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Ronald B. Smith

jets, gap jets, wakes, thermally driven slope winds, and cold pools. In section 5 , orographic precipitation is summarized. In section 6 , we describe the generation of mountain waves that propagate deep into the upper atmosphere. In section 7 , we consider the global effects of mountains on climate over Earth’s history. 2. Atmospheric reference heights: How high is a mountain? Mountains are usually ranked by their peak heights. Citizens take pride in their nation’s highest peaks. Climbers

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Mark P. Baldwin, Thomas Birner, Guy Brasseur, John Burrows, Neal Butchart, Rolando Garcia, Marvin Geller, Lesley Gray, Kevin Hamilton, Nili Harnik, Michaela I. Hegglin, Ulrike Langematz, Alan Robock, Kaoru Sato, and Adam A. Scaife

are caused by adiabatic heating and cooling processes, which are driven by waves ( Leovy 1964 ). The three principal theoretical paradigms that are applied to middle atmosphere dynamics are as follows: 1) wave propagation, 2) wave mean–flow interaction, and 3) the mean overturning circulation response to radiative forcing and wave driving. The most important wave modes for middle atmosphere theory are atmospheric gravity waves (see section 6 ), whose restoring force is buoyancy due to gravity and

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Isaac M. Held

speeds become comparable to characteristic eddy velocities. At this scale, the inverse cascade becomes very anisotropic, with energy flowing into zonal jets. In this regime the flow can be characterized as consisting of nonlinear waves energizing jets, not a bad qualitative description of extratropical atmospheric flows. Scaling arguments based on the Rhines scale as a mixing length have been developed by Held and Larichev (1996) , for example, and explain some aspects of idealized two

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Kenneth S. Gage and Earl E. Gossard

Abstract

This review begins with a brief look at the early perspectives on turbulence and the role of Dave Atlas in the unfolding of mysteries concerning waves and turbulence as seen by powerful radars. The remainder of the review is concerned with recent developments that have resulted in part from several decades of radar and Doppler radar profiler research that have been built upon the earlier foundation.

A substantial part of this review is concerned with evaluating the intensity of atmospheric turbulence. The refractivity turbulence structure-function parameter C 2 n , where n is radio refractive index, is a common metric for evaluating the intensity of refractivity turbulence and progress has been made in evaluating its climatology. The eddy dissipation rate is a common measure of the intensity of turbulence and a key parameter in the Kolmogorov theory for locally homogeneous isotropic turbulence. Much progress has been made in the measurement of the eddy dissipation rate under a variety of meteorological conditions including within clouds and in the presence of precipitation. Recently, a new approach using dual frequencies has been utilized with improved results.

It has long been recognized that atmospheric turbulence especially under hydrostatically stable conditions is nonhomogeneous and layered. The layering means that the eddy dissipation and eddy diffusivity is highly variable especially in the vertical. There is ample observational evidence that layered fine structure is responsible for the aspect sensitive echoes observed by vertically directed very high frequency VHF profilers. In situ observations by several groups have verified that coherent submeter-scale structure is present in the refractivity field sufficient to account for the “clear air” radar echoes. However, despite some progress there is still no consensus on how these coherent structures are produced and maintained.

Advances in numerical modeling have led to new insights by simulating the structures observed by radars. This has been done utilizing direct numerical simulation (DNS) and large eddy simulation (LES). While DNS is especially powerful for examining the breaking of internal waves and the transition to turbulence, LES had been especially valuable in modeling the atmospheric boundary layer.

Internal gravity waves occupy the band of intrinsic frequencies bounded above by the Brunt–Väisälä frequency and below by the inertial frequency. These waves have many sources and several studies in the past decade have improved our understanding of their origin. Observational studies have shown that the amplitude of the mesoscale spectrum of motions is greater over mountainous regions than over flat terrain or oceans. Thus, it would appear that flow over nonuniform terrain is an important source for waves. Several numerical studies have successfully simulated the generation of internal waves from convection. Most of these are believed to result from deep convection with substantial wave motion extending into the upper troposphere, stratosphere, and mesosphere. Gravity waves known as convection waves are often seen in the stable free atmosphere that overlay convective boundary layers.

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Yukari N. Takayabu, George N. Kiladis, and Victor Magaña

realized that when enough data became available “to establish a physical interpretation of the time series data, we have to analyze the spectral estimates into different wave modes in a manner similar to the analysis of atmospheric tides” ( Yanai and Murakami 1970a , p. 196) as was done, for example, by Longuet-Higgins (1968) . This statement foresaw the matching of theoretical dispersion properties of equatorial waves to space–time characteristics of observed fields. 5. Convection and equatorial

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Baode Chen, Wen-wen Tung, and Michio Yanai

convergent in Fig. 8-17d , likely reflecting the distinction between the geostrophic (Rossby) mode and the inertia–gravity mode of atmospheric motions. This gradual transition is more obvious in the Eastern Hemisphere than the Western Hemisphere where background westerlies permit extratropical Rossby waves to propagate through (cf. Fig. 8-5 ). In the total and 30–60-day period ranges ( Figs. 8-17a and 8-17b ), two rotational centers straddle the equator over the Indian Ocean. From the Pacific to the

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Guoxiong Wu and Yimin Liu

of the eastern Asian precipitation are presented in section 8 . Perspectives on future study are given in section 9 . 2. Relative importance of mechanical and thermal forcing induced by large-scale mountains The response of the atmospheric circulation to a thermal forcing could be considered as the response to a topography with a so-called equivalent mountain height H Q ( Held 1983 ), which is inversely proportional to the intensity of the basic flow u . For stationary waves forced by a

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Lance F. Bosart and, Alicia C. Wasula, Walter H. Drag, and Keith W. Meier

Abstract

This paper begins with a review of basic surface frontogenesis concepts with an emphasis on fronts located over sloping terrain adjacent to mountain barriers and fronts located in large-scale baroclinic zones close to coastlines. The impact of cold-air damming and differential diabatic heating and cooling on frontogenesis is considered through two detailed case studies of intense surface fronts. The first case, from 17 to 18 April 2002, featured the westward passage of a cold (side-door) front across coastal eastern New England in which 15°–20°C temperature decreases were observed in less than one hour. The second case, from 28 February to 4 March 1972, featured a long-lived front that affected most of the United States from the Rockies to the Atlantic coast and was noteworthy for a 50°C temperature contrast between Kansas and southern Manitoba, Canada.

In the April 2002 case most of New England was initially covered by an unusually warm, dry air mass. Dynamical anticyclogenesis over eastern Canada set the stage for a favorable pressure gradient to allow chilly marine air to approach coastal New England from the east. Diabatic cooling over the chilly (5°–8°C) waters of the Gulf of Maine allowed surface pressures to remain relatively high offshore while diabatic heating over the land (31°–33°C temperatures) enabled surface pressures to fall relative to over the ocean. The resulting higher pressures offshore resulted in an onshore cold push. Frontal intensity was likely enhanced prior to leaf out and grass green-up as virtually all of the available insolation went into sensible heating.

The large-scale environment in the February–March 1972 case favored the accumulation of bitterly cold arctic air in Canada. Frontal formation occurred over northern Montana and North Dakota as the arctic air moved slowly southward in conjunction with surface pressure rises east of the Canadian Rockies. The arctic air accelerated southward subsequent to lee cyclogenesis–induced pressure falls ahead of an upstream trough that crossed the Rockies. The southward acceleration of the arctic air was also facilitated by dynamic anticyclogenesis in southern Canada beneath a poleward jet-entrance region. Frontal intensity varied diurnally in response to differential diabatic heating. Three types of cyclogenesis events were observed over the lifetime of the event: 1) low-amplitude frontal waves with no upper-level support, 2) low-amplitude frontal waves that formed in a jet-entrance region, and 3) cyclones that formed ahead of advancing upper-level troughs. All cyclones were either nondeveloping or weak developments despite extreme baroclinicity, likely the result of large atmospheric static stability in the arctic frontal zone and unfavorable alongfront stretching deformation. Significant frontal–mountain interactions were observed over the Rockies and the Appalachians.

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Howard B. Bluestein and Roger M. Wakimoto

Abstract

Most severe convective storms are too remote, occur too infrequently, or translate too rapidly to be resolved at high enough spatial and temporal scales by fixed-site, ground-based radars. Fortunately, the recent introduction of mobile radar platforms into the field has had a major impact on advancing our understanding of the internal structure of severe convection. These systems can be broadly divided into airborne, spaceborne, and ground-based mobile platforms. The National Oceanic and Atmospheric Administration (NOAA) P-3, Electra Doppler Radar (ELDORA) ER-2 Doppler Radar (EDOP) are examples of aircraft equipped with radars that have successfully collected data on supercell storms, tornadoes, microbursts, and intense squall lines. Spaceborne platforms might be considered of limited use for studying severe convective storms owing to their high altitude, poor temporal resolution over a particular geographic region, and narrow swath with respect to the earth. However, an example of a synthetic-aperture radar detecting microbursts over the ocean and the ability of the Tropical Rainfall Measuring Mission (TRMM) radar to provide the first global data of severe convective storms are discussed.

Ground-based radars have been used to map the wind field near and within severe convective features close to the ground at very high update cycles. Doppler spectra in tornadoes suggesting F5 wind speeds were collected by a continuous wave radar developed by the Los Alamos National Laboratory. The University of Massachusetts—Amherst built a W-band radar that was mounted in a van and later a truck. This radar was designed with a beamwidth of 0.18°, allowing for ultrahigh spatial resolution. Moreover, a polarization diversity pulse-pair technique was implemented so that the maximum unambiguous Doppler velocity was large enough to be useful in determining maximum wind speeds in tornadoes. The University of Massachusetts radar and an X-band system, developed jointly by the University of Oklahoma, the National Severe Storms Laboratory, and the National Center for Atmospheric Research and also mounted on a truck, known as the Doppler on Wheels (DOW), have collected unprecedented data on the finescale structure of tornadoes. “Eyes” and spiral bands in the radar reflectivity fields were shown to be ubiquitous. For the first time, data suggesting the existence of multiple vortices within tornadoes have been collected.

There has been a burgeoning growth of mobile radar systems that continues to this day. Two C-band systems have been built and are in the early stages of being tested. These two systems known as the Shared Mobile Research and Teaching Radars (SMART-Rs) and the Seminole hurricane hunter are both equipped with polarization diversity. These radars will be able to provide more details of the precipitation physics within severe storms and the range and velocity ambiguities will be reduced. A DOW radar capable of rapid scanning is under development by the University of Oklahoma and an X-band phased array is being converted for meteorological use by the University of Massachusetts. Other examples are provided.

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Boualem Khouider and Andrew J. Majda

. (right) Vertical structure of (a) the total heating, with the u – w velocity overlaid, and (b) the zonal velocity for the moving average of the planetary-scale envelope. [Figures 2 and 4 from Majda et al. (2007) . ©2007 National Academy of Sciences, USA.] 5. GCM simulation of the MJO and convectively coupled waves Here, we show an MJO solution produced by the multicloud model when implemented in the next generation climate model of the National Center for Atmospheric Research (NCAR), namely, the

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