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Charles W. C. Rogers

Abstract

A technique for estimating the low-level wind field from satellite photographs of cumuliform cellular cloud patterns is presented. This technique applies to areas of such patterns, which are frequently photographed behind major oceanic cyclones. The wind speed estimates are obtained by correlating the different cumuliform cellular patterns with three wind speed categories, 0–7, 8–22, and 23–37 knots. This categorization was suggested by results of laboratory experiments, performed by Chandra, in which convection occurred in a layer of air containing vertical shear. The wind direction can be estimated with at worst a ±180° ambiguity; larger scale synoptic considerations can frequently be used to eliminate this ambiguity. Illustrative examples comparing TIROS photographs of cumuliform cellular patterns and surface wind reports are presented.

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W. Erick Rogers and David W. C. Wang

Abstract

A methodology for quantitative, directional validation of a long-term wave model hindcast is described and applied. Buoy observations are used as ground truth and the method does not require the application of a parametric model or data-adaptive method to the observations. Four frequency ranges, relative to the peak frequency, are considered. The validation of the hindcast does not suggest any systematic bias in predictions of directional spreading at or above the spectral peak. Idealized simulations are presented to aid in the interpretation of results.

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CHARLES W. C. ROGERS and PAUL E. SHERR

Abstract

No Abstract Available.

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R. J. Pilié, E. J. Mack, W. C. Kocmond, W. J. Eadie, and C. W. Rogers

Abstract

Extensive measurements were made of the microphysics of valley fog in the Chemung River Valley near Elmira, New York. This paper discusses data on drop size distributions, drop concentrations, liquid water contents, and haze and cloud nucleus concentrations obtained on eight fog nights.

The behavior patterns of the microphysical variables were found to be extremely consistent. Shallow ground fog usually occurs prior to the formation of deep valley fog. The data show that ground fog is characterized by droplet concentrations of 100 to 200 per cubic centimeter in the 1 to 10 μm radius range with mean radii of 2 to 4 μm. As deep fog forms aloft, droplet concentration near the surface decreases to less than 2 cm−3 and the mean radius increases from 6 to 12 μm. Droplets of radii <3 μm disappear. Thereafter, droplet concentration and liquid water content increase gradually until the first visibility minimum at the surface when typical values range from 12 to 25 cm−3 and 50 to 150 mg cm−3, respectively. The small droplets reappear at first visibility minimum. Subsequently, bimodal drop size distributions occur in approximately half of the fogs with one mode at 2–3 μm radius and a second mode between 6 and 12 μm. Aloft, drop size distributions become narrower and the mean radius decreases with both increasing altitude and increasing age of the fog. The cloud nucleus concentration active at S = 3.0% is usually between 800 and 1000 cm−3 near the surface and decreases to 500–800 cm−3 at 300 m.

It is argued from the data that supersaturation in the thin ground fog exceeds that in deep fog. The initial surface obscuration in deep fog appears to be due to droplets that form aloft and are transported downward into unsaturated air by turbulent diffusion. New droplets are apparently not generated near the surface until after the first visibility minimum.

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R. J. Pilié, E. J. Mack, W. C. Kocmond, C. W. Rogers, and W. J. Eadie

Abstract

Extensive measurements were made of micrometeorological variables associated with eleven fogs in the Chemung River Valley near Elmira, N.Y. Temperature was measured at five levels on a 17 m tower, dew point at three levels, wind speed and direction at two levels, and net radiation and vertical wind at one level. Visibility was measured at three locations, and dew deposition and evaporation at one location near the surface. Vertical temperature distributions were also measured using an aircraft. The microphysical variables are discussed in Part II of this paper.

Consistent patterns of behavior of all micrometeorological variables were observed. The formation of ground fog may be explained by radiational cooling of the surface and associated low-level heat exchange. To explain observed temperature behavior (maximum cooling rate near 100 m in the 6 h preceeding fog) and the initial formation of a thin fog layer slightly below that level, it seems necessary to invoke Defant's model of valley circulation. Radiation divergence at the fog layer aloft then produces an inversion near the fog top and unstable conditions at lower levels. The fog base therefore propagates downward.

Dew deposition is responsible for formation of a low-level dew point inversion before fog forms, a necessary condition for initial fog formation aloft. The inversion breaks as fog forms and dew weight is constant from that time until sunrise. Evaporation of dew after sunrise maintains saturation throughout the fog depth as fog temperature increases, and is therefore responsible for fog persistence. Dissipation of fog occurs when evaporation rate is no longer adequate to maintain saturation.

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R. J. Pilié, E. J. Mack, C. W. Rogers, U. Katz, and W. C. Kocmond

Abstract

This paper summarizes the results of seven field expeditions aboard the Naval Postgraduate School's R/V Acania, designed specifically to study the formation of marine fog along the California coast. On the basis of observations and analyses, physical models have been formulated for the formation and persistence of at least four different types of marine fog which occur off the West Coast: 1) fog triggered by instability and mixing over warm water patches; 2) fog developed as a result of lowering (thickening) stratus clouds; 3) fog associated with low-level mesoscale convergence; and 4) coastal radiation fog advected to sea via nocturnal land breezes. In addition, it has been found that the triggering of embryonic fogs and further downwind development produces a synoptic-scale fog-stratus system and is responsible for redevelopment of the unstable marine boundary layer after Santa Ana events.

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W. Erick Rogers, Paul A. Wittmann, David W. C. Wang, R. Michael Clancy, and Y. Larry Hsu

Abstract

It is a major challenge to determine whether bias in operational global wave predictions is predominately due to the wave model itself (internal error) or due to errors in wind forcing (an external error). Another challenge is to characterize bias attributable to errors in wave model physics (e.g., input, dissipation, and nonlinear transfer). In this study, hindcasts and an evaluation methodology are constructed to address these challenges. The bias of the wave predictions is evaluated with consideration of the bias of four different wind forcing fields [two of which are supplemented with the NASA Quick Scatterometer (QuikSCAT) measurements]. It is found that the accuracy of the Fleet Numerical Meteorology and Oceanography Center’s operational global wind forcing has improved to the point where it is unlikely to be the primary source of error in the center’s global wave model (WAVEWATCH-III). The hindcast comparisons are specifically designed to minimize systematic errors from numerics and resolution. From these hindcasts, insight into the physics-related bias in the global wave model is possible: comparison to in situ wave data suggests an overall positive bias at northeast Pacific locations and an overall negative bias at northwest Atlantic locations. Comparison of frequency bands indicates a tendency by the model physics to overpredict energy at higher frequencies and underpredict energy at lower frequencies.

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C. E. Dorman, D. P. Rogers, W. Nuss, and W. T. Thompson

Abstract

An instrumented C-130 aircraft flew over water around Point Sur, California, on 17 June 1996 under strong northwest wind conditions and a strong marine inversion. Patterns were flown from 30- to 1200-m elevation and up to 120 km offshore. Nearshore, marine air accelerated past Point Sur, reaching a surface maximum of 17 m s−1 in the lee. Winds measured over water in and above the marine layer were alongshore with no significant cross-shore flow. Sea level pressure, 10-m air temperature, and air temperature inversion base generally decreased toward the coast and were an absolute minimum just downcoast of the wind speed maximum. The sea surface temperature also decreased toward the coast, but was an absolute minimum directly off Point Sur. The near-coast, air temperature inversion base height was 400 m north of Point Sur, decreased to a minimum of 50 m in the lee of Point Sur, then increased farther to the south. Wind speeds were at a maximum centered along the air temperature inversion base; the fastest was 27 m s−1 in the lee of Point Sur.

Using a Froude number calculation that includes the lower half of the capping layer, the marine layer in the area is determined to have been supercritical. Most of the marine layer had Froude numbers between 1.0 and 2.0 with the extreme range of 0.8–2.8. Temperatures in the air temperature inversion in the lee were substantially greater than elsewhere, modifying the surface pressure gradient. The overall structure was a hydraulic supercritical expansion fan in the lee of Point Sur under the influence of rotation and surface friction.

The Naval Research Laboratory nonhydrostatic Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS) indicated a broad, supercritical marine boundary layer moving to the south along central California and Point Sur during the aircraft flight. The marine boundary layer thinned and accelerated into the lee of Point Sur, which was the site of the fastest sea level wind speed along central California. Isotherms dip and speeds decreased in the lee of Point Sur in the capping inversion well above the marine layer. COAMPS forecasted a compression shock wave initiating off the upwind side of the topography behind Point Sur and other coastal points to the north. Evidence from the model and the aircraft supports the existence of an oblique hydraulic jump on the north side of Point Sur.

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Thomas Carl Peterson, L. O. Grant, W. R. Cotton, and D. C. Rogers

Abstract

Mountains often act as barriers to low-level flow creating regions of stagnant, decoupled flow within thermally stratified air masses. This paper addresses the question: how does a region of low-level decoupled flow affect the overlying orographic cloud?

Three different methodologies were used to examine this problem. The first method involved analysis of one and a half months of precipitation and wind data from a 24-station mesonetwork located in the Yampa River valley and surrounding mountains of northwest Colorado during the winter of 1981/1982 as part of the third Colorado Orographic Seeding Experiment (COSE III). The second method was a case study analysis of two orographic storms using data from an instrumented cloud physics aircraft to supplement the data from the mesonetwork. The third method involved two-dimensional numerical simulations using Colorado State University's Regional Atmospheric Modeling System (RAMS).

The results show that the presence of extensive low-level decoupled flow causes part of the orographic lift of the mountain barrier to be experienced upstream of the barrier. This changes the location of condensate production which in turn shifts precipitation upstream and appears to enhance the precipitation efficiency for the entire barrier.

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David H. Bromwich, Aric N. Rogers, Per Kållberg, Richard I. Cullather, James W. C. White, and Karl J. Kreutz

Abstract

The El Niño–Southern Oscillation (ENSO) signal in Antarctic precipitation is evaluated using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses and ECMWF 15-yr (1979–93) reanalyses. Operational and reanalysis datasets indicate that the ENSO teleconnection with Antarctic precipitation is manifested through a close positive correlation between the Southern Oscillation index and West Antarctic sector (75°–90°S, 120°W–180°) precipitation from the early 1980s to 1990, and a close negative correlation after 1990. However, a comparison between the operational analyses and reanalyses shows significant differences in net precipitation (PE) due to contrasts in the mean component of moisture flux convergence into the West Antarctic sector. These contrasts are primarily due to the mean winds, which differ significantly between the operational analyses and the reanalyses for the most reliable period of overlap (1985–93). Some of the differences in flow pattern are attributed to an error in the reanalysis assimilation of Vostok station data that suppresses the geopotential heights over East Antarctica. Reanalysis geopotential heights are also suppressed over the Southern Ocean, where there is a known cold bias below 300 hPa. Deficiencies in ECMWF reanalyses result in a weaker ENSO signal in Antarctic precipitation and cause them to miss the significant upward trend in precipitation found in recent operational analyses. Ice-core analyses reflect both an upward trend in ice accumulation and the ENSO teleconnection correlation pattern seen in the operational analyses. This study confirms the results of a previous study using ECMWF operational analyses that was the first to find a strong correlation pattern between the moisture budget over the West Antarctic sector and the Southern Oscillation index.

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