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Nicolas Viltard and Frank Roux

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.

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Frank Roux and Nicolas Viltard

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.

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Samo Diatta, Frédéric Hourdin, Amadou Thierno Gaye, and Nicolas Viltard

Abstract

Vertical rainfall profiles obtained with TRMM-PR 2A25 standard products are compared with rain profiles deduced from the Laboratoire de Météorologie Dynamique second generation global climate model (LMDZ, the Z stands for zoom capability) with two parameterization schemes: Emanuel’s and Tiedke’s. This paper focuses on the low layers of the atmosphere over West Africa during the monsoon [June–September (JJAS)]. The precipitation decrease above 4 km is systematically not represented in rainfall profiles generated by Emanuel’s parameterization scheme. However, Emanuel’s scheme shows a decrease similar to the observation from 4 km down to the surface, especially in the Sahel (proper depth of the layer dominated by reevaporation). As for Tiedtke’s scheme, it best describes the downward increase in the upper levels of the atmosphere, whereas the downward decrease in the lower levels begins too low when compared to the observations.

Tiedtke’s parameterization shows an overestimation of liquid water production over the ocean and over the Guinean region and a slightly too strong reevaporation in the Sahara and Sahel. The zonal distribution of vertical rain profiles is then biased with this model scheme compared to the 2A25-PR product. On the other hand, although Emanuel’s scheme detects too much reevaporation over the Sahara and underestimates liquid water production over the ocean compared to PR observation, it shows a good meridional distribution of these parameters. This is especially true in the Sahel where Emanuel’s scheme gives the best representation of reevaporation.

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