Robert Rosset

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 ...

The Numerical Simulation of Clouds, Rains and Airflow over the Vosges and Black Forest Mountains: A Meso-β Model with Parameterized Microphysics

Abstract A three-dimensional meso-β model with parameterized microphysics is presented. The model is capable of simulating orographically forced clouds, rain, and airflow. Tests using a two-dimensional version confirm the ability of the model to replicate the linear and nonlinear mountain wave simulations of previous authors. The model is applied to the Rhine valley and surrounding mountainous areas, the Vosges in France and the Black Forest in Germany. Model-predicted rainfall over the mountainous areas is in good agreement with observations in both magnitude and location; however, an absence of model-predicted cloud cover over the Rhine valley suggests the need for an improved mesoscale initialization procedure.

Physico-chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 1. A moving column approach

During Lagrangian experiment B (LB in the following) of the First Aerosol Characterization Experiment (ACE 1), a clean maritime air mass was followed over a period of 28 hours. During that time span, the vertical distribution of aerosols and their gas phase precursors were characterized by a total of nine aircraft soundings which were performed during three research flights that followed the trajectory of a set of marked tetroons. The objective of this paper is to study the time evolution of gas phase photochemistry in this Lagrangian framework. A box model approach to the wind shear driven and vertically stratified boundary layer is questionable, since its basic assumption of instantaneous turbulent mixing of the entire air column is not satisfied here. To overcome this obstacle, a one-dimensional Lagrangian boundary layer meteorological model with coupled gas phase photochemistry is used. To our knowledge, this is the first time that such a model is applied to a Lagrangian experiment and that enough measurements are available to fully constrain the simulations. A major part of this paper is devoted to the question of to what degree our model is able to reproduce the time evolution and the vertical distribution of the observed species. Comparison with observations of O3, OH, H2O2, CH3OOH, DMS, and CH3I, made on the nine Lagrangian aircraft soundings shows that this is in general the case, although the dynamical simulation started to deviate from the observations on the last Lagrangian flight. In agreement with experimental findings reported by Q. Wang et al. (unpublished manuscript, 1998b), generation of turbulence in the model appears to be most sensitive to the imposed sea surface temperature. Concerning the different modeled and observed chemical species, a number of conclusions are drawn: (1) Ozone, having a relatively long photochemical lifetime in the clean marine boundary layer, is found to be controlled by vertical transport processes, in particular synoptic-scale subsidence or ascent. (2) Starting with initally constant vertical profiles, the model is able to “create” qualitatively the vertical structure of the observed peroxides. (3) OH concentrations are in agreement with observations, both on cloudy and noncloudy days. On the first flight, a layer of dry ozone rich air topped the boundary layer. The model predicts a minimum in OH and peroxides at that altitude consistent with observations. (4) Atmospheric DMS concentrations are modeled correctly only when using the Liss and Merlivat [1986] flux parameterization, the Wanninkhof [1992] flux parameterization giving values twice those observed. To arrive at this conclusion, OH is assumed to be the major DMS oxidant, but no assumptions about mixing heights or entrainment rates are necessary in this type of model. DMS seawater concentrations are constrained by observations.

Ozone peaks associated with a subtropical tropopause fold and with the trade wind inversion: A case study from the airborne campaign TROPOZ II over the Caribbean in winter

Aircraft measurements of ozone, methane, carbon monoxide, relative humidity, and equivalent potential temperature were performed during the TROPOZ II campaign. During the aircraft descent down to Pointe-a-Pitre (16.3°N, 61.5°W), at 2100 UTC on January 12, 1991, two ozone peaks (75 ppb) are observed, one at an altitude of 7.5 km and the other at 3.0 km. A physicochemical interpretation for each ozone peak is proposed in connection with the meteorological context, using radiosounding data, total ozone content from TOMS/NIMBUS 7 and diagnoses issued from analyses by the European Centre for Medium-Range Weather Forecasts, Reading, England. The stratospheric origin of the 7.5-km ozone peak is inferred from negative correlations between ozone and its precursors and from diagnoses based on potential vorticity and ageostrophic circulations depicting the structure of the tropopause fold embedded in the subtropical jet front system. Using an appropriate method to isolate cross- and along-front ageostrophic circulations, we show that much of the observed structure of the tropopause fold can be ascribed to transverse and vertical circulations associated with the irrotational part of the flow. Though the downward extent of the subtropical tropopause fold (400 hPa) is restricted in comparison with typical extratropical tropopause ones (700 hPa), the present results suggest that subtropical tropopause folds may significantly contribute to the global stratosphere-troposphere ozone exchange. The origin of the 3.0-km ozone peak trapped just below the trade wind inversion cannot be ascribed precisely. Analogies with other measurements of dust and aerosols transported over the Atlantic or Pacific in the summer season are discussed. Various possibilities are examined: (1) an earlier stratospheric intrusion event, (2) long-range transport by the trade winds of biomass burning species emitted over West Africa, and (3) fast photochemical ozone formation occurring just below the trade wind inversion within already polluted air parcels originating from remote regions (United States and Gulf of Mexico) after eastward and southward transport around the western Atlantic anticyclone.

One‐dimensional modeling of sulfur species during the First Aerosol Characterization Experiment (ACE 1) Lagrangian B

A one-dimensional Lagrangian model is used to simulate vertical profiles and temporal evolution of dimethylsulfide (DMS), sulfur dioxide (SO2), aerosol methane sulfonate, and non-sea-salt sulfate (nss sulfate) that were measured during the three flights of the second First Aerosol Characterization Experiment (ACE 1) Lagrangian (Lagrangian B) experiment. Entrainment rate, mixing heights, and cloud occurrence are calculated prognostically in this type of model. The model is forced by geostrophic winds and large scale subsidence from European Centre for Medium-Range Weather Forecasts (ECMWF) analysis and sea surface temperature measured on board Research Vessel Discoverer. Gas phase oxidation and heterogeneous oxidation of SO2 to nss sulfate in clouds and sea-salt particles are considered. The evolution of dynamical variables in the column is found to be well reproduced by the model. The model captures 82% of the variance of observed DMS assuming OH is the only oxidant and a DMS flux term calculated from Liss and Merlivat [1986] parameterization and seawater DMS concentrations measured aboard R/V Discoverer. However, uncertainties in DMS oxidation rates and regional seawater concentrations are too great to identify a best fit wind speed-transfer velocity relationship. SO2 mixing ratios are correctly represented in the model (least squares correlation coefficient r2 = 75%) using a DMS to SO2 conversion efficiency of about 70%. Oxidation of SO2 in sea-salt particle appears to be a dominant process and controls SO2 lifetime during the Lagrangian B at least in the well mixed lower layer. Removing heterogeneous loss of SO2 in sea salt significantly deteriorates the simulation (r2 = 50%). Under cloudy conditions, heterogeneous loss in cloud droplets and in sea-salt particles are competitive (relative rates are 35% and 41%, respectively, during flight 26). Model-generated aerosol methane sulfonate mixing ratios agree with the observations (r2 = 62.5%) when high branching ratio for an addition oxidation pathway is used. The model estimates nss sulfate mixing ratios with little bias (median simulated-to-observed concentration ratio 1.03 and slope of the regression line 0.7) but captures only one third of the observed variance of nss sulfate. Part of the discrepancy could be due to the assumption of a decrease of nss sulfate mixing ratios with altitude in the model, whereas observations revealed high concentrations at 4500 m during the last two flights suggesting that horizontal transport could be more important than vertical mixing in this region. Nss sulfate is found to be produced photochemically under non cloudy, low wind speed conditions encountered during the first flight. During the last two flights, nss sulfate is produced mainly by oxidation in cloud droplets (48% during flight 25 and 69% during flight 26) and sea-salt particles (50% during flight 25 and 22% during flight 26).

A Comparative Study of Various Parameterizations of the Planetary Boundary Layer in a Numerical Mesoscale Model

Abstract Various parameterizations of the planetary boundary layer (PBL) currently used in three-dimensional (3D) mesoscale models are compared with a more complex scheme including a turbulent kinetic energy (TKE) equation. In the first set of simulations made with a ID model against the classical Wangara data, the mean wind, temperature and moisture calculated in the PBL are nearly insensitive to the choice of the parameterization. In the second set of simulations, the TKE parameterization is used in a 3D mesoscale model to simulate sea breeze flows over south Florida. A comparison is presented with previous simulations of Pielke, and Pielke and Mahrer, for the mean flow, and with the third-order turbulence closure model of Briere for the turbulent variables, including a discussion of the turbulent energy budget, The analysis of the results obtained with the TKE scheme shows that the predicted turbulent fields are qualitatively realistic and interact significantly with the sea breeze circulation. Finally...

Photodissociation and UV radiative transfer in a cloudy atmosphere: Modeling and measurements

We present an analysis of UV radiative transfer under cloudy conditions in relation to the photochemistry of the hydroxyl radical (OH) by means of modeling and measurements. The measurements, which are part of the First Aerosol Characterization Experiment (ACE 1) campaign, consist of four different ascents/descents selected from research flight (RF) 12 and 28. The ascents/descents give vertical profiles of UV irradiances, microphysical properties, standard meteorological parameters, and OH concentration in the presence of one or more cloud layers. In order to assess the photochemical conditions for these (complex) cloudy cases we first compare the UV irradiance measurements with modeled profiles. We model the UV irradiances using cloud optical properties which we derive from the measured microphysical properties. Second, we use the simultaneously modeled actinic flux to calculate the rate constant of the photodissociation of ozone to the O( 1 D) radical (J O3 ). This reaction initiates the primary OH-production. Finally, we compare the measured OH concentrations with those derived from the radiative transfer calculations. For single-cloud layer cases we successfully simulated UV radiative transfer, J O3 and OH. For more complex multiple-cloud layer cases the UV radiative transfer could only be explained allowing large variations of the cloud optical thickness (from zero to double the measurement derived values). The impact of such variations on the modeled radiation-derived photochemical properties, J O3 and OH, was, with a variation of 25%, found to be relatively small. As a consequence, we were able to simulate the general profile of OH for these complex cloudy conditions.

Physico‐chemical modeling of the First Aerosol Characterization Experiment (ACE 1) Lagrangian B: 2. DMS emission, transport and oxidation at the mesoscale

A three-dimensional mesoscale meteorological model was used to study the interplay between the dynamical (turbulent mixing and advection) and physico-chemical (sea-air flux and photochemical sink by OH) processes that control dimethylsulfide DMS concentrations and their distribution in the marine boundary layer (MBL) during the First Aerosol Characterization Experiment ACE 1. Atmospheric DMS concentrations were constrained using observed seawater DMS concentrations and box model derived OH concentrations. Lateral boundary values of dynamical parameters were derived from the 6-hourly meteorological analysis of the European Centre for Medium-Range Weather Forecasts. Calculated DMS concentrations, wind speed and direction, and cloud cover were compared with measurements made aboard the R/V Discoverer and on the three NCAR/C130 aircraft flights during the LagB experiment. Model-generated atmospheric DMS concentrations agreed with the DMS observations from the NCAR/C130 aircraft flights during the LagB experiment (R 2 = 0.69) assuming OH is the only oxidant and DMS flux parameterization based on Liss and Merlivat [1986]. Comparison with Eulerian measurements made aboard the R/V Discoverer showed that the model simulated the range of observed values but not the hour-to-hour variation observed in the atmospheric DMS concentrations. Part of the discrepancy was attributed to uncertainties in DMS sea-to-air transfer velocity, small scale features of seawater DMS that are beyond the model resolution, and uncertainties in the venting of the boundary layer by shallow clouds. A quantitative budget at the ship location revealed a strong impact of advection processes in determining DMS levels and temporal evolution. The three-dimensional mesoscale meteorological model was also used to estimate the effect of the low spatial resolution used in global models on seawater DMS concentrations and atmospheric OH concentrations.

Fractal modelling of carbonaceous aerosols—application to car exhaust plumes

Most commonly, atmospheric aerosol particles are ideally modelled as spheres allowing for straightforward calculations of their geometrical properties (diameter, surface and volume) and ensuing radiative, dynamical and chemical characteristics. However, particles issued particularly from combustion processes display various structural types, from linear clusters to quasi-spherical ones. Such various shapes result in quite different physical and chemical particle characteristics: e.g. the processes of absorption, coagulation/sintering and deposition are strongly affected by the fractal morphology of such aerosols. Whereas only one discretization parameter (diameter, d) is required for the spectral distribution n(d) of spherical particles, it is necessary to use a 2D distribution n(v,a) for fractal ones, v and a being, respectively, the volume and area of particles. For the coagulation process the associated governing aerosol population balance equation not only involves a collision term but also another term for more or less complete coalescence (sintering). For example, in view of the general texture of carbonaceous aerosols proposed by Strommen and Kamens (Environmental Science and Technology 31 (1997) 2983), let us consider two colliding particles each made of elementary carbon spherules immersed within a viscous organic liquid. They tend to more or less rapidly and completely merge, according to their bulk viscosities and surface tensions. In a first limit case, particles only stick together, displaying a maximum overall area. The other limit case is obtained through full particle merging into spherical structures. The new fractal model proposed, appropriate for describing such diverse particle morphologies, is then applied to car exhaust plumes.

Atlantic subtropical potential vorticity barrier as seen by Measurements of Ozone by Airbus In-Service Aircraft (MOZAIC) flights

The existence and the seasonal variability of the Atlantic subtropical potential vorticity barrier controlling the stratosphere-troposphere exchanges between the lowermost extratropical stratosphere and the upper equatorial troposphere are investigated using Measurements of Ozone by Airbus In-Service Aircraft (MOZAIC). The methodology is based on the relationship between strong gradients of potential vorticity and of ozone mixing ratio situated on the cyclonic-shear side of the subtropical jet stream. Episodes of high ozone mixing ratio sampled along MOZAIC flight tracks over the subtropical Atlantic with mixing ratio exceeding 100 ppbv on length scales larger than 200 km on flight levels between 11 and 12 km are studied. A total of 154 high ozone episodes is extracted from the MOZAIC database over the period August 1994 to April 1997. All these high ozone episodes are observed north of 15°N and have lengths ranging from subsynoptic to synoptic scales. It is shown that this barrier effect at 15°N over the central/eastern Atlantic fits with the southernmost latitude of the subtropical jet stream during the period of interest. South of the subtropical jet stream within the latitude band where the Intertropical Convergence Zone oscillates, tens of ozone-rich transients (high-ozone episodes with length scales smaller than 80 km) are sampled within the upper equatorial Atlantic troposphere (9 to 12 km). At present, the origin of these tropical ozone-rich transients is still not clear. Some outlooks are given to investigate the possibility that some of the ozone-rich transients may be interpreted as the final result of tropopause foldings and small scale mixing processes. The seasonal variability of the subtropical barrier is captured when determining the subtropical tropopause break (STB) point for each flight, that is, the southernmost latitude of the southernmost high-ozone episode of a flight, and classifying STB points on a monthly basis. A sinusoidal evolution of STB points appears, reflecting the northernmost (southernmost) position of the dynamical barrier in summer (winter) boreal months. This seasonal variability clearly agrees with that of the position of the subtropical jet stream as derived from mean isotach analysis.