B. Bénech

Atmospheric boundary layer response to sea surface temperatures during the SEMAPHORE experiment

The sensitivity of the marine atmospheric boundary layer (MABL) subjected to sea surface temperatures (SST) during the Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment in 1993 has been studied. Atmospheric analyses produced by the Action de Recherche, Petite Echelle, Grande Echelle (ARPEGE) operational model at the French meteorological weather service assimilated data sets collected between October 7 and November 17, 1993, merged with the Global Telecommunication System (GTS) data. Analyses were validated against independent data from aircraft instruments collected along a section crossing the Azores oceanic front, not assimilated into the model. The responses of the mean MABL in the aircraft cross section to changes in SST gradients of about 1°C/100 km were the presence of an atmospheric front with horizontal gradients of 1°C/100 km and an increase of the wind intensity from the cold to the warm side during an anticyclonic synoptic situation. The study of the spatiotemporal characteristics of the MABL shows that during 3 days of an anticyclonic synoptic situation the SST is remarkably stationary because it is principally controlled by the Azores ocean current, which has a timescale of about 10 days. However, the temperature and the wind in the MABL are influenced by the prevailing atmospheric conditions. The ocean does not appear to react to the surface atmospheric forcing on the timescale of 3 days, whereas the atmospheric structures are modified by local and synoptic-scale advection. The MABL response appears to be much quicker than that of the SSTs. The correlation between the wind and the thermal structure in the MABL is dominated by the ageostrophic and not by the geostrophic component. In particular, the enhancement of the wind on either side of the SST front is mainly due to the ageostrophic component. Although the surface heat fluxes are not the only cause of ageostrophy, the surface buoyancy flux Q b appears to be an important local source.

Use of the Doppler Spectral Width to Improve the Estimation of the Convective Boundary Layer Height from UHF Wind Profiler Observations

Abstract 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.

Comparison of Radar Reflectivity and Vertical Velocity Observed with a Scannable C-Band Radar and Two UHF Profilers in the Lower Troposphere

Abstract A C-band meteorological Doppler radar has been used to investigate the dynamic processes and the coherent organizations within a clear air atmospheric boundary layer (ABL). Depending on the atmospheric conditions, this moderate sensitive radar was able to provide a coherent and continuous velocity field from 0.1 km up to 3 km above ground, and over a horizontal range reaching at least 30 km. Here the focus is on the profiling capability of this Doppler radar in a clear air boundary layer. The velocity volume processing method was used to deduce vertical profiles of the wind field from the panoramic conical scannings. A comparison between the observations of this C-band radar and two UHF wind profilers is presented. Good agreement was obtained in the measurements of the wind velocity and of the vertical and temporal evolution of the reflectivity. In particular, as for UHF wind profilers, the ABL top was found coincident for the C-band radar with a bright band of reflectivity maximum, and both type...

Structure of the marine atmospheric boundary layer over an oceanic thermal front: SEMAPHORE experiment

The Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE) experiment, the third phase of which took place between October 4 and November 17, 1993, was conducted over the oceanic Azores Current located in the Azores basin and mainly marked at the surface by a thermal front due to the gradient of the sea surface temperature (SST) of about 1° to 2°C per 100 km. The evolution of the marine atmospheric boundary layer (MABL) over the SST front was studied with two aircraft and a ship in different meteorological conditions. For each case, the influence of the incoming air direction with respect to the orientation of the oceanic front was taken into account. During the campaign, advanced very high resolution radiometer pictures did not show any relation between the SST field and the cloud cover. The MABL was systematically thicker on the warm side than on the cold side. The mean MABL structure described from aircraft data collected in a vertical plane crossing the oceanic front was characterized by (1) an atmospheric horizontal gradient of 1° to 2°C per 100 km in the whole depth of the mixed layer and (2) an increase of the wind intensity from the cold to the warm side when the synoptic wind blew from the cold side. The surface sensible heat (latent heat) flux always increased from the cold to the warm sector owing to the increase of the wind and of the temperature (specific humidity) difference between the surface and the air. Turbulence increased from the cold to the warm side in conjunction with the MABL thickening, but the normalized profiles presented the same structure, regardless of the position over the SST front. In agreement with the Action de Recherche Programme te Petite Echelle and Grande Echelle model, the mean temperature and momentum budgets were highly influenced by the horizontal temperature gradient. In particular, the strong ageostrophic influence in the MABL above the SST front seems linked with the secondary circulation due to the SST front.

Comparison of sea surface flux measured by instrumented aircraft and ship during SOFIA and SEMAPHORE experiments

Two major campaigns (Surface of the Oceans, Fluxes and Interactions with the Atmosphere (SOFIA) and Structure des Echanges Mer-Atmosphere, Proprietes des Heterogeneites Oceaniques: Recherche Experimentale (SEMAPHORE)) devoted to the study of ocean-atmosphere interaction were conducted in 1992 and 1993, respectively, in the Azores region. Among the various platforms deployed, instrumented aircraft and ship allowed the measurement of the turbulent flux of sensible heat, latent heat, and momentum. From coordinated missions we can evaluate the sea surface fluxes from (1) bulk relations and mean measurements performed aboard the ship in the atmospheric surface layer and (2) turbulence measurements aboard aircraft, which allowed the flux profiles to be estimated through the whole atmospheric boundary layer and therefore to be extrapolated toward the sea surface level. Continuous ship fluxes were calculated with bulk coefficients deduced from inertial-dissipation measurements in the same experiments, whereas aircraft fluxes were calculated with eddy-correlation technique. We present a comparison between these two estimations. Although momentum flux agrees quite well, aircraft estimations of sensible and latent heat flux are lower than those of the ship. This result is surprising, since aircraft momentum flux estimates are often considered as much less accurate than scalar flux estimates. The various sources of errors on the aircraft and ship flux estimates are discussed. For sensible and latent heat flux, random errors on aircraft estimates, as well as variability of ship flux estimates, are lower than the discrepancy between the two platforms, whereas the momentum flux estimates cannot be considered as significantly different. Furthermore, the consequence of the high-pass filtering of the aircraft signals on the flux values is analyzed; it is weak at the lowest altitudes flown and cannot therefore explain the discrepancies between the two platforms but becomes considerable at upper levels in the boundary layer. From arguments linked to the imbalance of the surface energy budget, established during previous campaigns performed over land surfaces with aircraft, we conclude that aircraft heat fluxes are probably also underestimated over the sea.

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.

Spatial variability in airborne surface flux measurements during HAPEX-Sahel

Abstract 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 .

tude exprimentale de la couche limite de montagne: Partie I: Influence sur l'coulement du relief moyenne et grande chelles

The characteristics of the boundary layer over complex terrain (Lannemezan - lat.: 43.7° N and, long.: 0.7 ° E) are analyzed for various scales, using measurements obtained during the COCAGNE Experiment. In this first part, the dynamic characteristics of the flow are studied with respect to atmospheric stability and the relief at small (~20 km) and medium scales (~100 km). These relief scales depend on the topographical profile of the Lannemezan Plateau along the dominant axis of the wind (E-W) and the Pyrenees Mountains located at the south of the experimental site. The terrain heterogeneities have a standard deviation of ~48 m and a wavelength of ~2 km.