Search Results

You are looking at 1 - 10 of 78 items for :

  • Waves, oceanic x
  • Meteorological Monographs x
  • All content x
Clear All
Carl Wunsch and Raffaele Ferrari

, where the ocean is a key element of the climate system. The turbulence is a mixture of classical three-dimensional turbulence, turbulence heavily influenced by Earth rotation and stratification, and a complex summation of random waves on many time and space scales. Stratification arises from temperature and salinity distributions under high pressures and with intricate geographical boundaries and topography. The fluid is incessantly subject to forced fluctuations from exchanges of properties with

Full access
Baode Chen, Wen-wen Tung, and Michio Yanai

growth of the MJO over the Indian Ocean–western Pacific warm pool, where the wave energy flux is clearly radiating upward and downward from the convective source region. In the central-eastern Pacific, where deep cumulus convection is suppressed, there are strong equatorward fluxes of wave energy from the subtropics of both hemispheres, causing horizontal convergence of wave energy flux in the equatorial upper troposphere. The results obtained from the climatological data indicate that the

Full access
Kazuyoshi Oouchi and Masaki Satoh

-moving buildup—of convection over the Indian Ocean; the inference was supported by a cloudiness data analysis in almost the same target period ( Julian and Madden 1981 ). The convection over the Indian Ocean illustrated in the schematic is decoupled from the underlying surface pressure anomaly, and the surface pressure and the low-level zonal flow are negatively correlated with each other. The negative correlation, being at odds with the eastward-propagating gravity wave dynamics, implies that some other

Full access
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.

Full access
Thomas J. Galarneau Jr.,, Lance F. Bosart, and, and Anantha R. Aiyyer

Abstract

The pioneering large-scale studies of cyclone frequency, location, and intensity conducted by Fred Sanders prompt similar questions about lesser-studied anticyclone development. The results of a climatology of closed anticyclones (CAs) at 200, 500, and 850 hPa, with an emphasis on the subtropics and midlatitudes, is presented to assess the seasonally varying distribution and hemispheric differences of these features. To construct the CA climatology, a counting program was applied to twice-daily 2.5° NCEP–NCAR reanalysis 200-, 500-, and 850-hPa geopotential height fields for the period 1950–2003. Stationary CAs, defined as those CAs that were located at a particular location for consecutive time periods, were counted only once.

The climatology results show that 200-hPa CAs occur preferentially during summer over subtropical continental regions, while 500-hPa CAs occur preferentially over subtropical oceans in all seasons and over subtropical continents in summer. Conversely, 850-hPa CAs occur preferentially over oceanic regions beneath upper-level midocean troughs, and are most prominent in the Northern Hemisphere, and over midlatitude continents in winter.

Three case studies of objectively identified CAs that produced heal waves over the United States, Europe, and Australia in 1995, 2003, and 2004, respectively, are presented to supplement the climatological results. The case studies, examining the subset of CAs than can produce heat waves, illustrate how climatologically hot continental tropical air masses produced over arid and semiarid regions of the subtropics and lower midlatitudes can become abnormally hot in conjunction with dynamically driven upper-level ridge amplification. Subsequently, these abnormally hot air masses are advected downstream away from their source regions in conjunction with transient disturbances embedded in anomalously strong westerly jets.

Full access
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.

Full access
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.

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

evidence of laterally forced equatorial Kelvin waves. For example, in Fig. 3-12 , a convectively coupled Kelvin wave signal originating over the west Pacific warm pool is preceded by an extratropical Rossby wave train originating many days earlier over South America and propagating through the southern Indian Ocean storm track. It is especially notable that this wave train is very similar to the one associated with YM waves in Fig. 3-11 . These events frequently occur even in the presence of a

Full access
Lee-Lueng Fu, Tong Lee, W. Timothy Liu, and Ronald Kwok

space was made in the late 1960s by the TIROS weather satellite program. A notable discovery from the early infrared observations of SST made by weather satellites was the tropical instability waves in the Pacific Ocean ( Legeckis 1977 ), illustrating the power of satellite observations of large-scale oceanographic phenomena. Although the infrared sensors have been improved by the Advanced Very High Resolution Radiometer (AVHRR) Program since the late 1970s, their limitation by cloud covers is a

Full access
Russ E. Davis, Lynne D. Talley, Dean Roemmich, W. Brechner Owens, Daniel L. Rudnick, John Toole, Robert Weller, Michael J. McPhaden, and John A. Barth

ocean is bigger than the atmosphere in terms of eddy scales and human movement; that it is opaque to light and radio waves; and that it has an unbreathable composition, high hydrostatic pressures, and harsh sea states. These complicate observing and increase cost. For example, harsh sea states and large oceans demand expensive large ships and crews. Indeed, large ships and crews may be why oceanography is so multidisciplinary. Most science cruises have carried projects in several areas of

Full access