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Robert L. Grossman

Abstract

New applications of conditional sampling using the bivariate joint frequency distribution (JFD) and conditional mean distribution (CMD) are introduced to analyze time series of water vapor flux obtained from aircraft gust-probe vertical velocity and water vapor data collected during free convective conditions (zi/L≈−46) in the Atlantic trade winds. The JFD can provide estimates of fractional time as well as mean and variance estimates for the four ways vertical velocity/moisture fluctuations can combine (up-moist, up-dry, down-moist, down-dry). These combinations were interpreted using a subcloud layer model consisting of upward and downward moving convective cells and a mixing zone between them. The data suggest detrainment from the up-moving cell becomes the predominant mixing process above 0.08zi. The fractional time and mean values were used to parameterize the moisture flux. Three parameterization methods are introduced; two estimated the moisture flux to within 10% while a third, simpler version gave estimates to within 13%, The CMD analysis subpartitioned the flux into direct and indirect components associated with up and downdrafts. Though the majority of direct flux was associated with updrafts, downward moving dry air was a substantial part of the direct flux even at 0.03zi. Given the differences in approach, comparison with other conditional sampling analyses was good. Using saturation point analysis, downward moving dry air sensed at 0.25zi was shown to originate above zi, while the upward moving moist air sampled at 0.25zi was shown to originate near the ocean surface.

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Robert L. Grossman and Oswaldo Garcia

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Tle highly reflective cloud (HRC) dataset is a daily index of organized deep convection, at one degree resolution, from 17 years of polar-orbiting, satellite imagery. These data are used to analyze and discuss the climatological geographical distribution of deep convection observed over the Asian summer monsoon season and its component months (June, July, August and September). Intraseasonal variations of convection for selected regions am examined using normalized pentad time series of regional median HRC values. We also compare HRC data over two regions (western coastal India and western coastal Burma/Thailand) with the results from a two-dimensional numerical model, consisting of a simple differentially heated land-ocean system which predicts that a preponderance of deep convection occurs over the coastal zone. The Buma/Thailand regional comparison supports the model result. Comparison of the model with the western coastal India region is less conclusive, which may be due to the limitations of the model.

We conclude that monsoon deep convection, and its attendant sources of latent heat momentum, and mass sources important to large-scale monsoon dynamics is localized and persistent from year to year. If, as hypothesized by others, tropical cumulonimbus activity is important to stratospheric-tropospheric exchange, this study shows the preferred areas of such exchange during the monsoon. The locations of areas with large HRC amounts are consistent with upstream lifting of low-level, conditionally unstable air by low, coastal mountains. Intraseasonal variability follows variations in sea surface temperature and low-level flow. Upper-level dynamics are also recognized as an important contribution.

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Dale R. Durran and Robert L. Grossman

Abstract

No abstract available.

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Robert L. Grossman and Dale R. Durran

Abstract

Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure deceleration result is further explored by applying an idealized monsoon flow to an analytical, nonliner, two-dimensional mountain-flow interaction model using a smoothed profile of the Western Ghats Mountains. The model qualitatively agrees with aircraft observations taken in the subcloud layer, and predicts large vertical wind shears over the coastal area and mountain crest which would inhibit deep convection. These shears are confirmed by earlier observations.

When the lifting predicted by the model is applied to mean dropwindsonde soundings, well upstream of the coast, for days with and without offshore convection, deep convection is predicted for the mean sounding associated with offshore convection. The mean sounding for days without deep convection shows more offshore lifting is needed to produce convection; even if the lifting were applied, the convection would not be very deep due to a cooler surface layer and a dry layer above the boundary layer which may have originated from the desert areas to the west and/or upper tropospheric downward motion. We conclude that the mountains, though not very high, play an important role in overall monsoon convection for India. It is suggested that, given the climatic character of offshore monsoon convection, interaction of the low-level flow with the western coastal mountains of India, Burma, Thailand, and the Philippines should be considered a factor in monsoon climatology.

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Robert L. Grossman and Richard Friesen
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Guang Jun Zhang and Robert L. Grossman

Abstract

This study is directed to evaluating the feedback between evaporation (FL) and sea surface temperature (Ts) in the equatorial Pacific Ocean by looking at the components that control dFL/dTs, the variation of evaporation with Ts. First eddy correlation evaporation estimates obtained during long (∼1000–1500 km), low-level (30 m) traverses of the central equatorial Pacific by research aircraft during the Central Equatorial Pacific Experiment (CEPEX) are analyzed. From this limited dataset, extension to climate space- and timescales is made by comparing the aircraft measurements to bulk aerodynamic estimates of FL, using mean values from both the aircraft and Tropical Atmosphere–Ocean buoys.

Variation of surface evaporation with Ts is shown to be affected not only by surface saturation humidity deficit and its dependence on Ts, but also by variations of wind speed with Ts. Depending on the relative importance of the two contributions, surface evaporation can either increase or decrease with Ts. Intercomparison between the aircraft data and the buoy data indicates that the humidity deficit effect is dominant during, CEPEX, and in low Ts, where surface winds are only weakly related to Ts; the effect of wind speed variation with Ts is much more important in the 2-yr buoy data for Ts ≥ 301 K. The discrepancy between the evaporation feedback in CEPEX and that from the 2-yr buoy data is shown to be largely due to oversampling of high winds and high evaporation during CEPEX for 302 ≤ Ts < 303 K. The long-tem buoy data show that for Ts < 301 K, dFL/dTs = +9 W m−2K−1, while for 304 K > Ts ≥ 301 K, dFL/dTs = −13 W m−2K−1. Furthermore, observations of FL are well below the values necessary for evaporation to be the primary limiting factor in the regulation of Ts in the equatorial Pacific.

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Robert L. Grossman and Carl A. Friehe

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The vertical structure of the low-level wind jet over the Arabian Sea during the southwest monsoon is modeled by using the geostrophic wind shear (thermal wind) and a two-layer boundary layer model. We show that geostrophic wind shear is dominant above the low-level wind maximum while turbulent momentum exchange is dominant below. Both effects combine to produce a low-level wind jet. Model results compare well with observations obtained from a research aircraft during the 1979 Summer Monsoon Experiment.

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Robert L. Grossman and Alan K. Betts

Abstract

The area east of the coast of North Carolina chosen for enhanced observation during the Genesis of Atlantic Lows Experiment (GALE) has one of the highest average wintertime energy transfers from ocean to atmosphere on earth. A substantial part of this transfer occurs in the aftermath of winter storms as cold, dry air flows off of the continent over the warm Gulf Stream. We report on an aircraft investigation of boundary layer mean and turbulent structure and evaluate the Lagrangian budgets of temperature and moisture in the subcloud layer following a streamline during an extreme cold air outbreak. The maximum sea-air temperature difference was 23 K. Two aircraft were used: the NCAR Electra, which measured turbulent fluxes and investigated subcloud layer conditions, while the NASA Electra, using a radar, measured the height of cloud tops. A stratocumulus overcast was found from about 60 km offshore to the Gulf Stream core with cloud top rising downstream. East of the Gulf Stream cumulus congestus and snow showers were observed. Cloud base decreased downstream and numerous steam plumes filled the subcloud layer. Temperatures were corrected for the substantial effects of diurnal variation in order to isolate air-sea interaction processes. Cross sections show most warning, and moistening of the subcloud layer occurred before the Gulf Stream core. Windspeeds increased downstream and maxima were observed near cloud top (inversion) and in the subcloud layer. Lagrangian budgets showed most warming, and moistening of the layer between 70 m and about 100 m below mean cloud base was due to turbulent flux divergence. At 70 m a maximum total heat flux of 1174 W m−2 (364 W m−2 sensible, 799 W m−2 latent) was observed over the Gulf Stream core. In the temperature budget, the radiative flux divergence term was relatively small and a residual condensation warming was inferred. Complementary drying was estimated from the residual of the moisture budget. The budgets were combined using a graphical technique on a conserved parameter (θ − q) diagram and were extrapolated into the surface layer with reasonable results. This technique was also applied to the entire subcloud layer with results that implied that east of the Gulf Stream entrainment fluxes at cloud base and evaporation of falling precipitation may act to cool and dry the subcloud layer, reducing the effects of flux convergence (which would warm and moisten). The Lagrangian warming and moistening rates we estimated indicate that cold, dry continental air can be transformed to air which can participate in deep convection (which appears to be an integral part of rapid cyclogenesis) in about 20–30 hours.

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Robert L. Grossman and Donald W. Beran

Abstract

The occurrence of extreme low-level wind shear at 10 stations in the conterminous United States is investigated by applying a bivariate frequency-distribution analysis to rawinsonde and pibal data. Data for Denver, Colo., received additional analysis and showed that extreme wind shears were associated with particular synoptic conditions. Many stations indicated variation of extreme shear with season. Large values of extreme wind shear were found to correlate well with mean storm tracks in the conterminous United States.

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L. Jay Miller, Margaret A. LeMone, William Blumen, Robert L. Grossman, Nimal Gamage, and Robert J. Zamora

Abstract

Observations taken over the period 8–10 March 1992 during the Storm-scale Operational and Research Meteorology Fronts Experiment Systems Test in the central United States are used to document the detailed low-level structure and evolution of a shallow, dry arctic front. The front was characterized by cloudy skies to its north side and clear skies to its south side. It was essentially two-dimensional in the zone of intense observations.

There was a significant diurnal cycle in the magnitude of the potential temperature gradient across both the subsynoptic and mesoscale frontal zones, but imposed upon an underlying, more gradual, increase over the three days. On the warm (cloudless) side., the temperature increased and decreased in response to the diurnal heating cycle, while on the cold (cloudy) side the shape of the temperature decrease from its warm-side value (first dropping rapidly and then slowly in an exponential-like manner) remained fairly steady. The authors attribute the strong diurnal variation in potential temperature gradient mostly to the effects of differential diabatic heating across the front due to differential cloud cover.

The front is described in terms of three scales: 1) a broad, subsynoptic frontal zone (∼250–300 km wide) of modest temperature and wind gradients; 2) a narrower mesoscale zone (∼15–20 km wide) with much larger gradients; and 3) a microscale zone of near-zero-order discontinuity (≤1–2 km wide). There was some narrowing (≲50 km) of the subsynoptic frontal zone, but the authors found no evidence for any significant contraction of this zone down to much smaller mesoscale sizes. In response to the differential diabatic heating, the strongest evolution occurred in the micro-mesoscale zone, where dual-Doppler radar and aircraft measurements revealed the development of a density-current-like structure in and behind the leading edge of cold air. Here the steepest gradients developed shortly after sunrise and then increased by an order of magnitude during the day, with leading-edge vorticity, divergence, and temperature gradients reaching maximum values of 10−2 s−1 and 8 K km−1. A narrow updraft, marked by cumulus clouds, grew in intensity above the leading edge through the day to a maximum of 5–8 m s−1. Stratus clouds lay in the cold air, their leading edge receding by noon to 10–20 km behind the cumulus line.

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