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Frank Roux

Abstract

The squall line observed during the night of 23-24 June 1981 was one of the most intense events during the COPT 81 (COnvection Profonde Tropicale) experiment, conducted in May and June 1981 at Korhogo in the northern Ivory Coast by French and Ivorian research institutes. The presquall environment possessed a very large convective instability with a stable layer in the lowest levels. The presence of dry air in the midlevels, which promoted the development of convective downdrafts, and the unusually low altitude of the strong easterly winds, which aided the development of a leading anvil above 6 km, are two noteworthy characteristics of the environment of this squall line. Its mesoscale structure, as deducted from radar data, is similar to that previously observed for tropical squall lines. Convective updrafts and downdraft occur in the leading heavy precipitation region (convective region) and a mesoscale updraft and downdraft is found within and below the trailing mid-to upper-level anvil clouds, respectively (stratiform region).

The analysis of dual-Doppler radar data shows that the convective region was composed of short-lived cells characterized by intense updrafts and high reflectivity values, with convective downdrafts between and behind the cells. An improved method to retrieve thermodynamic fields from the radar data documents a Cold low-level frontward flow, inducing a dynamic pressure high in front of the line, which in turn forces the initial lifting of the inflowing air in the low levels. Positive temperature perturbations at midlevels and the hydrostatic pressure low beneath the convective region result from the large convective instability of the entering monsoon flow.

Comparisons of vertical fluxes of horizontal momentum and thermodynamic properties in the convective region and vertical gradients of the environmental values show that, owing to the internal sources/sinks and two-dimensionality of the line, the component of momentum normal to the line and the “virtual-cloud” potential temperature are transported against their vertical gradients, while the parallel component of momentum is transported down its vertical gradient. Horizontal exchange rates of mass, momentum and sensible heat (also deduced from the radar data) illustrate the important role of the convective cells in modifying the kinematic and thermodynamic structure of the middle and upper troposphere.

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Joël Arnault and Frank Roux

Abstract

The so-called “perturbation D” was a nondeveloping West African disturbance observed near Dakar (Senegal) during special observing period (SOP) 3 of the African Monsoon Multidisciplinary Analysis (AMMA) in September 2006. Its mesoscale environment is described with the dropsonde data obtained during flights on three successive days with the Service des Avions Français Instrumentés pour la Recherche en Environnement Falcon-20 aircraft. Processes involved in this evolution are studied qualitatively with ECMWF reanalyses and Meteosat-9 images. The evolution of perturbation D was the result of an interaction between processes at different scales such as the African easterly jet (AEJ), a midtropospheric African easterly wave (AEW), a series of mesoscale convective systems, the monsoon flow, dry low- to midlevel anticyclonic Saharan air, and a midlatitude upper-level trough. The interaction between these processes is further investigated through a numerical simulation conducted with the French nonhydrostatic Méso-NH model with parameterized convection. The growth of the simulated disturbance is quantified with an energy budget including barotropic and baroclinic conversions of eddy kinetic energy, proposed previously by the authors for a limited domain. The development of the simulated system is found to result from barotropic–baroclinic growth over West Africa and baroclinic growth over the tropical eastern Atlantic. It is suggested that these energy conversions were the result of an adjustment of the wind in response to the pressure decrease, presumably caused by convective activity, and other synoptic processes. A comparison with the developing case of Helene (2006) reveals that both perturbations had similar evolutions over the continent but were associated with different synoptic conditions over the ocean. For perturbation D, the anticyclonic curvature of the AEJ, caused by the intensification of the eastern ridge by a strong flow of dry Saharan air, prohibited the formation of a closed and convergent circulation. Moreover, a midlatitude upper-level trough approaching from the northwest contributed to increase the northward stretching and then weakened the perturbation. It is therefore suggested that at least as important as the intensity of the AEW trough and associated convection leaving the West African continent are synoptic conditions associated with the Saharan heat low, the subtropical high pressure zone, and even the midlatitude circulation, all of which are instrumental in the (non)cyclogenetic evolution of AEWs in the Cape Verde Islands region.

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Joël Arnault and Frank Roux

Abstract

The West African perturbation that subsequently evolved into Hurricane Helene (2006) during NASA’s African Monsoon Multidisciplinary Analysis (NAMMA), 15 August–14 September 2006, and AMMA’s third special observing period (SOP-3), 15–29 September 2006, has been simulated with the nonhydrostatic Méso-NH model using parameterized convection. The simulated disturbance evolved over West Africa and the adjacent eastern tropical Atlantic through interactions between different processes at the convective scale, mesoscale, and synoptic scale. The aim of this paper is to quantify the energetics of the simulated disturbance. A set of energy equations is first developed in the hydrostatic case to solve the limitations of Lorenz’s analysis when applied to a finite domain. It is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the Méso-NH results. Application to the simulated pre-Helene disturbance allows one to determine the most important terms in these equations. These simplifications are taken into account to derive an energy cycle including barotropic and baroclinic conversions of eddy kinetic energy. The development of the simulated system was found to result from barotropic–baroclinic growth over West Africa and barotropic growth over the tropical eastern Atlantic. It is suggested that most of these energy conversions were the result of an adjustment of the wind field in response to the pressure decrease, presumably caused by convective activity.

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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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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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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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Marie-Dominique Leroux, Matthieu Plu, David Barbary, Frank Roux, and Philippe Arbogast

Abstract

The rapid intensification of Tropical Cyclone (TC) Dora (2007, southwest Indian Ocean) under upper-level trough forcing is investigated. TC–trough interaction is simulated using a limited-area operational numerical weather prediction model. The interaction between the storm and the trough involves a coupled evolution of vertical wind shear and binary vortex interaction in the horizontal and vertical dimensions. The three-dimensional potential vorticity structure associated with the trough undergoes strong deformation as it approaches the storm. Potential vorticity (PV) is advected toward the tropical cyclone core over a thick layer from 200 to 500 hPa while the TC upper-level flow turns cyclonic from the continuous import of angular momentum.

It is found that vortex intensification first occurs inside the eyewall and results from PV superposition in the thick aforementioned layer. The main pathway to further storm intensification is associated with secondary eyewall formation triggered by external forcing. Eddy angular momentum convergence and eddy PV fluxes are responsible for spinning up an outer eyewall over the entire troposphere, while spindown is observed within the primary eyewall. The 8-km-resolution model is able to reproduce the main features of the eyewall replacement cycle observed for TC Dora. The outer eyewall intensifies further through mean vertical advection under dynamically forced upward motion. The processes are illustrated and quantified using various diagnostics.

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