Jean-Pierre Pinty

Extending Twomey’s Analytical Estimate of Nucleated Cloud Droplet Concentrations from CCN Spectra

A fundamental but approximate formula has been established by Twomey in order to compute the nucleated cloud droplet number concentration as a function of the vertical velocity and the CCN (cloud condensation nuclei) characteristics expressed by the C and k parameters. The derivation of such a relationship employs the widespread power-law dependence NCCN 5 of the total activable CCN number NCCN at a given supersaturation k Csy,w of water vapor over cloud droplets sy,w. In this paper the authors follow Twomey’s theoretical approach but using a more realistic four-parameter CCN activation spectrum as shaped by the physicochemical properties of the accumulation mode in a natural aerosol population. The analytical scheme is shown to improve the estimate of cloud droplet number concentration because it accounts for the limited availability of small-sized CN (condensation nuclei) with increasing supersaturation. A first validation and calibration of the new generic CCN activation spectrum is made by comparison with activation spectra, either produced by growing realistic lognormal distributions of CN or measured by a CCN spectrometer over the mid-Atlantic Ocean.

An investigation of Mesoscale Flows Induced by Vegetation Inhomogeneities Using an Evapotranspiration Model Calibrated Against HAPEX-MOBILHY Data

Abstract Many recent studies have suggested that heterogeneities in soil properties or vegetation characteristics many trigger mesoscale circulations in planetary boundary layer (PBL). Unfortunately, these flows appear to be very sensitive to the choice of the model characteristics and therefore require a careful calibration of the parameterization representing the vegetation/atmosphere interface. In this paper, the micrometeorological data from the HAPEX-MOBILHY field experiment are used to calibrate an evapotranspiration parameterization scheme over three types of dense vegetation typical of western Europe. This parameterization is then used a 2D mesoscale model to investigate the atmospheric response to a discontinuity in vegetation type (cereal crop to conifer forest). The results show a significant circulation when the soil is moist, associated with substantial PBL modification, whereas only a negligible atmospheric response is obtained when the soil is dry in the conifer forest). The results show a ...

A Numerical Investigation of the Influence of Large-Scale Winds on Sea-Breeze- and Inland-Breeze-type Circulations

Abstract A two-dimensional mesoscale model is used to study the influence of large-scale background winds on sea-breeze- and inland- (vegetation) breeze-type circulations. It is found that the intensity (vertical velocity) of the sea breeze is at its maximum when the Propagation speed of the sea-breeze front is canceled out by the background wind speed. Using the gravity current theory, we get a fair prediction of this optimum background wind value. The intensity and extent of the inland breeze, forming between a forecast and an adjacent crop area, do not vary over a large range of values for the large-scale wind. The location of the ascending branch of the inland breeze is stationary with respect to the interface between the two vegetation types. It is suggested that it is not friction drag but rather turbulent mixing that leads to a moon horizontally uniform boundary layer and which is responsible for the different behavior of the inland breeze, i.e., a weak and nonpropagating circulation.

Canopy resistance formulation and its effect in mesoscale models : a HAPEX perspective

Abstract The canopy resistance of the vegetation determines the Bowen ratio at the Earths surface when a vegetation cover is present. This control of the Bowen ratio indirectly determines the sensible heat flux distribution at the surface and, consequently, is a factor of prime importance for the development of mesoscale thermal circulations in the atmospheric boundary layer. In this paper, data from the HAPEX-MOBILHY experiment are used to examine the most important factors controlling the stomatal resistance, and the role of the stomatal resistance, in mesoscale models is discussed using two examples: a two-dimensional simulation of the atmospheric response to a discontinuity in vegetation type and a three-dimensional simulation of a clear day of the HAPEX-MOBILHY experiment (16 June 1986) which is compared with experimental data.

On the parameterization of activation spectra from cloud condensation nuclei microphysical properties

A simple parametric relationship is established between factors describing the shape of cloud condensation nuclei (CCN) activation spectra and observable properties of the aerosol population they grow on (size distribution and solubility). This is done independently for maritime and continental aerosol types because of their very different characteristics. The data used for the multiple statistical adjustments in the procedure described in this paper are generated by running a numerical model of aerosol growth coupled to a simple cloud droplet activation scheme. Each aerosol population (maritime and continental) is assumed to be of homogeneous chemical composition, lognormally distributed and with variable solubility. The parameterization is then evaluated using a large set of aerosol populations with randomized properties. Finally, the study presents a preliminary analysis of the most important aerosol properties that influence the shape of the CCN spectra. An idealized scenario of a clean maritime boundary layer cloud perturbed by anthropogenic emissions (such as the ship track problem) illustrates the capability of the parameterization to selectively increase the cloud droplet concentration in a partially polluted cloud. The calibration results presented in this paper are not meant to be the definitive activation spectra produced by any lognormally distributed aerosols. These results are indeed a step toward an objective initialization of CCN spectra and hence toward the computation of cloud droplet concentrations based on measurable multimodal aerosol features, as required by three-dimensional numerical models with a coupled interactive aerosol module.

Simple Tests of a Semi-Implicit Semi-Lagrangian Model on 2D Mountain Wave Problems

Abstract The fully compressible 3D nonhydrostatic semi-implicit semi-Lagrangian MC2 (mesoscale compressible community) model described by Tanguay et al. has been modified in order to incorporate orography through the Gal-Chen and Somerville transformation of the vertical coordinate by Denis. In this study, a 2D version of the model is tested against classical solutions covering various mountain-wave regimes for continuously stratified flows. A close inspection of the propagation of the vertical momentum flux is performed to asses the accuracy and stability of the numerical method. The study emphasizes also the fact that a correct representation of forced hydrostatic gravity waves is reliable for Courant numbers less than 0.5. This limitation may be less severe as the flow becomes more nonhydrostatic. Furthermore, the sensitivity of the isothermal reference state for flows with realistic static stability and over steep slope mountain is explored.

Modeling of passive microwave responses in convective situations using output from mesoscale models: Comparison with TRMM/TMI satellite observations

[1] Passive microwave observations are sensitive to the whole hydrometeor column, in contrast to infrared and visible observations, which essentially sense cloud tops. Therefore passive microwave observations are a very promising tool to study the internal structure of precipitating clouds. A microwave radiative transfer model (Atmospheric Transmission at Microwaves (ATM)) has been developed to accurately simulate brightness temperature TB fields using output from nonhydrostatic mesoscale atmospheric model, Meso-NH, simulations. The radiative transfer code takes the detailed description of the hydrometeor properties (as simulated by the Meso-NH model) into account. The sensitivity of the predicted brightness temperature TB to the hydrometeor properties is carefully analyzed. Depending on the frequency, the passive microwave simulations show different sensitivities to the hydrometeor and surface properties: The low frequencies (10–30 GHz) sense essentially the surface properties and the liquid water column, whereas the higher frequencies (30–90 GHz) are most sensitive to the large icy hydrometeors (graupel and snow). TB simulations are generated for two real convective situations studied with

A Radar Simulator for High-Resolution Nonhydrostatic Models

A full radar simulator for high-resolution (1–5 km) nonhydrostatic models has been developed within the research nonhydrostatic mesoscale atmospheric (Meso-NH) model. This simulator is made up of building blocks, each of which describes a particular physical process (scattering, beam bending, etc.). For each of these blocks, several formulations have been implemented. For instance, the radar simulator offers the possibility to choose among Rayleigh, Rayleigh–Gans, Mie, or T-matrix scattering methods, and beam bending can be derived from an effective earth radius or can depend on the vertical gradient of the refractive index of air. Moreover, the radar simulator is fully consistent with the microphysical parameterizations used by the atmospheric numerical model. Sensitivity experiments were carried out using different configurations for the simulator. They permitted the specification of an observation operator for assimilation of radar reflectivities by high-resolution nonhydrostatic numerical weather prediction systems, as well as for their validation. A study of the flash flood of 8–9 September 2002 in southeastern France, which was well documented with volumetric data from an S-band radar, serves to illustrate the capabilities of the radar simulator as a validation tool for a mesoscale model.

Radiative Transfer Simulations Using Mesoscale Cloud Model Outputs: Comparisons with Passive Microwave and Infrared Satellite Observations for Midlatitudes

Abstract Real midlatitude meteorological cases are simulated over western Europe with the cloud mesoscale model Meso-NH, and the outputs are used to calculate brightness temperatures at microwave frequencies with the Atmospheric Transmission at Microwave (ATM) radiative transfer model. Satellite-observed brightness temperatures (TBs) from the Advanced Microwave Scanning Unit B (AMSU-B) and the Special Sensor Microwave Imager (SSM/I) are compared to the simulated ones. In this paper, one specific situation is examined in detail. The infrared responses have also been calculated and compared to the Meteosat coincident observations. Overall agreement is obtained between the simulated and the observed brightness temperatures in the microwave and in the infrared. The large-scale dynamical structure of the cloud system is well captured by Meso-NH. However, in regions with large quantities of frozen hydrometeors, the comparison shows that the simulated microwave TBs are higher than the measured ones in the window...

A Midlatitude Precipitating Cloud Database Validated with Satellite Observations

The simulations of five midlatitude precipitating events by the nonhydrostatic mesoscale model Meso-NH are analyzed. These cases cover contrasted precipitation situations from 30° to 60°N, which are typical of midlatitudes. They include a frontal case with light precipitation over the Rhine River area (10 February 2000), a long-lasting precipitation event at Hoek van Holland, Netherlands (19 September 2001), a moderate rain case over the Elbe (12 August 2002), an intense rain case over Algiers (10 November 2001), and the “millennium storm” in the United Kingdom (30 October 2000). The physically consistent hydrometeor and thermodynamic outputs are used to generate a database for cloud and precipitation retrievals. The hydrometeor vertical profiles that were generated vary mostly with the 0°C isotherm, located between 1 and 3 km in height depending on the case. The characteristics of this midlatitude database are complementary to the GPROF database, which mostly concentrates on tropical situations. The realism of the simulations is evaluated against satellite observations by comparing synthetic brightness temperatures (BTs) with Advanced Microwave Sounding Unit (AMSU), Special Sensor Microwave Imager (SSM/I), and Meteosat observations. The good reproduction of the BT distributions by the model is exploited by calculating categorical scores for verification purposes. The comparison with 3-hourly Meteosat observations demonstrates the ability of the model to forecast the time evolution of the cloud cover, the latter being better predicted for the stratiform cases than for others. The comparison with AMSU-B measurements shows the skill of the model to predict rainfall at the correct location.