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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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Nicolas Viltard, Corinne Burlaud, and Christian D. Kummerow

Abstract

This study focuses on improving the retrieval of rain from measured microwave brightness temperatures and the capability of the retrieved field to represent the mesoscale structure of a small intense hurricane. For this study, a database is constructed from collocated Tropical Rainfall Measuring Mission (TRMM) precipitation radar (PR) and the TRMM Microwave Imager (TMI) data resulting in about 50 000 brightness temperature vectors associated with their corresponding rain-rate profiles. The database is then divided in two: a retrieval database of about 35 000 rain profiles and a test database of about 25 000 rain profiles. Although in principle this approach is used to build a database over both land and ocean, the results presented here are only given for ocean surfaces, for which the conditions for the retrieval are optimal. An algorithm is built using the retrieval database. This algorithm is then used on the test database, and results show that the error can be constrained to reasonable levels for most of the observed rain ranges. The relative error is nonetheless sensitive to the rain rate, with maximum errors at the low and high ends of the rain intensities (+60% and −30%, respectively) and a minimum error between 1 and 7 mm h−1. The retrieval method is optimized to exhibit a low total bias for climatological purposes and thus shows a high standard deviation on point-to-point comparisons. The algorithm is applied to the case of Hurricane Bret (1999). The retrieved rain field is analyzed in terms of structure and intensity and is then compared with the TRMM PR original rain field. The results show that the mesoscale structures are indeed well reproduced even if the retrieved rain misses the highest peaks of precipitation. Nevertheless, the mesoscale asymmetries are well reproduced and the maximum rain is found in the correct quadrant. Once again, the total bias is low, which allows for future calculation of the heat sources/sinks associated with precipitation production and evaporation.

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Nicolas Viltard, Estelle Obligis, Virginie Marecal, and Claude Klapisz

Abstract

The aim of this paper is to report on the retrieval of the vertically averaged liquid cloud water content and vertically averaged precipitation rates (rain and ice) from microwave airborne radiometric observations in a two-plane parallel layer atmosphere. The approach is based on the inversion of a simple radiative transfer model in which a raindrop size distribution derived from microphysical measurements is introduced. The microwave data (18.7, 21, 37, and 92 GHz) used were acquired by the Airborne Multichannel Microwave Radiometer and Advanced Microwave Moisture Sounder on board NASA DC8 within a mesoscale convective system on 6 February 1993 during the Tropical Oceans Global Atmosphere Coupled Ocean–Atmosphere Response Experiment.

Before interpreting the results, the quality of the inversion is checked. The fit between the measured and the model-retrieved brightness temperatures is good when compared to the model and measurements uncertainties. Doppler radar data from three other aircraft help the result’s interpretation, providing reflectivity and wind fields. The cloud liquid content seems to be difficult to retrieve. The ice and liquid rain rates are consistent with the other data sources: order of magnitude for convective and stratiform regions, presence of ice and liquid precipitation correlated with cell structure, and presence of cloud particles in the lighter precipitating regions.

A quantitative comparison is done between the radiometric rainfall rates and those derived from the Airborne Rain Mapping Radar observations (also on board NASA DC8). There is a good agreement between the two from the statistical point of view (mean and standard deviation values). Moreover, the finescale rain structures that appear in radar results are rather well reproduced in the radiometric results. The importance of the new drop size distribution introduced in the radiative transfer model is emphasized by this last comparison.

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Nicolas Viltard, Christian Kummerow, William S. Olson, and Ye Hong

Abstract

A combination of passive microwave and radar observations from the Tropical Rainfall Measuring Mission (TRMM) satellite is used to investigate the consistency between the two sensors. Rather than relying on some absolute “truth” to verify retrievals, this paper focuses on one assumption—namely, the drop size distribution (DSD)—and how different DSDs lead to improved or reduced consistency. Results from a case in the central Pacific suggest that a crude consistency may be achieved if a different drop size is used for the radiometer and the radar. In this particular case, a Marshall–Palmer or a gamma distribution with the shape parameters properly set leads to similar results. Although this study offers no independent validation of its conclusions, it does demonstrate that rainfall validation need not be confined to surface rainfall measurements, which are only loosely related to the volumetric observations made by most sensors. As mean size distributions of raindrops are measured in the TRMM field experiments by disdrometers, profilers, multiparameter radars, and direct aircraft observations, the technique presented in this paper can be applied on a storm-by-storm basis, and conclusions can be verified directly.

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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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Thomas Garot, Hélène Brogniez, Renaud Fallourd, and Nicolas Viltard

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The spatial and temporal distribution of upper-tropospheric humidity (UTH) observed by the Sounder for Atmospheric Profiling of Humidity in the Intertropics by Radiometry (SAPHIR)/Megha-Tropiques radiometer is analyzed over two subregions of the Indian Ocean during October–December over 2011–14. The properties of the distribution of UTH were studied with regard to the phase of the Madden–Julian oscillation (active or suppressed) and large-scale advection versus local production of moisture. To address these topics, first, a Lagrangian back-trajectory transport model was used to assess the role of the large-scale transport of air masses in the intraseasonal variability of UTH. Second, the temporal evolution of the distribution of UTH is analyzed using the computation of the higher moments of its probability distribution function (PDF) defined for each time step over the domain. The results highlight significant differences in the PDF of UTH depending on the phase of the MJO. The modeled trajectories ending in the considered domain originate from an area that strongly varies depending on the phases of the MJO: during the active phases, the air masses are spatially constrained within the tropical Indian Ocean domain, whereas a distinct upper-tropospheric (200–150 hPa) westerly flow guides the intraseasonal variability of UTH during the suppressed phases. Statistical relationships between the cloud fractions and the UTH PDF moments of are found to be very similar regardless of the convective activity. However, the occurrence of thin cirrus clouds is associated with a drying of the upper troposphere (enhanced during suppressed phases), whereas the occurrence of thick cirrus anvil clouds appears to be significantly related to a moistening of the upper troposphere.

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Virginie Marecal, Taoufik Tani, Paul Amayenc, Claude Klapisz, Estelle Obligis, and Nicolas Viltard

Abstract

The first part of this paper is dedicated to establishing relations among rain-integrated parameters representative of west Pacific precipitation. This is achieved by using airborne microphysical data gathered within a rain event on 6 February 1993 during the Tropical Ocean and Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). The relations between the rain rate R, the reflectivity factor Z, and the attenuation coefficient K are calculated for moderate to heavy precipitation at 13.8 GHz. They give twice as much attenuation for a given Z than the relations obtained for an exponential distribution with N 0 = 8 × 106 m−4. This effect is related to the large number of small size particles observed in TOGA COARE convective systems.

In the second part of the paper, these relations are used to check the reliability of a rain-profiling method applied to ARMAR (airborne radar-mapping radar) observations at 13.8 GHz in the same rain event. This method provides a bulk correction factor that can be interpreted primarily in terms of a change of the initial ZK relation. Then, the algorithm provides modified ZR and KR relations while assuming a gamma or an exponential-shaped distribution for raindrops with a constant N 0. For the selected case study, the adjusted relations agree very well with those derived from the microphysical measurements. An exponential shape model with constant N 0 for the DSD is found to provide results that are consistent with the microphysical measurements. Moreover, the derived N 0 value is close to that inferred from the radar algorithm. The impact of modifying the initial rain relations in the radar algorithm on the rain-rate estimates is also discussed. The retrieved rain rates are not very sensitive to the choice of initial relations except for very high values. Finally, the results are found more representative of convective rain than stratiform precipitation.

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William S. Olson, Peter Bauer, Nicolas F. Viltard, Daniel E. Johnson, Wei-Kuo Tao, Robert Meneghini, and Liang Liao

Abstract

In this study, a 1D steady-state microphysical model that describes the vertical distribution of melting precipitation particles is developed. The model is driven by the ice-phase precipitation distributions just above the freezing level at applicable grid points of “parent” 3D cloud-resolving model (CRM) simulations. It extends these simulations by providing the number density and meltwater fraction of each particle in finely separated size categories through the melting layer. The depth of the modeled melting layer is primarily determined by the initial material density of the ice-phase precipitation. The radiative properties of melting precipitation at microwave frequencies are calculated based upon different methods for describing the dielectric properties of mixed-phase particles. Particle absorption and scattering efficiencies at the Tropical Rainfall Measuring Mission Microwave Imager frequencies (10.65–85.5 GHz) are enhanced greatly for relatively small (∼0.1) meltwater fractions. The relatively large number of partially melted particles just below the freezing level in stratiform regions leads to significant microwave absorption, well exceeding the absorption by rain at the base of the melting layer. Calculated precipitation backscatter efficiencies at the precipitation radar frequency (13.8 GHz) increase with particle meltwater fraction, leading to a “bright band” of enhanced radar reflectivities in agreement with previous studies. The radiative properties of the melting layer are determined by the choice of dielectric models and the initial water contents and material densities of the “seeding” ice-phase precipitation particles. Simulated melting-layer profiles based upon snow described by the Fabry–Szyrmer core-shell dielectric model and graupel described by the Maxwell-Garnett water matrix dielectric model lead to reasonable agreement with radar-derived melting-layer optical depth distributions. Moreover, control profiles that do not contain mixed-phase precipitation particles yield optical depths that are systematically lower than those observed. Therefore, the use of the melting-layer model to extend 3D CRM simulations is likely justified, at least until more-realistic spectral methods for describing melting precipitation in high-resolution, 3D CRMs are implemented.

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Shuyi S. Chen, Brandon W. Kerns, Nick Guy, David P. Jorgensen, Julien Delanoë, Nicolas Viltard, Christopher J. Zappa, Falko Judt, Chia-Ying Lee, and Ajda Savarin

Abstract

One of the most challenging problems in predicting the Madden–Julian oscillation (MJO) is the initiation of large-scale convective activity associated with the MJO over the tropical Indian Ocean. The lack of observations is a major obstacle. The Dynamics of the MJO (DYNAMO) field campaign collected unprecedented observations from air-, land-, and ship-based platforms from October 2011 to February 2012. Here we provide an overview of the aircraft observations in DYNAMO, which captured an MJO initiation event from November to December 2011. The National Oceanic and Atmospheric Administration (NOAA) WP-3D aircraft was stationed at Diego Garcia and the French Falcon 20 aircraft on Gan Island in the Maldives. Observations from the two aircraft provide a unique dataset of three-dimensional structure of convective cloud systems and their environment from the flight level, airborne Doppler radar, microphysics probes, ocean surface imaging, global positioning system (GPS) dropsonde, and airborne expendable bathythermograph (AXBT) data. The aircraft observations revealed interactions among dry air, the intertropical convergence zone (ITCZ), convective cloud systems, and air–sea interaction induced by convective cold pools, which may play important roles in the multiscale processes of MJO initiation. This overview focuses on some key aspects of the aircraft observations that contribute directly to better understanding of the interactions among convective cloud systems, environmental moisture, and the upper ocean during the MJO initiation over the tropical Indian Ocean. Special emphasis is on the distinct characteristics of convective cloud systems, environmental moisture and winds, air–sea fluxes, and convective cold pools during the convectively suppressed, transition/onset, and active phases of the MJO.

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