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Daniele Hauser and Paul Amayenc

Abstract

Raindrop-size distributions and vertical air motions are deduced from vertically pointing C-band Doppler radar data by using a new method (described in Hauser and Amayenc, 1981) which assumes the raindrop sizes are exponentially distributed [N 0 exp(−λD)]. Results gathered in stratiform rain precipitation after the passage of a cold front are presented and compared with those obtained by using the well-known Rogers approach. The new method which does not a priori require the knowledge of N 0 values is capable of detecting spatial and temporal variations much better than the Rogers method. Taking this variability into account allows one to find relationships involving three rainfall parameters with good accuracy.

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Michel Chong and Daniele Hauser

Abstract

The role of moist convective processes in the heat and moisture budgets of the 22 June 1981 tropical squall line is investigated. Detailed kinematic structure from Doppler radar observations, and thermodynamic and microphysical fields diagnosed from a steady-state model respectively presented in Parts I and II of this paper, are used to estimate the total apparent heat source and moisture sink. The relative contributions of the system components (the convective-scale component associated with the leading convective region, and the mesoscale component of the trailing stratiform region) and of the physical processes (latent heat release and eddy transports) are examined.

Consistent results are examined which are obtained in qualitative agreement with those of previous studies. Condensation and evaporation, through the release of latent heat, are the dominant terms in the total apparent heating profile although the melting process is also important. The total apparent moisture sink is also dominated by condensation and evaporation. However, the eddy moisture flux has an important influence on the vertical structure.

Although the stratiform-region heating and moistening profiles display common features with other studies (i.e., upper level heating and drying due to condensation in the mesoscale updraft, and lower level cooling and moistening due to rain evaporation in the mesoscale downdraft), the total heat and moisture budgets present significant differences. The total apparent heat source is marked by a deep layer of cooling, while the total apparent moisture sink has a single peak observed in the middle troposphere. This differs fundamentally from the net heating and the double-peak structure of the moisture sink, often seen in tropical budget studies. The differences primarily lie in the structure of the convective-region profiles.

The convective-scale heating profile indicates a net cooling in the lower troposphere, mainly due to the evaporation of precipitation. This cooling is explained by a deep subsaturated layer where convective precipitation evaporates. The convective-region moisture sink profile exhibits drying through the full depth of the troposphere, but with a peak at middle levels. It is found that this relatively high position of the drying peak results from the specific contribution of the eddy moisture transport in the convective region, which is concentrated in the low-to-middle troposphere and which tends to raise the drying peak due to condensation in convective updrafts. From this particular impact of convective eddies, an interpretation of the occurrence of a double-peak structure in the total apparent moisture sink is proposed. It is also found that ambient dry air entering the system at midlevels strongly modifies the vertical exchange by convective updrafts by limiting it to the lower part of the troposphere.

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Daniele Hauser and Paul Amayenc

Abstract

Using the hypothesis of an exponential hydrometeor-size distribution characterized by the two classical intercept (N 0) and slope (λ) parameters, a method is presented for simultaneously deducing N 0, λ and the vertical air velocity w from a Doppler spectrum measured at vertical incidence. It is based on a least-squares fitting of a theoretical spectrum depending on N 0, λ and w, which are the adjusted parameters, to the measured one. The principle, sensitivity and limitations of the method are discussed in detail. It is expected that this method applies mainly to the case of stratiform rain.

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Michel Chong and Daniele Hauser

Abstract

The relative contributions of the different processes involved in the water budget of the 22 June 1981 tropical squall line are investigated. The kinematic and thermodynamic fields derived from Doppler radar data are used to calculate the sources and sinks of condensate in the convective and trailing stratiform regions of the system. Both regions play an important role in providing the surface rain. Fifty-five to 65% of the total rain precipitated at the surface is convective, while the remaining 35–45% is supplied by the trailing stratiform cloud. This partition corresponds to a precipitation efficiency of 47–57% for the convective region, and 45-57% for the stratiform region. Though these efficiencies are of the same order of magnitude, the sink of water substance in each region (before reaching the surface) is attributable to different processes. In the convective region, low of water substance is mainly due to the transfer of condensate into the trailing anvil cloud. This transfer represents 32% of the condensate formed in the convective updrafts and constitutes an important source (47%) of condensate for the stratiform cloud. In the stratiform region, the evaporation of precipitation which occurs beneath the trailing anvil cloud is predominant: 33% of the total condensate supplied to the cloud is lost in the mesoscale evaporative downdraft. Cooling effects due to this important evaporation, together with those associated with melting, help to maintain the observed mesoscale downdraft. Finally, this study indicates that the transfer of condensate from the convective to the stratiform region and mesoscale evaporation of precipitation may have a direct influence on the apparent feedback between convective-scale and mesoscale airflows.

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Danièle Hauser and Paul Amayenc

Abstract

This paper describes the retrieval of cloud water and water vapor contents from Doppler radar data. The convective part of a tropical squall line (22 June 1981) observed during the COPT 81 (Convection Profonde Tropicale 1981) West African experiment, was chosen for developing a two-dimensional and steady state model for the retrieval of these parameters. The model is based upon the solution of the continuity equation for the total water substance, with wind and rain water fields specified from Doppler radar observations. The results are consistent with the previous kinematic analysis of the convective part of this squall line. Cloud water mixing ratios up to 4 g kg−1 are found in the warm and unstable inflow while unsaturated air is observed in the low level frontward cold flow. At high altitude, an important amount of condensate is transferred rearward into the stratiform part of the squall line. The paper also presents sensitivity tests of the model in order to discuss the main assumptions used to cope with the problem.

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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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Anthony J. Illingworth, Daniele Hauser, and Paul Amayenc

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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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Béatrice Fradon, Danièle Hauser, and Jean-Michel Lefèvre

Abstract

Numerical wave prediction models presently used in the meteorological institutes are still of two types: the so-called second-generation and third-generation models. In this paper, the authors present a comparison of the performance of a second-generation model—the VAG model from Météo-France—and of the third-generation WAM model. These two models have been run with similar characteristics (same wind input, same resolution). Simple tests show the differences between the behaviors of VAG and WAM in typical situations (constant wind, rotating wind). Hindcasts have been performed in the general context of the SEMAPHORE experiment. A one-month hindcast over the North Atlantic domain has been run by driving both models with the same wind fields. A comparison between the model output and the available observations, including significant wave height from satellite measurements, is presented. The results show that VAG and WAM results are in a general good agreement with the observations, but also that WAM results are a little better than VAG when the satellite data are taken as a reference. A modification of VAG is then proposed, which allows the performances of VAG to be closer to those of WAM. This study shows that (i) the second-generation VAG model is nearly as good in predicting wave heights as the third-generation model WAM in spite of its poor representation of the nonlinear interactions and (ii) VAG has been improved when introducing the growth and dissipation terms of WAM instead of parameterizations taken from Golding.

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