weather data to anyone outside documented so well by radar and other the NOAA structure. Only vigorous lobbying observations published in the last two prevented that outcome. decades. Today, students in our field enjoy access One area where theoretical progress has to real-time and archived data that would lagged is in understanding the origins of the have caused heart palpitations in the mesoscale kinetic energy spectrum. There is students of my time. So, too, have the still no generally accepted theory for this, political barriers to data distribution fallen; though attention has focused on stratified today NOAA makes available in near real turbulence and the possibility that the time virtually everything it produces. The atmosphere has a saturated spectrum of civilized world has seen fit to regard internal waves, much as the ocean is environmental data as a public good; in due thought to possess. It would seem that little course, Europe is sure to follow. The progress can be made on this problem until benefits to research and forecasting of the and unless we are successful in identifying revolution in data storage and the particular phenomena that contribute to communication should not be this spectrum. Doing so may have important underestimated. ramifications for the degree of predictability of mesoscale phenomena. 3. A LOOK AHEAD There is increasing evidence that cloud- radiation interactions can play a significant “Prediction is difficult, especially about the role in mesoscale weather phenomena. It is future.” So said Yogi Berra. The safest already well known that this has a profound course is just to extrapolate current trends. effect on MCCs, contributing to their diurnal cycle, among other things. The more One trend that is sure to continue is the general question of how cloud-radiation evolution of mesoscale models to ever finer interaction may affect other kinds of resolution, as computing power continues to mesoscale systems is still open. increase. This offers the hope that we will no longer have to concern ourselves with the On the more practical side, cheap and difficult and controversial job of powerful computers, the proliferation of parameterizing moist convection. Already, mesoscale models, and the easy models with grid spacing of a few kilometers communication of real-time weather data are referred to as “cloud-resolving models”, has made it feasible for small organizations, though observations of real clouds show such as small companies and university important structure down to a few tens of departments, to do real time mesoscale meters. Experience with the modeling of NWP and to post forecasts on the web. At entrainment into boundary layers, which has the same time, work on ensemble modeling shown that extremely fine resolution is is beginning to show the benefits of multi- necessary to obtain numerical convergence model ensembles. In the field of hurricane of entrainment, should caution us that track prediction, for example, the superiority processes important in convective physics, of multi-model ensembles over single control such as entrainment and the upward forecasts and single-model ensembles has advection of boundary layer turbulence into been clearly demonstrated. We are clouds, may still be badly underresolved witnessing what may prove to be the even with grid spacings of 100 m. The beginnings of a radical de-centralization of question of when underresolved explicit NWP in general. Although the WRF project clouds are to be preferred to provides a badly needed framework parameterizations is by no means settled. whereby myriad organizations can contribute to model development, the Questions of mesoscale variability and benefits of model diversity should not be predictability have long focused on the overlooked. In addition to continuing its dynamical variables of temperature, crucial mission of running large, centralized pressure and wind. The variability of water NWP models, NOAA could also choose to vapor has been somewhat neglected by act as a clearinghouse for the dissemination comparison. Studies of tracer transport of NWP products generated outside NOAA, show that passive tracers can develop very thereby helping us reap the benefits of multi- fine structure even in flows that vary slowly model ensembles. in space an time. The time-evolving, three- dimensional distribution of water vapor in “All politics is local” quipped Tip O’Neil. In unsaturated air poses a particular challenge the same vein, we may say that all weather for mesoscale meteorology, since the is mesoscale. mesoscale structure of clouds and precipitation ultimately depend on it and because it is poorly observed. Though satellite methods can yield high horizontal resolution, they so far have not been able to provide the needed vertical resolution. Here, high-resolution mesoscale models can help by quantifying the kinds of water vapor variability that matter for weather and by defining observational requirements for specifying its distribution.