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Bernard A. Walter

Abstract

NOAA-4 satellite photos of roll clouds over the Bering Sea are used to derive quantitative information as to how the interval between the clouds (λ) changes with distance south of the edge of the ice pack. Radiosondes are utilized to obtain values of the boundry-layer height (h) at St. Paul Island. The magnitude of /h is calculated for a number of cases and is compared to that expected from theory. A number of values obtained (6–7:1) differ significantly from the normally expected values (2–3:1) for circular roll vortices.

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Steven Businger and Bernard Walter

Abstract

The NOAA P-3 aircraft was used to collect data in a genesis region for mesoscale comma clouds over the Gulf of Alaska. Aircraft measurements in the genesis region showed that rainbands with spacings of 65–75 km and orientations along the mean wind shear were present. Possible mechanisms for the formation of the rainbands, including conditional symmetric instability (CSI) and modified wave-CISK were investigated, but the data did not allow the formation of the rainbands to be conclusively ascribed to a particular mechanism. The existence of low static stability in the genesis region was also documented and its role in mesoscale comma-cloud development explored.

Careful analysis of images from NOAA polar orbiter and GOES satellites together with synoptic analyses made it possible to trace the life cycles of several mesoscale comma clouds as the genesis region moved across the Gulf of Alaska. As the genesis region approached a preexisting polar frontal cloud band, a wave cyclone formed on the front and absorbed one of the comma clouds. The resulting cyclone central pressure dropped 25 mb in 12 hours. The intensity of this development was underestimated by operational forecast models.

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Bernard A. Walter Jr. and James E. Overland

Abstract

The behavior of stratified air flowing around an isolated mountain is dependent on an internal Froude number (F), which indicates the relative importance of upstream velocity and vertical stratification. Three cases of the flow in the lee of the Olympic Mountains in the State of Washington are studied where the measured F was in the range 1.0–1.4 but apparently dominated by stable stratification. This study combined measurements of spatial variation of low-level winds and other parameters from a NOAA P-3 research aircraft with a dense network of surface stations including eight meteorological buoys and six upper-air stations. Results from these cases show the presence of an area of light winds in the lee of the Olympic Mountains. The characteristics of the flow are shown to be similar to laboratory results for low Froude number flow around an isolated obstacle where the flow is confined to quasi-horizontal planes. These cases are contrasted with a situation which led to the formation of a mesoscale low-pressure area and high surface winds in the lee of the mountains. The latter case was the Hood Canal Bridge storm on 13 February 1979 where local winds in the lee of the Olympic Mountains were in excess of 50 m s−1. The flow at the surface was produced by down-pressure-gradient acceleration in the confined channels of Puget Sound toward the orographically produced low-pressure center. The measured internal Froude number in this situation was 4.6, and the pressure fields are shown to agree with the linear hydrostatic model developed by Smith (1980) for F > 1. It is suggested that the Froude number calculated from routine, upper-air sounding data is an index that forecasters can use to determine the potential for severe wind conditions over the inland waters of Puget Sound.

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Pierre D. Mourad and Bernard A. Walter

Abstract

The existence of synoptically distributed, coherent, linear mesoscale features with wavelengths of 12–18 km in a shallow (z l ≈ 150 m) atmospheric boundary layer is documented. These were observed north of Alaska over the ice-covered Beaufort Sea during the LEADEX program in April 1992. These banded features appear both in satellite infrared (but not visible) images and in concomitant in situ aircraft measurements. Those measurements were of cloud condensation nuclei (CCN), potential temperature (θ), and meridional velocity (v) and were taken within and above the arctic atmospheric boundary layer. These aircraft data also exhibit smaller-scale circulations with scales of 3–8 times the boundary layer depth. Based on analysis of our dataset, we argue that the 12–18-km motions may be due to a hybrid form of slantwise convection within the boundary layer. The authors use the term hybrid because some of the energy, scale selection, and orientation of the linear features may be modulated by the nonlinear mean wind profile in the boundary layer. The strongest arguments for slantwise convection are as follows: 1) a significant meridional (cross-band) flux of beat and CCN; 2) long-wavelength, two-dimensional circulation patterns aligned perpendicular to the strong, horizontal temperature gradient; 3) thin, warm bands parallel to thick, cold bands in the IR image, consistent with convection in the boundary layer; and 4) generally weak correlations between lead signals in a downward-looking radiometer and θ, v, and CCN. The data also suggest that at least the influence of the circulations can reach up beyond the well-mixed boundary layer into the stable, lower troposphere. (However, this signal cannot be dismissed as, nor definitely identified with, gravity waves.) It is noted that if slantwise convection is present as described, then it represents another mechanism with mesoscale organization over synoptic-scale regions by which the Arctic's atmospheric boundary layer and the overlying, stably stratified lower troposphere may exchange heat, momentum, and particulates. This is in addition to large leads and shear-generated turbulence in the boundary layer, both of which create vertical mixing in the Arctic's lower atmosphere that is spatially and temporally intermittent.

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Bernard A. Walter Jr. and James E. Overland

Abstract

Aircraft and satellite data an used to study the structure of longitudinal roll vortices in a nearly neutral (zi/L=-1.2, where zi is the inversion height and L is the Monin-Obukhov length) boundary layer over the ice-covered Bering Sea during February. Steam fog, formed over cracks and leads in the ice, was used as a tracer to delineate the various scales of roll motion seen in satellite images. The satellite information combined with aircraft data collected by the NOAA P-3 indicated the presence of a hierarchy of roll vortex motions. It is suggested that interactions of the various scales of motion resulted in certain scales dominating in one area and other scales dominating in another. Two-kilometer wavelength variations an attributed to the inflection point instability mechanism while 12–15 km variations seen to have been reinforced by the upstream topography on the Chukotka Peninsula. Organization of the fog banks on scales of 30 km was also present and may be attributable to resonant subharmonics of the basic boundary layer instability or to a mesoscale entrainment instability.

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Nicholas A. Bond and Bernard A. Walter

Abstract

A NOAA P-3 research aircraft collected measurements in a storm off of the U.S. West Coast that featured a low-level jet with wind speeds approaching 50 m s−1. These measurements have been used to document mean and turbulent boundary layer structures in the vicinity of the jet along a pair of transects (∼150 and ∼25 km off the coast). Large magnitudes of turbulent kinetic energy (TKE) and a lack of thermal wind balance were found in the region of the jet, indicating that mixing was an important process. The turbulent fluxes of buoyancy were generally negative, implying that the turbulence near the jet was generated by wind shear. The observed momentum fluxes associated with this wind shear were compared with a TKE-based parameterization for these fluxes. In certain locations, the momentum fluxes were up the vertical wind shear gradient, whereas the parameterization specifies that these fluxes should be down the gradient. Nevertheless, in general the observed and parameterized momentum fluxes agreed favorably, and this agreement was improved with the inclusion of factors related to the magnitude of the TKE and buoyancy effects. The terms in the TKE budget equation were evaluated, revealing substantial variations in shear production and even larger variations in the turbulent transport of TKE. These variations probably are responsible for the highly patchy distribution of TKE in the vicinity of the jet. Because relatively few turbulence measurements have been collected in such strong low-level winds, this case represents a unique opportunity for validating the boundary layer processes simulated by numerical weather prediction models in the storm environment.

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James E. Overland and Bernard A. Walter Jr.

Abstract

Gap winds can be defined as a flow of air in a sea level channel which accelerates under the influence of a pressure gradient parallel to the axis of the channel. In February 1980 two distinct cases of gap winds were observed in the Strait of Juan de Fuca between western Washington State and British Columbia during a study that measured spatial variation of low-level marine winds and other parameters from the NOAA P-3 research aircraft and a dense network of surface stations which included eight meteorological buoys. These two cases were a high-pressure region over central British Columbia and a low-pressure system propagating northward, seaward of the Washington coast. Both cases produced strong easterly winds of 13–15 m s−1 at the western end of the Strait of Juan de Fuea. The high-pressure region provided a drainage air mass from the interior of British Columbia which flowed through the Straits of Georgia and Juan de Fuca and eventually into the Pacific Ocean. This air mass remained nearly homogeneous and was capped by a well-defined inversion. For the offshore low-pressure center, the lower atmosphere was stably stratified throughout the region, and weak winds were observed at the eastern end of the Strait of Juan de Fuca with strong winds at the western end. Although the features of the flow fields were complex, major characteristics of the wind fields can be accounted for by the combined effect of topography and the synoptic pressure field. Local winds were in approximate ageostrophic equilibrium between the inertia term and the imposed sea level pressure gradient.

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Wendell Brown, Walter Munk, Frank Snodgrass, Harold Mofjeld, and Bernard Zetler

Abstract

Pressure fluctuations on the deep seafloor at frequencies below inertial and tidal have been measured. Between 0.1 and 1 cycle per day the variance is about 2 mb2, spectra diminish with increasing frequency as ωn, n=1.5 to 2, and a signal-to-instrument noise ratio of 10 dB is achieved. Fluctuations are in phase and highly coherent within the MODE area (>0.95 at 200 km) and even with inferred (atmosphere plus sea level) Bermuda subsurface pressures (0.8 at 700 km). Station differences (to which MODE-sized eddies would make the principal contribution) are relatively small. The large horizontal scale of the recorded bottom pressure fluctuations resembles that of atmospheric pressure, yet the coherence locally between atmospheric and bottom pressure is slight; the recorded fluctuations may be related to a barotropic ocean response to a variable wind stress on the subtropical gyre. Bottom temperature records show “sudden” (1 day) changes of order 30 millidegrees Celcius separated by long intervals (20 days) of uniform temperatures. The changes are much larger than have been observed in the Pacific. They are correlated at horizontal separations of 2 km, but uncorrelated to bottom pressure and to temperatures 1 km above the seafloor.

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Bernard Zetler, Walter Munk, Wendell Brown, Florence Dormer, and Harold Mofjeld

Abstract

IGPP and AOML bottom pressure measurements at four MODE stations constitute a unique set of deep-sea tidal measurements (although deployed for other purposes). A response analysis relative to a Bermuda reference has been optimized with regard to the number of complex weights and the makeup of gravitational and radiational inputs. Duplicate instrumentation on EDIE capsule gave 32.067, 2.5° 32.074, 2.6° for M2 amplitude (cm) and Greenwich epoch, thus attesting the reality of measured small station differences (order 1 cm, 1°). M2 tidal currents (calculated from the M2 surface and bottom slopes) have u and v speeds of 0.5 and 0.8 em s−1, respectively, in rough agreement (both amplitude and phase) with preliminary estimates from current measurements. M2 and K1 tides are in accord with sonic existing cotidal and co-range charts. M2 tides are a fraction of equilibrium magnitude, whereas M4, M5 and M6 (typically 0.07, 0.05, O.03 cm) vastly exceed equilibrium values. Presumably these overtides are generated by nonlinear coupling in the world's shallow basins, from where they radiate into the global oceans to attain a level where radiative and dissipative processes are somehow balanced.

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David Atlas, Bernard Walter, Shu-Hsien Chou, and P. J. Sheu

Abstract

The combination of vertical lidar and in situ meteorological observations from two aircraft provide an unprecedented view of the marine atmospheric boundary layer (MABL) during a cold air outbreak. To a first approximation, the lidar reflectivity is associated with the concentration of sea salt aerosols. Across the capping inversion, the lidar reflectivity contours approximate isentropes and streamlines thereby defining the inversion. Within the mixed layer, high reflectivity cores are associated with updrafts carrying aerosol-rich air upward and conversely. These effects are enhanced by increasing humidity in updraft and decreasing humidity in downdrafts that operate to increase and decrease aerosol sizes. Narrow high reflectivity columns extend upward from the ocean indicating that organized flow exists all the way to the surface. Entrainment across the inversion is manifested by small scale perturbations (∼200–500 m) superimposed upon the large scale (&sim 1–2 km) undulations of the inversion. These occur where the local entrainment zone is sharpest; generally, this is on the upshear side of the, convective. domes where Kelvin-Helmholtz instability is triggered by local compression of the inversion,

The MABL on 20 January 1983 is highly organized. The organization takes the form of 1–2 km scale roll vortices and corresponding undulations of the inversion with amplitude of 150–200 m peak to trough. The roll circulation is very strong with up and downdrafts of 2–4 in s−-1 at the 210 m level. The axes of the rolls are essentially north-south along the direction of the strong northerly low-level winds. The rising arm of the roll coincides with a column of high lidar reflectivity and with the updraft which transport aerosols, moisture, and heat up from the surface. The presence of the rolls, driven mainly by the combination of strong transverse sheer and buoyancy, serves to produce low-level convergence which concentrates the small-scale buoyant eddies to form a single well-ordered updraft in the manner previously postulated by LeMone.

The fluxes measured by the covariance method in the undulating inversion are unreliable because of the sensitivity to detrending and inadequate sampling of the exchanges across the interfaces of the dames and troughs. The partitioning method of Wilczak and Businger provides improved insight as to the mechanisms responsible for the downward flux in the inversion. However, unlike Wilczak and Businger, who find the downward flux dominated by cold updrafts we find that it is due mainly to the entrainment of warm eddies which are then transported downward by the larger-scale roll circulations on the downshear side of the domes.

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