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Frank Roux, Virginie Marécal, and Danièle Hauser

Abstract

The kinematic and thermodynamic structure of a narrow cold-frontal rainband (NCFR) observed during the English–French–German MFDP/FRONTS 87 experiment is presented. Radiosonde data indicated a very weak convective instability below 1500-m altitude and a low-level jet of 30 m s−1 from SSW before the arrival of the front; a cooling of about 1°C associated with an airflow of 13 m s−1 from WSW after its passage. A composite of wind and reflectivity fields from 17 dual-Doppler radar analyses shows high reflectivity values, large convergence, and relatively intense vertical motions associated with this NCFR at the 1000-m altitude.

A mean vertical cross section perpendicular to the surface front, derived from three successive high-resolution dual-Doppler scans, is used to examine the general characteristics of the air circulation. A structure apparently similar to that of a density current is observed. Above the 2-km altitude, however, air flowing at a speed faster than the surface front appreciably modified the kinematic structure as compared to classical schemes. The budgets of the associated mass and momentum fluxes show that only 20% of their vertical divergences were due to the alongfront variations. As deduced from the retrieved pressure and temperature fields, the frontal updraft was essentially maintained by the vertical pressure gradient force since buoyancy remained very small. Examination of the different frontogenetic terms indicates that diabatic heating, with a necessary contribution of the convergence term, was the most important one for maintaining the surface temperature gradient. These results are consistent with those previously deduced for other NCFRs, except the values are smaller here due to the less intense features.

Analysis of the successive wind and reflectivity fields reveals some three-dimensional and time-dependent features. In particular, the frontal updraft underwent some evolution related to the formation and fall of precipitation. The pressure and temperature perturbations retrieved from these three-dimensional fields are qualitatively similar to the two-dimensional ones. Their larger amplitudes are, however, closer to those observed during the passage of the front.

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Danièle Hauser, Frank Roux, and Paul Amayenc

Abstract

Microphysical and thermodynamic retrieval studies using a specified wind field can provide a means for analysing the different processes occurring within an observed precipitating system. Up to now, the retrieval of microphysical variable fields or thermodynamic fields have been performed separately, though the interest of associating both types of retrieval has been already noted by several authors.

The research reported here presents a new retrieval method allowing consistent and simultaneous derivation of the microphysical and thermodynamic variable fields using the whole set of governing equations (momentum, thermodynamic, and microphysical equations) with the wind field specified from Doppler radar observations. The microphysical retrieval makes use of the continuity equation for the total water substance and for the precipitating substance. Two types of precipitating particles are considered (rain and graupel), and a parameterization derived from that proposed by Kessler is chosen. In practice, the microphysical retrieval is coupled to the retrieval of thermodynamic variables, which is derived from Roux. A more classical approach taking into account the thermodynamic equation and the microphysical equations, but not the momentum equation is also used for comparison.

Results obtained from both approaches in the convective region of a tropical squall line observed during COPT81 (22 June 1981) are presented and discussed. It is found that both approaches provide results in mutual agreement, and which are consistent with the observed reflectivity structure, and with surface measurements. The respective advantages and drawbacks of each approach are also discussed.

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Stéphanie Pradier, Michel Chong, and Frank Roux

Abstract

During the special observing period of the Mesoscale Alpine Programme, a narrow north–south-oriented frontal precipitation band was observed by two airborne and one operational Doppler radar on 4 October 1999, over the Friuli target area in northeast Italy. Convective precipitation was mostly concentrated to the south of the eastern Alps in regions of smooth orography and warm southerly flow ahead of the cold front, while stratiform rain formed above the mountainous chain where a marked cold northerly flow, wrapping around the eastern Alps, progressed southwestward. Doppler data collected in both convective and stratiform regions of the precipitation line, as well as high-resolution numerical simulations performed with the nonhydrostatic Meso-NH model, are used to investigate the associated flow structure in connection with the underlying topography.

The mutually consistent observed and simulated precipitation and wind fields clearly identify low-level flow convergence relative to the cold front and cold air from the north of the Alps as important factors in setting up the upward motions, in addition to orographic forcing. This cold air delimits a warm frontal surface that is overrun by the warm southerly air. Its southward progression has a blocking effect and contributes to modulating the airflow over the mountains. It is observed that the maximum updraft cores occur upstream of the steepest slopes, as a consequence of the convergence between the warm southerly and cold northeasterly flows. Moreover, observed wind acceleration from south to north is related to a horizontal pressure pertubation gradient force, and dynamic pressure perturbation is closely linked to the mountain shape. Additional numerical experiments with smoother orography are performed to identify the relative roles of broader and finer-scale topography. It is found that the smoother the terrain is, the faster the cold and warm fronts progress southeastward, highlighting the role of at least 1-km orography scale in retarding the frontal system.

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Virginie Marécal, Danièle Hauser, and Frank Roux

Abstract

The microphysics of a narrow cold-frontal rainband (NCFR) observed during the MFDP/FRONTS87 experiment is investigated by using a microphysical retrieval model. The equations of evolution of the water substance and of the temperature are solved using a wind field prescribed from dual-Doppler radar observations.

Different runs of the model were performed to investigate the role of various microphysical processes. All of them use a two-dimensional version of the model and give a solution for the steady state corresponding to the input wind field. The validity of this approach was checked a posteriori by comparing the results obtained from vertical cross sections at two different locations and two different times. In each case, the consistency of the results was controlled through comparisons with in situ measurements (aircraft, ground stations, and radiosondes) and radar reflectivity observations.

The main result obtained from this study was that the precipitation associated with the NCFR was mostly composed of graupel particles, essentially formed by riming. Rain was produced by accretion of cloud water in the condensation zone and by melting of graupel. The choice of the type of ice-precipitating particles introduced in the model appeared very important. Only rimed particles (graupel) could reproduce observed precipitation. The precipitation efficiency was rather high (73%). The zone of light precipitation in which the NCFR was embedded seemed to play no-seeder role in the growth of precipitation in the NCFR, probably due to the overturning airflow located in the prefrontal zone.

Another important result concerns the role of the microphysical processes on the thermodynamics. The temperature drop observed at low levels just behind the frontal discontinuity could be explained at the time of the observations by two cooling effects of equal importance: the melting of graupel and the evaporation of precipitation.

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Marie-Dominique Leroux, Matthieu Plu, and Frank Roux

Abstract

This study is part of the efforts undertaken to resolve the “bad trough/good trough” issue for tropical cyclone (TC) intensity changes and to improve the prediction of these challenging events. Sensitivity experiments are run at 8-km resolution with vortex bogusing to extend the previous analysis of a real case of TC–trough interaction (Dora in 2007). The initial position and intensity of the TC are modified, leaving the trough unchanged to describe a realistic environment. Simulations are designed to analyze the sensitivity of TC prediction to both the variety of TC–trough configurations and the current uncertainty in model analysis of TC intensity and position.

Results show that TC intensification under upper-level forcing is greater for stronger vortices. The timing and geometry of the interaction between the two cyclonic potential vorticity anomalies associated with the cutoff low and the TC also play a major role in storm intensification. The intensification rate increases when the TC (initially located 12° northwest of the trough) is displaced 1° closer. By allowing a gradual deformation and equatorward tilting of the trough, both scenarios foster an extended “inflow channel” of cyclonic vorticity at midlevels toward the vortex inner core. Conversely, unfavorable interaction is found for vortices displaced 3° or 4° east or northeast. Variations in environmental forcing relative to the reference simulation illustrate that the relationship between intensity change and the 850–200-hPa wind shear is not systematic and that the 200-hPa divergence, 335–350-K mean potential vorticity, or 200-hPa relative eddy momentum fluxes may be better predictors of TC intensification during TC–trough interactions.

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John F. Gamache, Frank D. Marks Jr., and Frank Roux

Abstract

Three different airborne Doppler radar sampling strategies were tested in Hurricane Gustav (1990) on 29 August 1990. The two new strategies were the fore-aft scanning technique (FAST) and airborne dual-platform Doppler sampling. FAST employs radar mans in cones pointing alternately fore and aft of the vertical plane that is perpendicular to the flight track. The airborne dual-platform sampling uses two Doppler radars, each aboard a separate aircraft. The Doppler radars scan strictly in the vertical plant normal to the flight track. The aircraft fly simultaneously along different, preferably perpendicular, tracks. The third strategy tested in Hurricane Gustav was single-platform sampling, which uses one Doppler radar on one aircraft that flies two consecutive, usually orthogonal, flight tracks. The antenna scans in the plane normal to the flight track. The third technique had been used previously in hurricanes and other disturbed weather.

The rms differences between the aircraft in situ winds and the Doppler winds derived near the aircraft by single-platform sampling, dual-platform sampling, and FAST are found to be 7.8, 5.1, and 2.5 m s−1, respectively. These results suggest that in hurricanes dual-platform flat-plane sampling and FAST both enable substantial improvements in the accuracy and temporal resolution of airborne Doppler wind fields over those obtained from single-platform, fiat-plane scanning. The FAST results should be applicable to dual-beam sampling, which began in 1991. The actual rms errors of Doppler winds far from the flight tracks, at levels well above flight level, and in highly sheared environments may be significantly higher than the above differences.

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Frank Roux, Jacques Testud, Marc Payen, and Bernard Pinty

Abstract

Pressure and temperature fields within a West African squall line, retrieved from dual-Doppler radar data collected during the “COPT 81” (Convection Profonde Tropicale) experiment are presented. The method for derivation of thew results is approximately similar to that proposed by Gal-Chen, based on the anelastic equation of motion.

Comparisons between pressure and temperature fields deduced from radar data at the lowest levels and surface network measurements show good agreement. The inferred thermodynamic structure displays the influence of a low-level frontward flow which is mainly due to a density current of cold air, generated in the stratiform region of the squall line and resulting from a mesoscale downdraft. This frontward flow contributes to initiate and maintain a frontal updraft through both nonhydrostatic pressure perturbation and temperature difference between entering air and colder frontward flow. At higher altitudes, mixing with the environment reduces buoyancy in the frontal updraft, while weaker convective updrafts develop in the inner region.

Comparisons between these results and the kinematic and thermodynamic structures deduced from a previous observation (Le Mone, 1983) display different types of dynamics of organized convective systems.

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Ching-Hwang Liu, Roger M. Wakimoto, and Frank Roux

Abstract

Pseudo-dual-Doppler analyses of two mesoscale circulations (20–40 km in horizontal dimensions) that formed along a warm front within a rapidly deepening extratropical cyclone over the ocean are presented. The circulations were analyzed using airborne Doppler radar during ERICA (Experiment on Rapidly Intensifying Cyclones over the Atlantic) IOP (intensive observing period) 4 and are believed to be a type of frontal instability that has not been addressed in past theoretical studies of frontal waves and cyclones. These features may be the shallow circulations that have been documented to play a role in cyclogenesis when they become coupled with an upper-level baroclinic wave. The present case is compared and contrasted with the only other well-analyzed event during ERICA IOP 5.

A retrieval of the perturbation pressure and buoyancy fields using the kinematic wind fields is performed in order to facilitate the interpretation of the dynamic structure of these vortices. In addition, a set of equations is derived in natural coordinates to examine the contributions to the total pressure by the cyclostrophic, buoyancy, updraft–vertical wind shear interaction, and the Bernoulli effects.

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Frank Roux, Fabrice Chane-Ming, Antoine Lasserre-Bigorry, and Olivier Nuissier

Abstract

Doppler radar observations of Tropical Cyclone Dina, as its eye passed at less than 100 km from the northern coast of La Réunion Island (21°S, 55.5°E) on 22 January 2002, are analyzed using the Ground-Based Extended Velocity Track Display (GB-EVTD) technique. This method is an extension of GB-VTD and it allows one to determine the full set of wavenumber-0 and -1 components of the tangential and radial winds in a tropical cyclone from a series of observations with a ground-based Doppler radar.

The results obtained for Dina reveal the presence of strong swirling winds (>65 m s−1) at 40–60-km radii from the storm center and below 3-km altitude. The observed changes in the location and intensity of the maximum winds, as well as the veering propagation of Dina, are shown to result probably from interaction between cyclonic winds and high topography of the island.

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Cheng-Ku Yu, David P. Jorgensen, and Frank Roux

Abstract

This study uses airborne Doppler radar measurements from the Special Observing Period of the Mesoscale Alpine Programme (MAP) to document the detailed airflow and precipitation structure over the mountainous regions near the border of northeastern Italy and Slovenia as a cold frontal system moved eastward and encountered the eastern Alps on 4 October 1999, during MAP IOP5. In contrast to previously documented MAP cases, the environmental conditions associated with this case are characterized by a deep layer of strong convective instability in the lower troposphere and by a cold, northeasterly continental flow coming down from the mountains (the so-called bora wind) along the southeastern Alps. Over the study region, there are two primary mountain barriers: the Julian Alps, oriented roughly west–east with a peak mountain height of ∼2500 m and a significant variation in terrain height along its length, and the other, the Dinaric Alps, a relatively lower mountain range oriented northwest–southeast, immediately adjacent to the south of the Julian Alps. How these two mountain barriers and the northeasterly continental flow influence the precipitation, and the nature of orographic precipitation developing in a potentially unstable environment, are explicitly addressed. Comprehensive analyses of airborne Doppler radar measurements reveal significant variations of precipitation in terms of its location, intensity, and type over the mountainous region. Particularly, five distinct forcing types of precipitation are evident for this case: slope convergence triggering, upslope triggering, weak slope convergence triggering, gap exit convergence triggering, and slope convergence forced stratiform. These precipitation characteristics are found to be closely related to the environmental thermodynamics, orographic features, and the complicated interactions between the southerly/southwesterly flow, northerly continental flow, and the orography. Details of these important observational aspects are elaborated upon in this paper.

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