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Dynamics Flux

Last update : 2011/07/06

The Lidar-ABC group addresses the issue of horizontal and vertical transport of scalars (particles, gaseous) in the atmospheric boundary layer (ABL). Particular interests are devoted to the use of lidar signals as dynamic tracers. The morning and afternoon transitions and the role of gravity waves are studied. Using a mass budget in the ABL, we estimate regional surface fluxes of scalar (CO2). These studies use a synergism between 0.5-1 µm backscatter Lidar, 2-µm Heterodyne Doppler Lidar and an array of in-situ sensors.

Convective Boundary Layer and Lidar signals

Transfer of mass, aerosols, heat and moisture from the surface to the overlying convective boundary layer (CBL) is mainly due to convection i.e., convective cells or thermals consisting of a sequence of plume updrafs and downdrafts. In [Gibert et al, 2007a], we questionned the correlation between vertical velocity (w) on the one hand and the occurrence of convective plumes in lidar reflectivity (i.e. range corrected backscatter signal P.z2) and depolarization ratio (Δ) on the other hand. The present study is motivated by the fact that transport processes associated with updrafts and downdratfs are treated as subgrid problems in large-scale models and their parametrization is still an issue.

Fig.1: Experimental site at IPSL-LMD, Ecole Polytechnique campus, France. EBL is an Elastic Backscatter Lidar at 0.532 µm and HDL is a Heterodyne Doppler Lidar at 2.064 µm.

Thermal vertical motion was directly investigated using vertical velocities measured by ground-based Heterodyne Doppler Lidar (HDL) operating at 2 µm. This lidar provides also simultaneous measurements of lidar reflectivity. In addition an Elastic Backscatter Lidar (EBL), 200 m away provides reflectivities at 0.53 and 1 µm and depolarization ratio (Δ) at 0.53 µm (Fig. 1).

Fig.2: June 10, 2005 - Lidar backscatter signals at (a) 2.064 µm and (b) 0.532 µm in log-scale and in arbitrary unit. (c) Depolarisation ratio Δ and (d) Vertical velocities uz as functions of altitude. Local time is UTC + 2 hours.

The time series from the two lidars are presented in Fig. 2 for June, 10, 2005. In [Gibert et al. 2007a], we made the analysis in terms of correlation coefficients of the observations recorded on six days. A positive correlation is found between the CBL height and vertical velocity measured near the CBL top. This result supports the conventionnal view, even if the correlation coefficient is lower than expected (< 0.5). On the other hand, the plume-like structures provided by lidar reflectivity within the CBL are not a clear signature of updrafts. The analysis showed that the lidar reflectivity (P.z2) is anti-correlated with the vertical velocity in most cases. On the contrary, the depolarization ratio is a fair tracer of updrafts, since the correlation coefficient between Δ and w is always positive i.e. 0< ρ(w, Δ) < 0.5.

An important factor that influences the lidar reflectivity is the relative humidity, which modifies substantially the aerosol size, shape and complex refractive index distributions. As a result, the backscatter reflectivity cannot be simply interpret as a passive scaler reprensenting the uplifting of aerosol particles from the surface by convective plumes or a downward motion of clean air from the free atmosphere by downdrafts.The importance of relative humidity fluctuations due to temperature differences between updrafts and downdrafts has been estimated. This phenomenon has been identified as a possible mechanism to produce the anti-correlation between lidar reflectivity and vertical velocity (i.e. ρ(w, P.z2) < 0). Nevertheless, it cannot completely explain the fluctuations in lidar reflectivity. Therefore, other phenomena such as RH hysteresis in particle size growth may enhance reflectivity variability, possibly explaining the remaining fluctuations.

Dynamical mechanisms controlling the west saharan boundary layer

The Saharan atmospheric boundary layer (SABL) plays a significant role in the atmospheric global circulation and directly affects the vertical redistribution of dust originated in the Sahara, the world’s largest dust source. Recent measurements have revealed a variety of new dynamical mechanisms that control the structure of the SABL, which are responsible for exchange between the Saharan convective and residual boundary layers (SCBL and SRL). Using new space-borne laser remote sensing data (CALIPSO) and recently published results, [Cuesta et al. 2009] provides an overview of the following known dynamical mechanisms: diurnal vertical mixing, dynamical lifting (density currents and cold air outbreaks) and topographic effects (mountains and albedo anomalies) (Fig. 3).

Fig. 3 : Schematic of the mechanisms (red arrows) which control the SABL structure and dust vertical redistribution (labels refer to Section numbers): I) Diurnal vertical mixing, II) Dynamical lifting (upgliding, gravity currents and cold air outbreaks) and III) Topographic effects (mountains and albedo anomalies). Shading (yellow or light blue) indicates air masses origin (i.e. from the SABL, Gulf of Guinea or mid-latitudes) and temperature. This representation does not correspond to any day nor suggest that the SABL is more likely to be well-mixed in the north than the south. Commonly, the SABL state is mostly horizontally homogeneous.

Fig. 4 shows a case in which the SABL is well-mixed throughout its depth over a large spatial domain. From 23 to 34ºN, the potential temperatures from the ECMWF model and radiosoundings (at 1200 UTC) as well as the measurements from the space-borne lidar (at 1307 UTC) all indicate a well-mixed state from the surface to the inversion, at about 5.5 km above mean sea level (msl). Lidar profiles north of 18°N show a rather moderate dust load up to ~5.5 km msl (β > 1.5 10-3 km-1 sr-1). From 13.5 to 16°N, relatively high aerosol loadings are mostly confined to the SCBL (with its top between ~2.5 and ~3.5 km msl), while lower aerosol contents are observed in the more stably stratified layers above. When the SCBL is well mixed throughout the depth of the SABL it can entrain air from the free troposphere (FT) above, with stronger inversions at the top of the SCBL yielding lower entrainment rates. Air from regions surrounding the Sahara may also move isentropically into the SRL, particularly at night and in the morning, and can then be entrained and mixed into the SCBL. These mechanisms imply a reduction in the concentration of dust due to dilution below the SABL top.

Fig. 4 : (a) CALIPSO lidar transect over West Africa on the 25th June 2006 at 1307 UTC: backscatter coefficient profiles at 532 nm with a 60-m (12-km) resolution in the vertical (horizontal). The outline of the topography appears in black. Dense clouds appear in white, the low backscatter values below them being due to lidar signal extinction. Superimposed on the CALIPSO lidar data are: i) 1200 UTC ECMWF analyses of horizontal winds (arrows), potential temperature (dark gray plain contours) and the location of the ITD (marked as a red triangle, derived using the 14°C-dew point temperature criterion) and ii) virtual temperature radiosonde profiles in pink (from 310 to 330 K on the horizontal axes) at Agadez (17°N 8°E), Tamanrasset (22.8°N 5.5°E) and In Salah (27.2°N 2.5°E). (b) Surface albedo retrieved from MODIS along the CALIPSO track. (c) SEVIRI-derived false colour images over West Africa at 1330 UTC, identifying dust (purple/red), clouds (orange/yellow) and differences in surface emissivity retrieved in absence of dust or clouds (green/magenta). The dashed black line is the CALIPSO track. (d) ECMWF 1200 UTC analysis of wind and water vapour mixing ratio at 925 hPa and the contour of near surface 14°C dew point temperature marking the position of the ITD.

Gravity waves

Excitation of internal gravity waves in the boundary layer is investigated from observations. Simultaneous measurements from a 2-µm Doppler lidar and a 0.5-µm backscatter lidar are combined to analyze the occurrence, or not, of internal gravity waves in the residual layer during the morning transition on two days, 10 and 14 June 2005.

Fig. 5: June 14, 2005 - (a) Lidar depolarisation ratio at 0.532 µm, Δ, and (b) Vertical velocities uz as functions of altitude. Local time is UTC + 2 hours.

Three cases are studied, illustrating three different flow configurations: no wave (Fig.2), evanescent wave (CASE II in Fig.5) and propagating wave (CASE III in Fig.5). Comparison of the three cases strongly suggests that a stably stratified residual layer and “mechanic oscillator” thermal forcing frequencies larger than the Brunt-Vaïsala frequencies are necessary conditions for the generation of gravity waves in the residual layer. The horizontal wind shear likely plays a role in the dynamics of the waves, but it is argued that it is not sufficient, alone, to generate the observed waves. In the case of wave propagation, the waves tilt upstream and against the wind shear, with typical horizontal wavelength and line phase direction with respect to the vertical are 2.4 km and 32 °, respectively. Unexpectedly, we found that measurements of the wave-associated vertical velocity and of tracers (0.5 µm-depolarization ratio or 2-µm-backcatter both indicative of relative humidity fluctuations) are in phase. Possible explanations include i) aerosol particles are not passive with respect to temperature or water vapour fluctuations or ii) a non-linear wave-turbulence interaction is at work and needs further investigation. This study is developped in [Gibert et al. 2011].

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