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Robert A. Mack, A. F. Hasler, and Robert F. Adler

Abstract

GOES stereoscopy is applied to the study of severe squall line cells. Short interval (3 min) GOES stereoscopic data from the 2–3 May 1979 SESAME case were used to measure cloud top heights of growing storms as a function of time. A one-dimensional cloud model was used to relate the stereoscopically derived cloud top ascent rates to thunderstorm updraft intensity. Results show ascent rates ranging from 4.4 to 7.7 m s−1 for intense cells in a squall line. These results compare well in magnitude with growth rates determined from simultaneous GOES infrared observations and previous estimates of visual cloud and radar echo top growth rates of other thunderstorms.

Detailed stereoscopic cloud top height contour maps of the mature squall line on 2–3 May 1979 were constructed and are discussed here in terms of the small-scale structure and its variability. Results show that for small-scale features (e.g., 5 km diameter tropopause penetrating towers) the short-interval GOES data are not sufficient for studying the life cycle of such features. The stereoscopic height contours are compared to infrared cloud top temperature patterns observed with intense thunderstorms and used to evaluate various theories on the cause of the infrared V-shaped signatures.

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Robert F. Adler and Robert A. Mack

Abstract

Observational studies of thunderstorm cloud height-rainfall rate and cloud height-volume rainfall rate relations are reviewed with significant variations being noted among climatological regimes. Analysis of the Florida (summer) and Oklahoma (spring) relations are made using a one-dimensional cloud model to ascertain the important factors in determining the individual cloud-rain relations and the differences between the two regimes. In general, the observed relations are well simulated by the model-based calculations. The generally lower predicted rain rates in Oklahoma (as compared to Florida) result from lower precipitation efficiencies which are due to a combination of larger entrainment (related to larger vertical wind shear) and drier environment. The generally steeper slope of the Oklahoma rain rate height curves is shown to be due to a stronger variation in maximum vertical velocity with cloud top height, which, in turn, is related to the greater static stability in the range of cloud tops. The impact of the regime-to-regime variations on empirical rain estimation schemes based on satellite-observed cloud height or cloud temperature information is discussed and a rain estimation approach based on model-generated cloud-rain relations is outlined.

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Robert F. Adler and Robert A. Mack

Abstract

A Lagrangian model applicable to the overshooting region of thunderstorm tops is used to describe the temperature-height path taken by updraft core parcels as they penetrate above the tropopause, reach their maximum height and descend in the periphery of the convective tower. The model is run under a variety of ambient and in-cloud conditions in order to simulate certain temperature-height relationships observed in satellite observations.

Observations indicate that in the majority of observed storm tops, the satellite-observed cold point in the IR brightness temperature (TB) field is collocated with the highest point in the convective overshooting region and the TB-height relations are near adiabatic. The parcel model quantitatively reproduces this type of relationship for model runs where the mixing parameter is relatively small.

Another type of storm has a close-in, cold-warm TB couplet with a dimension of approximately 20–40 km and a V-shaped cold TB pattern. In some cases of these V-shaped storms, the cold point is clearly located upwind of the high point. Model runs have been made to reproduce a number of these salient features for these types of storms. With larger values of the mixing parameters (presumably related to larger shear), the model produces temperature-height relationships that are, of course, much closer to ambient than to adiabatic, as is observed in these cases. With the larger mixing parameter, the cold-high offset is also produced, for model runs having a relatively large initial vertical velocity and under conditions of a strong inversion. The amount of the cold-high offset is shown to be a direct function of the strength of the inversion.

The cause of the close-in warm point is also explored with the simple model. As has been shown in three-dimensional cloud model results, the warm point in the cold-warm couplet can be related to internal cloud subsidence on the downwind side in association with mixing with the environment. This effect is also reproduced in the parcel model with the occurrence of a warm point being related to conditions of an intense updraft and strong mixing. The model also points to parcels subsiding from their maximum height and crossing the ambient lapse rate from negative to positive buoyancy on the downwind side and then coming into equilibrium at a relatively high level above the tropopause on the downwind side. This effect may be related to the top of the downwind anvil cloud being elevated significantly above the equilibrium point or tropopause. Another interpretation of this model result may be related to the above-anvil cirrus noted by a few investigators.

The temperature-height distributions produced by the model in a Lagrangian framework are converted to the spatial domain by the assumption of steady state conditions and are compared to temperature-height cross sections determined from GOES IR and stereoscopic height fields. The locations of cold points, high points, warm points, and the magnitude of cold-high offsets compare favorably between the model and the satellite observations.

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Bradley F. Smull and Robert A. Houze Jr.

Abstract

The mesoscale structure of a squall-line system that passed over Oklahoma on 22 May 1976 is investigated by dual-Doppler radar analysis. The mature storm consisted of a leading line of deep convection, which exhibited organized multicellular structure, trailed by an extensive region of stratiform precipitation marked by a radar bright band at the melting level. These contrasting radar echo regimes were separated by a narrow band of weak reflectivity at lower levels, which has been termed the “transition zone.” While conventional and single-Doppler radar analyses documented the persistence of this precipitation structure and revealed the corresponding kinematic structure in one part of the mature storm, the dual-Doppler analysis demonstrates the pervasiveness of these features over much of the squall line's length.

The structure of a midtropospheric maximum of rearward, system-relative flow crossing the system is particularly well described by the dual-Doppler data. This mesoscale current originated ahead of the storm. gained strength while passing through the convective line, spanned the transition zone, and extended to near the back edge of the stratiform region. It strongly influenced precipitation growth and radar echo structure by promoting the transfer of ice particles from convective cells across the transition zone into the trailing stratiform region. Deep, intense updrafts occurred in association with convective cells along the leading edge of the system. Convective downdrafts were apparently active both in the lower troposphere, where thermodynamic data showed they were a source of air feeding the leading gust front, and at upper levels, where the Doppler analysis indicated they were forced by convergence of air detrained from the tops of the updrafts with slower moving ambient air. Horizontal momentum transported vertically by convective motions converged at midlevels, accelerating parcels rearward and so bolstering the front-to-rear flow.

Profiles of radar-derived mean vertical motion confirm the presence of a mesoscale updraft overlying a mesoscale downdraft in the transition and trailing stratiform regions. The mean descent in the lower troposphere was particularly deep and intense in the transition zone and may have contributed to the decreased reflectivity values observed there.

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John F. Gamache and Robert A. Houze Jr.

Abstract

Composites of radar and wind observations in a coordinate system attached to a moving tropical squall line confirm that such a squall system is composed of two separate circulation features: a convective squall-line region and a stratiform anvil region. The squall-line region is characterized by mesoscale boundary-layer convergence, which feeds deep convective updrafts, and mid-to-upper-level divergence associated with outflow from the cells. The anvil region is characterized by mid-level convergence, which feeds both a mesoscale downdraft below the anvil and a mesoscale updraft within the anvil cloud. Before this study, the mesoscale updraft in the anvil cloud of the tropical squall system had been somewhat speculative, and both the anvil updraft and downdraft had been inferred only qualitatively. The occurrence of the anvil updraft is now proven and quantitative profiles of the mesoscale anvil updraft and downdraft have been obtained.

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Robert A. Lynch and E. F. Bradley

Abstract

An improved shearing stress meter (drag plate) of 6 ft (183 cm) diameter has been constructed for micro-meteorological work. This size should enable representative sampling of rough surfaces and is intended particularly for farmland experimental sites where, after harvesting, the ground is either left to grain stubble or ploughed.The sensors are inductive proximity probes, whose advantages of robustness and stability enable the instrument to be handled without undue precautions against damage and to withstand gusts of over 20 dyn cm−2 while achieving a resolution of 0.01 dyn cm−2.Comparison experiments involving two drag plates installed in a ploughed field with a sonic anemometer mounted nearby indicate good agreement in stress measurement among all three instruments.

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Robert A. Dalrymple and Philip L-F. Liu

Abstract

The problem of water waves propagating over a mud bottom, characterized as a laminar viscous fluid, is treated in several ways. First, two complete models are present, each valid for different lower (mud) layer depths, and second, a boundary layer model is presented as an appendix for the case where the lower layer is thick with respect to the boundary layer.

These models are compared to the shallow water model and experimental results of Gade (1957, 1958) and agree well. The results show that extremely high wave attenuation rates are possible when the thickness of the lower layer is the same order as the internal boundary layer thickness and when the lower layer is thick.

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David A. Short and Robert F. Cahalan

Abstract

The interannual variability (IAV) in monthly averaged outgoing infrared radiation (IR, from the NOAA polar orbiting satellites) is observed to be larger during summer than during winter over the north Pacific Ocean. A statistical analysis of the daily observations shows the daily variance to be similar during both seasons while the autocorrelation function is quite different. This leads to a seasonal difference in estimates of the climatic noise level, i.e., the variances expected in summer and winter monthly averages due to the number of effectively independent samples in each average. Because of a less vigorous tropospheric circulation, monthly means of IR during summer are affected by the passage of fewer synoptic-scale disturbances and their attendant cloudiness. Fewer independent samples imply a larger variance in the time averages. While the observed IAV is less in winter, the ratio of the observed IAV to the climatic noise level is larger, suggesting that signals of climatic variability in outgoing IR may be more readily diagnosed during winter in this region. The climatic noise level in monthly averaged IR and cloudiness is also estimated for two other climatic regimes—the quiescent subtropical north Pacific and the ITCZ in the western Pacific.

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Bradley F. Smull and Robert A. Houze Jr.

Abstract

A squall line exhibiting an extensive trailing region of stratiform precipitation passed over the observational network of the National Severe Storms Laboratory on 22 May 1976. Satellite imagery and conventional radar observations document its evolution from a broken line of thunderstorms to a system of mesoscale proportions, and single-Doppler radar observations describe aspects of its mature structure. Satellite measurements of cloud-top temperature showed the system to be a mesoscale convective complex (MCC). The life cycle of the system exhibited the stages of development seen in tropical cloud clusters.

At maturity, two prominent mesoscale flow regimes were identified at midlevels: one marked by inflow into the system's front and continuing toward its rear, and another associated with inflow entering the extreme rear of the system.

The rear inflow was associated with a cyclonic midlevel vortex in the stratiform precipitation region. It produced a concavity, or “notch”, in the back edge of the precipitation echo. Shortly after the appearance of the notch, a downwind segment of the leading convective line accelerated forward. The notch persisted through the dissipating stage, at which time secondary notches also formed. The last remnant of the stratiform precipitation area took the form of a chain of three comma-shaped vortices, whose origin could be traced in time back to the primary and secondary notches.

The inflow at the front of the system spanned both the leading convective and trailing stratiform regions. Convective-scale velocity maxima were superimposed on this front-to-rear flow in the convective region, while a broad maximum of the rearward current occurred in the stratiform region, just above the melting layer. This rearward system-relative flow apparently promoted the broad structure of the precipitation area. Slowly falling ice particles originating at convective cell tops were evidently advected rearward and dispersed over a 50–100 km wide region, whereupon their melting produced a prominent radar bright band.

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Robert A. Maddox and Hugo F. Bezdek

Abstract

An extended series of surface observations is used to compare observed surface winds with winds computed using the geostrophic relationship. These computations are done for both steady and unsteady wind regimes. Large differences are found in the comparisons of observed to computed winds. The differences exhibit pronounced seasonal and diurnal variability that appear to reflect both boundary layer stability and small-scale wind and pressure fields-for example, those attending land-sea breezes and thunderstorms.

The results of this study may be useful to those engaged in studying global datasets and to modelers, who are continually challenged to improve the treatment of parameterization of turbulent processes. However, it is not obvious that any simple parameterization can be applied to obtain an accurate estimate of the surface wind in central Florida, given only the large-scale pressure gradient or a model-predicted wind above the surface as input. The use of the pressure field to estimate surface winds is an uncertain exercise at best.

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