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Robert Cifelli, Steven A. Rutledge, Dennis J. Boccippio, and Thomas Matejka

Abstract

Vertical motion profiles can be diagnosed with the mass continuity equation using horizontal divergence fields derived from various single-Doppler radar techniques such as EVAD (extended velocity-azimuth display), CEVAD (concurrent extended velocity-azimuth display), and VVP (volume velocity processing). These methods allow for the retrieval of mesoscale air motions in precipitating regions when the wind field is relatively homogeneous. In contrast, VHF wind profiler data can provide a direct measurement of vertical motion, albeit across a much smaller domain compared to the single-Doppler radar techniques. In this study, we compare horizontal divergence and vertical motion patterns derived from the various single-Doppler methods with those obtained from VHF profiler data.

The diagnosed profiles of horizontal divergence and vertical velocity from the single-Doppler (scanning radar) techniques are in qualitative agreement in the lower troposphere but often exhibit large variability at higher levels. Because of less stringent radar echo requirements, the VVP technique often analyzed data above the top of the EVAD-CEVAD analysis domain, resulting in a deeper layer of upper-level divergence. The CEVAD technique often produced a deeper and larger region of upward motion despite similar profiles of divergence, probably due to the CEVAD top boundary condition specification of particle terminal fall speed as opposed to the vertical air motion, as well as to the adjustment procedure employed during the regression solution.

The wind profiler data showed much larger vertical gradients and magnitudes of divergence and vertical velocity when averaged over the same time interval required to collect data for a single-Doppler retrieval. However, when all the available data were composited, the high-frequency variability in the wind profiler retrievals was reduced resulting in relatively good agreement between all analysis methods. The wind profiler usually sampled vertical motion (divergence) several kilometers above the single-Doppler retrievals, which the authors attribute to the stringent precipitation echo coverage requirements imposed by the scanning radar analysis techniques, thus limiting their vertical extent new echo top.

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Peter V. Hobbs, Robert A. Houze Jr., and Thomas J. Matejka

Abstract

An occluded front moving over Washington State was investigated with serial rawinsonde ascents, aircraft penetrations, raingage measurements and conventional observations.

The rawinsonde data showed that the frontal system contained alternate mesoscale tongues of air with low and high values of wet–bulb potential temperature (θω). These tongues included a pre–frontal surge of low–θω air aloft, centered at about the 700 mb level, a low–evel high–θω tongue along the front, and two post–frontal high–θω tongues. The low– and high–θω tongues were of the order of 50–100 km in width.

The frontal precipitation was confined to a mesoscale band, 80 km in width, along the front. The cloud associated with this hand was characterized by a vertical circulation similar to, but much less vigorous than, an organized convective system. Cloud microphysical data indicated that a narrow cumulus–scale updraft zone was located near the leading edge of the frontal cloud. The concentrations of ice particles in the frontal cloud were probably on the order of 50 ℓminus;1 but could have been as high as 500 ℓminus;1. These values exceed the optimum for the efficient release of precipitation by the Bergeron–Findeisen process. However, cloud particles collected in flight revealed that both riming and aggregation played important roles in particle growth within the frontal cloud.

As the frontal system passed over the Cascade Mountains, the amount of cloud and precipitation ahead of the front decreased while that behind the front increased. The decrease in cloudiness ahead of the front is attributed to the low–level moisture source being cut off by the mountains. The increase in clouds behind the front was apparently due to orographic lifting of the cold air.

This study confirms that mesoscale processes play an important role in the production of frontal precipitation. It also indicates that the microphysical aspects of precipitation growth are more complex than classical models would suggest.

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Stanley B. Trier, David B. Parsons, and Thomas J. Matejka

Abstract

The passage of shallow cold fronts during the late spring and early summer months over the island of Taiwan is often accompanied by heavy rainfall and occasional flash flood episodes. Previous studies have emphasized the weak baroclinicity of these fronts and their possible modification by fluxes from the air-sea interface. In this study a cold frontal passage in the vicinity of Taiwan is analyzed using data gathered during the Taiwan Area Mesoscale Experiment (TAMEX) on 8 June 1987. At the northern extent of the TAMEX network the cold front was shallow (1–2 km deep) and moderately baroclinic with 5°-7°C temperature contrasts at the surface. A Doppler radar cross section of radial velocity reveals a structure similar to that of a density current at the leading edge of the shallow front. The postfrontal air man was substantially modified by oceanic heat fluxes as it moved southward over the warm ocean waters. This led to a 60%–70% decrease in the temperature contrast across the front between ocean stations at the northern and southern ends of the island, a distance of ∼400 km.

Frontal passages across Taiwan are also influenced by the presence of the Central Mountain Range (CMR), which has an average ridge elevation of ∼2500 m, and is oriented NNE-SSW along the major axis of the island. In the case described in this paper the CMR, 1) acts as a barrier to both the pre- and postfrontal flows, and 2) is influential by inducing thermally-driven diurnal circulations associated with differential heating of the sloped terrain and the nearby ocean. Terrain influences on the kinematics of the flow in the vicinity of the front are also shown to locally modify the frontal intensity.

The inhomogeneous distribution of precipitation attending the frontal passage is related to strong regional variations in thermodynamic stability across the island. These variations in stability are linked to the mesoscale effects of terrain, and to the larger-scale influence of advection of an unstable tropical air mass into the region by a low-level wind maximum.

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Peter V. Hobbs, Thomas J. Matejka, Paul H. Herzegh, John D. Locatelli, and Robert A. Houze Jr.

Abstract

Detailed information is deduced on the mesoscale organization of precipitation, the structures of the clouds, the air flows associated with mesoscale rainbands, and the precipitation efficiencies and the mechanisms producing precipitation in the rainbands associated with a cold front. Measurements were obtained with quantitative reflectivity and Doppler radars, two instrumented aircraft, serial rawinsondes and a network of ground stations.

The regions of heaviest precipitation were organized into a complex mesoscale rainband in the warm-sector air ahead of the front, a narrow band of precipitation at the surface cold front, and four wide cold-frontal rainbands. The wide cold-frontal rainbands and the smaller mesoscale areas of precipitation within them moved with the velocities of the winds between ∼3—6 km. The narrow rainband, which was produced by strong convergence and convection in the boundary layer, moved with the speed of the cold front at the surface. A coupled updraft and downdraft was probably responsible for the heavy precipitation on the cold front being organized, on the small mesoscale, into ellipsoidal areas with similar orientations.

The precipitation efficiencies in the warm-sector and narrow cold-frontal rainbands were ∼40–50% and ∼30–50%, respectively. One of the wide cold-frontal rainbands, in which there was a steady production of ice panicles in the main updraft, had a precipitation efficiency of at least 80%, whereas another wide cold-frontal band, in which some precipitation evaporated before reaching the surface, had a precipitation efficiency of ∼20%.

Ice particles from shallow convective cells aloft played important roles in the production of precipitation in the wide cold-frontal rainbands and in some regions of the warm-sector rainband. These “seed” ice particles grew by aggregation and by the deposition of vapor as they fell through lower level “feeder” clouds. About 20% of the mass of the precipitation reaching the ground in the wide cold-frontal rainbands originated in the upper level “seeder” zones and ∼80% in the “feeder” zones.

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Robert A. Houze Jr., Steven A. Rutledge, Thomas J. Matejka, and Peter V. Hobbs

Abstract

Doppler radar data and airborne cloud microphysical measurements obtained in the CYCLES PROJECT indicate that a warm-frontal rainband in an extratropical cyclone was characterized by a precipitation process in which clouds at low levels were enhanced by a mesoscale updraft. Ice particles, apparently formed in shallow convective cells aloft and then drifted downward, undergoing aggregation just above the melting layer. This study demonstrates the crucial role of the low-level mesoscale updraft in condensing a sufficient amount of cloud water for particles to accrete as they fell through the lower portion of the frontal cloud.

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