Spatial variability in airborne surface flux measurements during HABEX-Sahel

The spatial variability of sensible and latent heat flux measured by aircraft over a 90-km × 75-km area, near the Central Supersites of HAPEX-Sahel (Hydrologic and Atmospheric Pilot Experiment, Sahel) is discussed. The data from six flights are presented four of which were obtained during the rainy season; the others being obtained at the beginning of the dry season.A basic difference in the behaviour of the latent heat transfer is revealed when the measurements under dry and wet conditions are compared: the latent heat flux is far more heterogeneous under drying conditions so that the estimation accuracy is reduced if the same integration length is used. It was found that under dry conditions the contribution of low frequency eddies is more important than that of local turbulence: the surface moisture is probably less homogeneous than in the wet period, but it is mostly the interaction between the marine and continental air masses linked to the closeness of the inter-tropical convergence zone that seems to drive the transfers.Two-dimensional fields of fluxes are constructed to study their spatial variability according to the hydrological conditions. These fields are systematically compared with those of the mean parameters likely to drive the transfers. Some characteristics, common to several fields, are revealed that can be considered specific to the climate in this region, at this time of year: an east-west gradient of albedo, a south-north gradient of the sensible heat flux, surface temperature and air temperature and a north-south gradient of specific humidity. However, the aerodynamic formula that relates flux to mean parameters, fails at a 25-km × 25-km scale but gives good results at a larger scale (90 km × 75 km): at this scale, the Dalton number is around 2.5 × 10−3

An observational study of othe Mesoscale mistral dynamics

We investigate the mesoscale dynamics of the mistral through the wind profiler observations of the MAP (autumn 1999) and ESCOMPTE (summer 2001) field campaigns. We show that the mistral wind field can dramatically change on a time scale less than 3 hours. Transitions from a deep to a shallow mistral are often observed at any season when the lower layers are stable. The variability, mainly attributed in summer to the mistral/land–sea breeze interactions on a 10-km scale, is highlighted by observations from the wind profiler network set up during ESCOMPTE. The interpretations of the dynamical mistral structure are performed through comparisons with existing basic theories. The linear theory of R. B. Smith [Advances in Geophysics, Vol. 31, 1989, Academic Press, 1–41] and the shallow water theory [Schär, C. and Smith, R. B.: 1993a, J. Atmos. Sci. 50, 1373–1400] give some complementary explanations for the deep-to-shallow transition especially for the MAP mistral event. The wave breaking process induces a low-level jet (LLJ) downstream of the Alps that degenerates into a mountain wake, which in turn provokes the cessation of the mistral downstream of the Alps. Both theories indicate that the flow splits around the Alps and results in a persistent LLJ at the exit of the Rhône valley. The LLJ is strengthened by the channelling effect of the Rhône valley that is more efficient for north-easterly than northerly upstream winds despite the north–south valley axis. Summer moderate and weak mistral episodes are influenced by land–sea breezes and convection over land that induce a very complex interaction that cannot be accurately described by the previous theories

Remote sensing and surface observations of the response of the atmospheric boundary layer to a solar eclipse

On 11 August 1999, a near-total solar eclipse (80%) was observed in Campistrous, France. The influence of this particular event on the atmospheric boundary layer was observed with a UHF-RASS radar, a sodar and an instrumented mast. The changes in turbulence intensity, radar reflectivity, and temperature on the radiative budget are described in relation to collocated ground meteorological data. The impact of the eclipse induces a clear response of the atmosphere, with a time lag of 15 to 30 min, perceptible in several mean and turbulent meteorological variables up to the top of the atmospheric boundary layer

Contribution of a profiler network in the monitoring of a polluted area

(2-4 février 2005

Use of the Doppler spectral with to improve the estimation of the convective boundary layer height from UHF wind Profiler observations

Enhancement of the air refractive index structure parameter C2n often occurs at the top of the convective boundary layer (CBL), where the absolute values of the vertical gradients of virtual potential temperature and mixing ratio have a peak. This well-known behavior of the C2n profiles is often used to locate the height of the mixed layer Zi from UHF wind profiler observations. In the present study, Zi determination with the C2n-based technique was investigated for a case of clear-air CBL and a case of cloud-topped CBL. In certain circumstances, such as multifold C2n peaks or poorly defined peaks, these techniques fail to correctly retrieve CBL height. In order to improve Zi determination, a new method based on the conjoint use of C2n and Doppler spectral width profiles is proposed and discussed

Reply to 'Comments on "Comparison of rafar reflectivity and vertical velocity observed with a scannable c-band doppler radar and two UHF profilers in the lower troposphere"'

The objective of Lothon et al.'s (2002, hereafter L2002) paper was to investigate the ability of a scannable C-band Doppler meteorological radar for the documentation of the lower clear atmosphere with an emphasis on the atmospheric boundary layer (ABL). Comparison with the observations of two UHF wind profilers provided the opportunity to address the problem of the so-called downward bias of UHF vertical velocity measurements made in a fully developed and turbulent convective ABL previously put in evidence and described by Angevine (1997). Worthington's (2003) reproach of a too-narrow review of the different aspects of this problem is valid, but a thorough survey of this question was beyond the scope of the paper. The ambition of the authors was to bring some elements of discussion to this open debate—elements that tend to confirm Angevine's conclusion on the existence of UHF downward bias of instrumental origin.In the reply presented in the following section we discuss Worthington's (2003) suggestion of the possible meteorological origin of the UHF vertical velocity bias observed in the ABL. Coming back to certain arguments developed previously in L2002, the importance of which seems underestimated in Worthington's comment, and with new data analysis, we reinforce our position on a most probable instrumental origin of the UHF vertical bias

Constant volume balloons measurements in the urban Marseille and Fos–Berre industrial ozone plumes during ESCOMPTE experiment

International audienceESCOMPTE programme aims at studying the emissions of primary pollutants in industrial and urban areas, their transport, diffusion and transformation in the atmosphere. This experiment, carried out in southeast France, can be used to validate and to improve meteorological and chemical mesoscale models. One major goal of this experiment was to follow the pollutant plumes, and to investigate its thermodynamic and physico-chemical time evolution. This was realized by means of constant volume balloons, located by global position satellite (GPS) and equipped with thermodynamic and ozone sensors, flying at constant density levels. During the two ESCOMPTE campaigns that took place in June and July 2000 and 2001, 40 balloons were launched, 17 of them equipped with ozone sensors during the day from 0800 to 1800 UTC. Balloons' altitudes flight levels ranged between 400 and 1200 m altitude with Mistral (northerly synoptic flow) and Sea Breeze (southerly breeze) conditions. The atmospheric boundary layer (ABL) topography of the experimental domain is complex and varies strongly from day to day. Its depth presents a large gradient from the sea coast to the north part of the ESCOMPTE domain, and also more complex variability within the domain. The balloons' trajectories describe the evolution of the pollutant plume emitted from the industrial area of Fos-Berre or from the Marseille urban area. Constant volume balloons give a good description of the trajectories of these two plumes. The balloons, which fly at an isopicnic level, cross different atmospheric layers chiefly depending on the ABL height in relation with the constant volume balloons flight level. Thus, each balloon flight is decomposed into different segments that correspond to the same atmospheric layer. In each segment, the ozone content variation is analyzed in relation to other thermodynamical parameters measured by the balloon and mainly to the vapor mixing ratio content. During ESCOMPTE campaign, the mean linear rate of chemical net ozone production at the top of the atmospheric boundary layer was found to be around 6 ppb h−1

Stratification of the lower troposphere during the ESCOMPTE campaign

Implementation of adapted air quality policies and prediction of pollution events require the improvement of our understanding of combined chemical and dynamical processes which lead to high pollutants concentration in the atmospheric boundary layer. The ESCOMPTE program was devoted to establish a detailed 3-D database of primary pollutant emissions together with the dynamics and chemical composition of the atmosphere in order to validate and improve chemistry transport models. The ESCOMPTE experiment took place in the early summer of 2001, in the south-eastern part of France where the combination of high urbanization and industrialization with hot and sunny weather enhances photochemical pollution events. Due to the complex terrain of this region (both coastal and moderately mountainous area, channeling effects of the Rhône and Durance valleys,…), a preliminary effort is necessary to study the atmospheric dynamics in this heterogeneous region in relation with larger scales processes. The main objectives of this paper is to present the vertical structure of the ESCOMPTE area during different meteorological conditions of the second intensive observation period (IOP2) which lasted from June 22 to June 26