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2 Status of Technology and Science in Mesoscale Meteorology EMERGING OBSERVING SYSTEMS AND NEW TECHNOLOGIES Since the first plans for the National STORM Program were formulated in the early 1980s, there has been progress in the development of new in situ observing systems as well as ground-based and space-based remote sensors. In addition to those described below, accurate wind reports from commercial jet aircraft and satellites are available for meteorological use, several national ground-based lightning detection networks are in place, and lightning detection from satellites is planned. Great progress has also been made in the ability to integrate and use these and similar observations. The spectacular advance of computer capabilities and digital electronics has dramatically improved the technical capability for processing heterogeneous meteorological data for use in forecasting and research. A key to progress is the ability to process so-called raw observations, such as radar reflectivity, radar Doppler velocity, and satellite-observed in- frared radiance, into meteorological information, such as three-dimensional wind velocity, temperature, and moisture. For example, fields of radial velocity from Doppler radars have been combined with data from mul- tiple wind profilers to derive upper-atmospheric wind fields having much higher horizontal resolution than do those obtainable from traditional wind-observing systems. Similarly, techniques to process satellite infrared soundings have improved, yielding higher horizontal resolution of temper- ature and moisture fields. 13
14 The new wind profiler provides an improved capability to observe winds through the troposphere and lower stratosphere. These wind measurements are normally derived every hour, although for special studies it is possible to increase the temporal resolution to 6 minutes. A demonstration network of such profilers, now being established in the central United States (Figure 2.1), will vastly increase information on the kinematic structure of the atmosphere. The next-generation weather radar (NEXRAD) system network will shortly become operational and will cover most of the continental United States (Figure 2.2). NEXRAD radars are over 100 times more sensitive than existing network radars and can detect both liquid- and ice-phase precipitation much more effectively. Therefore, the NEXRAD radars will be far more useful than existing systems in defining wintertime snowstorms. The enhanced sensitivity of NEXRAD systems also permits the measure- ment of air motions in parts of the boundary layer where precipitation is not occurring. Recent research indicates that these measurements will be useful in detecting the initiation of convective storms as much as 2 hours before the first clouds appear. NEXRAD system capabilities are summarized in Appendix Table A.1. Another area of significant progress has been the development of data- assimilation procedures, which allow the integration of data of differing accuracies and spatial and temporal resolutions into consistent meteorolog- ical fields. Workshops on data assimilation have been held, and a national effort is being coordinated to bring together data from both standard syn- optic and nonstandard heterogeneous sources. Computerized atmospheric models are planned that will integrate diverse observational data in a phys- ically and dynamically consistent manner, resulting in improved accuracy and predictive capability, but much additional research and development are needed to achieve this goal. The prospects for rapid technological progress in the observing and processing of mesoscale meteorological data are bright, and new ideas continue to be proposed. The Radio Acoustic Sounding System (RASS) is a simple and inexpensive addition to a radar wind profiler that produces very accurate measurements of temperature in the lower several kilometers of the atmosphere. The Doppler velocity of air motion excited by an acoustic wave is measured by a wind-profiling radar. The measurement gives the speed of sound as a function of height . Since the speed of sound is directly related to air temperature, computations of temperature with accuracies from 0.15 to 0.30Â° C are possible. Excellent vertical and temporal resolution allows delineation of important lower-tropospheric structures such as temperature inversions and fronts. The Japanese have used a powerful RASS to obtain accurate temperature soundings from the surface to an altitude of over 20 km.
16 Another promising technique is the use of interferometers to increase the amount of meteorological information that can be derived from infrared radiation measurements. Plans have been proposed that would use the GOES NEXT satellite platform in the late 1990s to sense radiation at infrared wavelengths in over 2000 channels rather than the 20 channels currently used. This 100-fold increase in channels will permit construction of vertical profiles of atmospheric temperature and moisture with much greater resolution than is currently possible. Infrared interferometers may also be very cost-effective as ground-based thermodynamic profilers. Advanced airborne Doppler radars are being developed to observe, over great distances, the evolution of mesoscale systems over both land and oceans. The latest design synthesizes three-dimensional air-motion fields through the use of innovative antenna scanning while the aircraft flies along a straight-line track safely outside of severe storms. A high- altitude, downward-pointing airborne radar design will measure vertical air motions in unprecedented detail. Advanced ground-based radar techniques use polarization diversity for discrimination of the ice and water phases, detection of hail, and better estimation of very heavy rainfall. Other remote sensing techniques being investigated include the mea- surement of temperature, moisture, and winds from ground-based lasers. Significant advances in laser technology promise much more powerful lasers at greatly reduced cost. A summary of the principal new observing system technologies and their current status is given in Appendix Table A.2. Clearly, the new observing technologies described above require con- comitant advances in data-processing technologies. The increase in com- puter performance per unit of cost during the 1960s and 1970s has main- tained its pace in the 1980s. Industry experts predict that personal com- puters with power comparable to that of the CRAY-1 will be available before the mid-1990s. Atmospheric analysis and prediction are particularly amenable to parallel processing, in which different parts of a program are run simultaneously on different processors. There are commercially available computers with high-speed performance in the range of several gigaflops (109 floating-point operations per second), and prospects are good for computers with processing capability in the teraflops range in the late 1990s. Important concurrent advances in computer software have been at least as important as improvements in hardware in increasing the speed and utility of computers. Examples include faster routines to solve elliptic boundary value problems, vectorization of computer codes, and algorithms for parallel processing of computer programs.
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18 NATIONAL WEATHER SERVICE MODERNIZATION Beginning in 1990, a next-generation series of geostationary weather satellites will provide improved continuous weather surveillance of North America and its surrounding oceans. This will be supported by an even more advanced polar-orbiting weather satellite system that will begin op- erations in 1992. On the ground, the National Oceanic and Atmospheric Administration (NOAA) will deploy a network of wind-profiling radars that will provide data in unprecedented detail on the small-scale structure of intense atmospheric phenomena. Combined with temperature and mois- ture data from satellites and balloons, wind profiler observations will lead to improved forecasts of local weather. Critically important for the detection of locally violent weather is the NEXRAD system, which is capable of sensing hazardous wind shears, tornadoes, and downburstsâmajor causes of loss of life and property in the United States. The implementation of the NEXRAD network will begin in 1990 and is to be completed by 1995. In order to exploit the new observing systems, NOAA will imple- ment a sophisticated data communication, analysis, and product display system in the 1990s. The Advanced Weather Interactive Processing System (AWIPS-90) will enable rapid communication and video display of weather observations and information to forecasters for the timely issuance of storm warnings. Also, to ensure the most effective use of the new technology, an organizational restructuring of the National Weather Service (NWS) is in progress. A major purpose of NWS modernization is to improve weather warn- ings. The implementation of AWIPS-90, NEXRAD, and new space-based sensors on the planned GOES NEXT geostationary satellite is amply jus- tified by the much-improved weather warnings these new systems will facilitate. Additionally, these same systems will serve as the basis for im- proving short-term forecasts, both directly and through their contribution to research studies that lead to improved weather predictions and warnings. Research results based on these new observing systems will permit trained forecasters to use appropriate conceptual models of mesoscale weather systems. The conceptual models help the forecaster understand and make proper use of weather depictions produced by the AWIPS-90 integrated displays. Additionally, the results of research will allow the new observations to be incorporated properly into a variety of diagnostic and predictive computer models, ranging from comparatively simple algorithms to comprehensive high-resolution mesoscale models. In particular, a new operational mesoscale-a model with a domain that covers most of North America is planned at the National Meteorological Center as part of the overall NWS modernization. This model will have 80-km horizontal
19 resolution, consist of 16 layers, and contain the best available physical and topographical representations. This and other models will provide the basis for significant improvements in weather forecasts and warnings. Thus the full value of the NWS modernization will be realized only by pursuing a vigorous national mesoscale research program directed toward the optimal use of the new systems. DEVELOPMENT OF NEW MODELS Along with the huge increase in computer power during the past 3 decades has come a commensurate increase in numerical modeling activity. This increase is particularly evident in mesoscale meteorology, where mod- els have been developed to investigate a multitude of different mesoscale phenomena. Regional models with domains ranging from a single state to the entire United States are able to simulate with fidelity hurricanes, thun- derstorm squall lines, and other mesoscale convective systems. They also can simulate mesoscale features such as precipitation bands, sea breezes, regions of stratiform precipitation, frontal zones, orographic systems, polar lows, mountain-valley circulations, lake-induced vortices, internal gravity waves, and areas of heavy precipitation. The large breadth and extent of mesoscale modeling activity are evident from the great number and diver- sity of mesoscale models active in 1988 (Appendix Table A.3). It is clear that modeling has become an important tool in mesoscale research. Recently some regional-scale models have been modified to assimilate continuing observational data, such as profiler winds, during a forecast period. These models have provided much-needed guidance for the design of data-assimilation routines and special observing networks. Other recent efforts in mesoscale modeling have focused on the refinement of existing computer models in order to better simulate individual storm systems. In particular, models of cumulus clouds have revealed subtle but complex interactions between gravity waves in the stable troposphere and the orga- nization of clouds in the boundary layer. Such models have also produced realistic simulations of microbursts and low-level wind shear. Three-dimensional models of cumulonimbus clouds have been used to simulate thunderstorm downdrafts, downbursts, and gust fronts pro- duced by the outflow from downdrafts near the surface. They have also been used to clarify the processes involved in the formation of rotating thunderstorms and to study the effects of thunderstorm rotation on the formation of tornadoes. Some of these models now also include detailed microphysics, for example, the physics of ice-phase changes and hail for- mation. Electrification processes have been added to some cloud models so that the interaction of cloud-charging processes with storm motions can be examined. Other models include chemical reactions that occur in clouds
20 and precipitation. Explicit simulation of radiative heating and cooling rates associated with clouds has been added to some mesoscale models. These models have revealed that convective circulations are highly sensitive to radiative effects. Cloud models have also been expanded in horizontal scale to allow, for example, simulation of squall-line thunderstorm systems and mesoscale convective systems over the Rocky Mountains. Recent in- creases in computer speed and capacity have made possible the use of nonhydrostatic models to simulate convection on such mesoscale domains. A new development in cloud and mesoscale modeling is the use of interactive, grid-nesting techniques. This approach allows larger scales of motion to affect smaller scales, and vice versa, and it permits otherwise impossible model simulations to be made by economically increasing res- olution in the principal region of interest. Using this technique, regional models on the scale of the United States are designed to interact with finer-resolution models of motion on the scale of squall lines and mesoscale convective complexes. Because interactive grid-nested models allow smaller scales of motions to affect larger scales, the impact of mesoscale convective systems on larger scales of motion can be examined. Interactive grid-nesting techniques have also been useful in elucidating some of the fundamental physical processes involved in the entrainment of environmental air into individual cumulus clouds. In recent years global-scale general circulation models (GCMs) have obtained sufficiently high resolution to permit exciting opportunities for studying interactions between global scales of motion and mesoscale weath- er systems. For example, the European Centre for Medium Range Weather Forecasts (ECMWF) has a global numerical weather prediction model with sufficient resolution for the routine prediction of relatively large mesoscale phenomena, for example, mesoscale convective complexes (MCCs). At the same time, cloud models have been expanded in scale so that they include much of the mesoscale domain. Thus we are entering a period when power- ful new modeling approaches can be used to increase our understanding of storms and mesoscale phenomena. Despite these promising developments, much additional research on modeling the mesoscale domain needs to be done in order to translate the new developments into models useful for operational mesoscale weather prediction. ADVANCES IN FUNDAMENTAL UNDERSTANDING Using sophisticated new observing tools, advanced high-speed com- puters, and complex computer models, meteorologists have advanced our understanding of storm structure and dynamics and the physical processes leading to precipitation and severe weather. (See Appendix Table A.4, which summarizes the principal U.S. mesoscale observational studies and field programs that have led to these advances.)
21 A greater understanding has been obtained about the processes in- volved in the formation and evolution of mesoscale convective storms. Synoptic features such as jet streaks, short waves in the global circulation pattern, low-level jets, surface fronts, and dry lines have been shown to play an important role in storm initiation. At the same tune, processes of storm initiation and evolution, such as the growth of convective cloud systems by individual cloud mergers, the behavior of intersecting gust fronts and gravity waves emitted by neighboring clouds, the development of large cold pools by evaporation of precipitation in downdrafts, the systematic release of latent heat in the upper troposphere by anvil clouds, and the role of topographic features such as mountains and land-water thermal contrasts, are all now better understood. Much has been learned about the structure of individual convective storms and the processes governing storm intensity. The importance of nonhydrostatic vertical pressure gradients in driving convective updrafts and downdrafts is now recognized. Likewise, the contribution of vertical wind shear (change in horizontal wind speed and direction with height) to the development of nonhydrostatic vertical pressure gradients in storms and in establishing rotating storms has been clarified. Recent research has shown that boundary layer forcing is frequently the dominant mechanism for convective storm initiation. Progress has been made in understand- ing the genesis of tornadoes and strong surface winds, the formation of hailstones, and the relationship between thunderstorm dynamics and cloud electrification processes. Significant advances have also been made in understanding the struc- ture of mesoscale convective storm systems. Observational analysis and numerical simulation of squall lines have revealed that lines in the tropics and midlatitudes exhibit many common features. Both consist of a line of vigorous convective cells and showers along with widespread regions of steady rainfall. However, in the midlatitudes, where winds usually in- crease in speed and change direction with height, some squall lines exhibit features distinctly different from those of squall lines in the tropics. In the midlatitudes, a line of convective cells may consist of one or more persistent, severe, rotating cells along with more transient ordinary cells typical of those found in the tropics. The presence of persistent, rotating cells alters the kinematics of a squall line and increases the likelihood of severe weather such as tornadoes, large hail, and strong straight-line winds occurring with it . A better understanding of the organization and structure of MCCs has also been achieved in recent years. An MCC is a comparatively large mesoscale weather system composed of both convective cloud regions and stratiform cloud zones. Seen from satellites, the MCC is characterized by a large, circular high (cold) cloud shield that may be several hundred kilometers in diameter (see Figure 1.1) and may persist at this size for 6
22 hours or more. The system frequently includes one or more squall lines and often exhibits a warm core vortex structure in the stratiform cloud region. In some instances, the vortex grows large enough and persists long enough to become incrtially stable and is instrumental in initiating several new mesoscale convective systems over a period of several days. MCCs typically produce heavy rains and prolific lightning, and occasionally tornadoes and haiL Some 25 percent or more of the MCCs produce severe straight-line winds in swaths that may be 50 to 100 km in width and several hundred kilometers in length. Progress has been made in understanding the role of radiative cooling at the tops of clouds in the formation and intensification of mesoscale convective systems. Also, the effects of gravity waves, excited by deep convective clouds penetrating the relatively stable middle and upper tropo- sphere, on mesoscale convective systems are better understood. Interactions of gravity waves generated by neighboring clouds are now believed to be important in the growth of convective systems. In general, mesoscale wind, cloud, and precipitation features are ini- tiated by two mechanisms: (1) instabilities in the larger-scale environment and (2) forcing by inhomogeneities at the surface (such as terrain features). Recent observations and model simulations have revealed a great deal about mesoscale instabilities in the larger-scale environment. For example, mesoscale precipitation bands, associated with locally intense precipitation, form in both extratropical and tropical cyclones, and evidence has accu- mulated that many of the bands are manifestations of gravity waves or conditional symmetric instability (CSI). It is now recognized that CSI is capable of generating strong vertical motions of several meters per second. Such motions can account for the anomalously heavy snow bands associated with extratropical cyclones. Terrain-forced phenomena, such as cold air damming, are also better understood. For example, subcloud-layer evaporational cooling not only enhances the thermodynamic conditions favorable for damming but also helps to establish a "wedge ridge" in the cold air. Rotationally trapped Kelvin waves can be instrumental in initiating cold surges that result in cold air damming. Sometimes the two general mechanisms for producing mesoscale fea- tures are combined. This is the case with amplifying mountain waves that produce severe downslope windstorms. Recent studies have shown that, in addition to atmospheric thermal and wind stratifications, vertical moisture stratification is also important in the formation of mountain waves. Progress has also been made toward understanding the processes in- volved in the genesis of intense cyclonic storms along coastal areas and in the lee of major mountain barriers. These storms often result in excessive snowfall and rainfall. In recent years field research programs along both the east and west coasts of the United States have revealed the potential
23 importance of scale-interaction processes in the explosive development of extratropical cyclones that produce severe winter storms. Besides confirm- ing the role of large-scale, upper-level waves in intensification of surface cyclones, such studies have shown the important contribution of mesoscale processes to the strength of a storm. Intensification of coastal winter storms has been related to energy transfers from the warm ocean surface, latent heat liberated in convective and stratiform clouds, and dynamic instabilities associated with both lower- and upper-tropospheric wind maxima. Scale- interactive mesoscale processes can cause severe winter storms to develop in a surprisingly short period of time. Although considerable progress has been made in the scientific un- derstanding of mesoscale weather processes and in their simulation by numerical models, there are many areas where additional study is required to gain sufficient understanding and simulation capability that, in con- junction with the new observing capabilities, should lead to significantly improved forecasts and warnings. The step from understanding to accurate prediction is large and will require a focused national effort, involving both research and operations, if it is to be achieved. RELATIONSHIPS TO CLIMATE AND ATMOSPHERIC CHEMISTRY The scientific challenges of mesoscale meteorology are linked to a wide range of other underlying problems in climate and atmospheric chemistry. Mesoscale weather systems transfer momentum, energy, and water vapor horizontally and vertically; thus they are important components of the global atmospheric circulation. The latent heat realized through moist convection is an important source of energy in the maintenance of the global circulation. Moreover, mesoscale weather systems are the dominant precipitation-producing systems in the tropics and much of the summer midlatitudes; thus they are major components in the global hydrological cycle. They are a link between tropical sea-surface temperature anomalies and anomalies in the global atmospheric circulation. A key factor for predicting global climate is the impact of cloudi- ness on the radiation budget of the earth-atmosphere system. Mesoscale weather systems often produce extensive layers of clouds that can persist for days, long after the systems themselves decay. Such residual clouds, along with clouds produced by active weather systems, greatly alter the ra- diative properties of the troposphere and, consequently, the global energy budget. Tbgether with the release of latent heat, the radiative heating of layered clouds in the upper tropical troposphere is a significant source of energy for driving the global circulation. Moreover, changes in the global composition of atmospheric aerosol particles can alter the concentration of
24 cloud droplets and ice particles, which in turn affect the radiative budget of the atmosphere. A detailed understanding of the behavior of mesoscale weather systems will contribute substantially to better formulations of the physical processes in general circulation models and to a better understanding of past, present, and future climates, including the influence of human activities on climate. Convective systems also transport large amounts of trace chemical species, such as sulfur dioxide, nitrogen oxides, ozone, methane, and chlo- rofluorocarbons, from the earth's boundary layer to the free atmosphere above. In the process of transporting boundary layer air upward, the con- vective systems bring water, ice, and lightning into proximity with trace chemicals and act as atmospheric chemical reactors to process and trans- form the chemicals. High in the atmosphere, the chemical species and reactants disperse and may be transported over great distances by jet streams. The large amounts of residual pollutants in the middle and up- per troposphere that are dispersed around the globe significantly alter the global chemistry of the troposphere. In the polar stratosphere, reactions involving the chlorine that derives from chlorofluorocarbons take place in the presence of ice crystals, thereby reducing the concentration of the earth's protective ozone. The deposition of acidic precipitation over a region is frequently dom- inated by a few major mesoscale events. Organized mesoscale circulations, such as clusters of individual mesoscale convective clouds within extrat- ropical cyclones, can sweep large volumes of polluted air into regions of moist convection, where the pollutants are transported aloft and scavenged through precipitation. One such system can scavenge enough acidic con- taminants to dominate the annual average chemistry of a given watershed or local ecological system. Lightning is also an important contributor to the production of certain chemical species such as nitrates and ozone. An assessment of the global chemical budget of these species requires a knowledge of the frequency of lightning events associated with different weather systems. Recent research has revealed that, for some areas, a single large mesoscale convective complex can account for as much as 25 percent of the total annual lightning occurrences in the area. A detailed description of mesoscale weather systems will aid in the diagnosis and prediction of chemical distributions and reactions and of acidic precipitation. Conversely, detailed atmospheric measurement of trace chemical species can provide information on air motions within and around mesoscale storm systems.