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.

Abstract

A new method is proposed to determine completely the thermodynamic fields from the relative pressure and temperature perturbations, retrieved from the processing of multiple-Doppler radar data through the equations of motion. A simplified thermodynamic equation is used to calculate the vertical gradients of the missing pressure and temperature constants. Then. provided that an initial value is known, the three-dimensional fields of pressure and temperature are deduced through adding, at each altitude, these constants to the relative pressure and temperature fields.

This method is tested with dual-Doppler radar data from the observation of the frontal convective part of a West-African squall-line on 22 June 1981. A first attempt is made with two-dimensional data derived from the actual radar data and from the environmental winds. Then the method is applied to the actual three-dimensional fields. Vertical cross-sections of pressure and temperature perturbations are deduced, and the different components of the vertical projection of the equation of motion are analyzed. The different hypotheses and possible further improvements are finally discussed.

Abstract

In Part I, the kinematic and precipitating fields of Hurricane Claudette have been analyzed, using airborne Doppler radar data collected on 7 September 1991 by the two National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft. Evidence of an incipient “eyewall replacement cycle” and its influence on Hurricane Claudette circulation have been revealed through the EVTD (extended velocity track display) method. This study has been conducted for six successive analyses in a domain of 200 km × 200 km × 12 km domain from 1700 to 2200 UTC.

A thermodynamic retrieval method is adapted here to the EVTD geometry to deduce the temperature and pressure perturbation fields from the previously EVTD-derived wind fields. The relation between the evolution of the circulation and the thermodynamic structure of Hurricane Claudette can now be studied. The main feature deduced from this method is a positive temperature perturbation about 8–9 K warmer than the environment at the center of the storm circulation, associated with a pressure deficit of about 25 hPa at the sea level. During the considered period, the temperature perturbation maximum changed according to the evolution of the inner eyewall, while warming in the middle part was related to intensifying external outward motions and cooling in the outer part, due to stronger inflow. Meanwhile, there is no distinct evolution of the pressure perturbation field. Comparisons between the retrieved thermodynamic fields and in situ data collected by both aircraft along their flight track show qualitatively good agreement, although the EVTD-retrieved values have substantially lower amplitudes, probably due to the strong spatial and temporal filtering. Analyses of fields with different time filtering confirms that inertia–gravity waves that may propagate outward from the system do not seem to affect the retrieved kinematic and thermodynamic fields.

Considering only the symmetric part (wavenumber 0) of this EVTD kinematic and thermodynamic description of Hurricane Claudette, the authors have verified that throughout most of the considered domain of the study, gradient wind balance, hydrostatic equilibrium, and thermal wind relation are nearly verified. Nevertheless, there are some indications that supergradient winds may be found locally in the lower inner part of the eyewall.

Abstract

On 7 September 1991, an experiment was conducted with the two National Oceanic and Atmospheric Administration (NOAA) WP-3D research aircraft to investigate the inner-core region of Hurricane Claudette. Both aircraft carried airborne Doppler radar, and one of them (N43RF) was equipped with a French dual-beam antenna that allows velocity measurements in two different directions. The aircraft flew simultaneously a series of rotated radial penetrations of the eyewall, the upper one being oriented 90° clockwise from the lower one. This dataset provided an extensive survey of the storm inner region (radius less than 100 km) for a period of about 7 h.

The Doppler velocity data from the two aircraft were analyzed with an EVTD (extended velocity track display) method. The precipitation content and the tangential, radial, and vertical wind components were calculated at 6 hourly intervals, in 50 rings at 1–99-km radii and 25 levels at 0.5–12.5-km altitudes, as wavenumbers 0, 1, and 2 with respect to the storm-relative azimuth. The limited angular resolution, associated with an efficient time filtering, ensured that the small-scale convective and inertial perturbations were virtually eliminated in the obtained wind fields. Comparisons with independent flight-level measurements showed that the EVTD-derived velocity values were representative of the storm kinematic structure.

During the airborne observations, Hurricane Claudette underwent a partial “eyewall replacement cycle,” as an inner crescent-shaped zone of high-reflectivity values progressively shrank, while at larger radii a ring of strong echoes built up. The first kinematic characteristic of Hurricane Claudette was its motion at a speed smaller, although in a similar direction, than the mean horizontal winds. For this reason, the inflow was mainly from the rear (south-southeast) in the lower and upper levels. The symmetric vortex structure was characterized by a decreasing inner updraft at 5–25-km radii and, at larger radii, an intensifying updraft with downward motions on its inner and outer sides in the upper levels. The low-level inflow and upper-level outflow consequently intensified in the outer part of the domain, while there was a slight decrease of the tangential wind close to the radius of maximum wind and an increase in the low levels below the developing updraft.

In the inner region (15–25-km radii), the updraft and low-level inflow were located upwind of the reflectivity maximum and progressively decreased while the downdraft intensified on the downwind side. In the outer ring (25–50-km radii), upward motions built up from the east-southeast in the low levels to the west-southwest in the upper levels. The asymmetric part of the horizontal wind (total wind minus the mean horizontal component and the symmetric vortex) displayed a wavenumber 1 eddy couplet below 6-km altitude and a mass source—sink pair above. These features, which probably resulted from divergence patterns associated with the vertical motions and from interactions between the storm motion and the mean vortex, were perturbed by the evolution of the kinematic structure. The asymmetric flow changed substantially during the considered period, but the flow across the storm center remained approximately constant. This could be related to the nearly linear motion of the storm.

Abstract

A large and intense West African squall line has been observed during the night of 27/28 May 1981 at Korhogo (in the north of Ivory Coast) during the COPT 81 experiment. The environmental conditions in which it occurred and its overall structure are slightly different from the previously analyzed COPT 81 cases on 22 and 23–24 June 1981. Unfortunately, in this situation, as only single-Doppler radar data available, it is not possible to obtain detailed information on the small scale structure of the leading convective region. Nevertheless talking advantage of the apparent stationarity of the mesoscale features, a new processing method (referred to as ABCD) has been developed for a combined processing of 12 successive conical scans, which allows deduction of three-dimensional wind and reflectivity fields in a domain of 260 × 60 × 15 km³, with resolutions of 20 km in the horizontal and 500 m in the vertical.

The deduced circulation displays classical characteristics of trailing straitform parts of mesoscale convective systems, with a mesoscale updraft above 4 km, a mesoscale downdraft below, and a relative rear-to-front flow of moderate intensity in the midlevels; all these features are slightly more intense than on 22 and 23–24 June 1981. One particularity here is that, because of the propagation of the squall line in a direction slightly different from that of the environmental winds, the velocity component parallel to the line is relatively important. Pressure and temperature fields retrieved from an improved analysis of the momentum equation and of the thermodynamic equation show a warming due to condensation in the mesoscale updraft, cooling due to evaporation below, hydrostatic low pressure in the midlevels and high pressure near the surface. The vertical component of vorticity is weak and correlated with the horizontal divergence, while there is an anticorrelation between the shearing deformation and the horizontal divergence. The cross-line component of vorticity is weak but not negligible, as compared with the along-line component. The baroclinic generation, due to the horizontal buoyancy gradient, is the dominant term in the budget of the along-line component of vorticity, however, the influence of the three-dimensional effects through the shearing-tilting term are ~ important in the leading part of the considered domain. The water budget, obtained through a microphysical mtdcvw in one vertical plane, provides values of the different components relatively similar to those obtained in previous studies, with, however, a ~r condensation rate due to the more important upward motions.

Abstract

Two West African disturbances observed in August and September 2006 during the National Aeronautics and Space Administration African Monsoon Multidisciplinary Analysis (NAMMA) and the Special Observing Period 3 (AMMA/SOP-3) have been simulated using the Méso-NH numerical model with explicit convection. The first disturbance spawned Hurricane Helene (2006) off the West African coast, and the second one, referred to as perturbation D, though relatively intense, failed to develop. Over the continent, each case was associated with a well-defined African easterly wave (AEW) trough with embedded growing and decaying convective activity of various size, duration, and intensity. The aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the AEW trough, with respect to the synoptic-scale processes. The absolute vorticity budgets are analyzed during the “continental” and “oceanic transition” stages of these AEW troughs in order to highlight the similarities and differences between the developing pre-Helene disturbance and the nondeveloping perturbation D. For the developing case, low- to midlevel cyclonic vorticity was produced by convective processes through tilting and stretching. Cyclonic vorticity was then transported upward through vertical advection associated with convection and outward through horizontal advection mostly induced by the large-scale midlevel diverging circulation related to the downstream AEW ridge. For the nondeveloping case, low- to midlevel cyclonic vorticity production through stretching and tilting, and its vertical transport were relatively similar over the continent but smaller over the oceanic transition because of weaker convective activity. The outward transport through horizontal advection was also weaker as there was little midlevel divergence induced by the downstream AEW ridge in this case.

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.

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.

Abstract

Doppler radar–derived fields of wind and reflectivity, retrieved temperature perturbations, estimated water vapor, and cloud water contents are used to initialize a nonhydrostatic cloud-resolving model. Airborne Doppler data collected in a tropical mesoscale convective system on 9 February 1993 during the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment are prepared for this purpose. Different numerical experiments are conducted to verify the reliability of the approach and to determine the most important parameters leading to the most realistic simulations possible.

The obtained results show that the numerical model can be initialized with two-dimensional wind and reflectivity fields describing a well-developed convective circulation. A major conclusion is that, apart from the horizontal and vertical wind, the most important parameter in the initial field is humidity. Although a relatively crude thermodynamic and microphysical description was sufficient, a complete description of the saturated/unsaturated conditions is essential to obtain simulations matching the observed characteristics. Finally, consequences of the results for the understanding of the 9 February 1993 mesoscale convective system and future applications of this technique are discussed.

Abstract

The authors present an improved version of the velocity track display (VTD) method, proposed by Lee et al., to deduce the primary vortex circulation in hurricanes from airborne Doppler radar data obtained during straightline legs through the storm center. VTD allows the derivation of one projection of the mean horizontal wind, the wavenumber 0, 1, and 2 components of the tangential wind and one projection of the radial wind, in a series of concentric rings centered on the storm circulation center. The extended VTD (EVTD) algorithm determines additional information through a combination of data collected during successive legs: the Cartesian components of the mean horizontal wind; the wavenumber 0, 1, and 2 components of the tangential wind; and the wavenumber 0 and 1 components of the radial wind.

Application of EVTD to airborne Doppler data collected on 17 September 1989 in Hurricane Hugo is discussed. Comparisons between the EVTD-derived winds, the flight-level measurements, and winds deduced from “pseudo-dual-Doppler” analyses show qualitatively good agreement. These results reveal the asymmetric structure of the storm and show that it was in a deepening stage, with increasing tangential wind, inflow, and upward velocity. Further applications are finally discussed.