Jean-Charles Dupont

Combined influence of atmospheric physics and soil hydrology on the simulated meteorology at the SIRTA atmospheric observatory

The identification of the land-atmosphere interactions as one of the key source of uncertainty in climate models calls for process-level assessment of the coupled atmosphere/land continental surface system in numerical climate models. To this end, we propose a novel approach and apply it to evaluate the standard and new parametrizations of boundary layer/convection/clouds in the Earth System Model (ESM) of Institut Pierre Simon Laplace (IPSL), which differentiate the IPSL-CM5A and IPSL-CM5B climate change simulations produced for the Coupled Model Inter-comparison Project phase 5 exercise. Two different land surface hydrology parametrizations are also considered to analyze different land-atmosphere interactions. Ten-year simulations of the coupled land surface/atmospheric ESM modules are confronted to observations collected at the SIRTA (Site Instrumental de Recherche par Télédection Atmosphérique), located near Paris (France). For sounder evaluation of the physical parametrizations, the grid of the model is stretched and refined in the vicinity of the SIRTA, and the large scale component of the modeled circulation is adjusted toward ERA-Interim reanalysis outside of the zoomed area. This allows us to detect situations where the parametrizations do not perform satisfactorily and can affect climate simulations at the regional/continental scale, including in full 3D coupled runs. In particular, we show how the biases in near surface state variables simulated by the ESM are explained by (1) the sensible/latent heat partitionning at the surface, (2) the low level cloudiness and its radiative impact at the surface, (3) the parametrization of turbulent transport in the surface layer, (4) the complex interplay between these processes. We also show how the new set of parametrizations can improve these biases.

Macrophysical and optical properties of midlatitude cirrus clouds from four ground-based lidars and collocated CALIOP observations

Ground-based lidar and Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) data sets gathered over four midlatitude sites, two U.S. and two French sites, are used to evaluate the consistency of cloud macrophysical and optical property climatologies that can be derived by such data sets. The consistency in average cloud height (both base and top height) between the CALIOP and ground data sets ranges from −0.4 km to +0.5 km. The cloud geometrical thickness distributions vary significantly between the different data sets, due in part to the original vertical resolutions of the lidar profiles. Average cloud geometrical thicknesses vary from 1.2 to 1.9 km, i.e., by more than 50%. Cloud optical thickness distributions in subvisible, semitransparent, and moderate intervals differ by more than 50% between ground- and space-based data sets. The cirrus clouds with optical thickness below 0.1 (not included in historical cloud climatologies) represent 30–50% of the nonopaque cirrus class. An important part of this work consists in quantifying the different possible causes of discrepancies between CALIOP and surface lidar. The differences in average cloud base altitude between ground and CALIOP data sets can be attributed to (1) irregular sampling of seasonal variations in the ground-based data, (2) day-night differences in detection capabilities by CALIOP, and (3) the restriction to situations without low-level clouds in ground-based data. Cloud geometrical thicknesses are not affected by irregular sampling of seasonal variations in the ground-based data but by the day-night differences in detection capabilities of CALIOP and by the restriction to situations without low-level clouds in ground-based data.

BASTA: A 95-GHz FMCW Doppler Radar for Cloud and Fog Studies

Doppler cloud radars are amazing tools to characterize cloud and fog properties and to improve their representation in models. However commercially-available cloud radars (35 and 95 GHz) are still very expensive, which hinders their widespread deployment. In this study we present the development of a lower-cost semi-operational 95 GHz Doppler cloud radar called BASTA for Bistatic rAdar SysTem for Atmospheric studies. In order to drastically reduce the cost of the instrument a different approach is used compared to traditional pulsed radars: instead of transmitting a large amount of energy for a very short time period (as a pulse), a lower amount of energy is transmitted continuously. In the paper we show that using specific signal processing technique the radar can challenge expensive radars and provide high-quality measurements of cloud and fog. The latest version of the instrument has a sensitivity of about -50 dBZ at 1 km for 3 s integration and a vertical resolution of 25 m. BASTA radar currently uses four successive modes for specific applications: the 12.5 m vertical resolution mode is dedicated to fog and low clouds, the 25 m mode is for liquid and ice mid-tropospheric clouds and the 100 m and 200 m are ideal for optically-thin high-level ice clouds. We also highlight the advantage of such a radar for calibration procedures and field operations. The radar comes with a set of products dedicated to cloud and fog studies. For instance, cloud mask, corrected Doppler velocity and multi mode products combining high sensitivity mode and high resolution modes are provided.

Cloud properties derived from two lidars over the ARM SGP site

Active remote sensors such as lidars or radars can be used with other data to quantify the cloud properties at regional scale and at global scale. Relative to radar, lidar remote sensing is sensitive to very thin and high clouds but has a significant limitation due to signal attenuation in the ability to precisely quantify the properties of clouds with a cloud optical thickness larger than 3. The cloud properties for all levels of clouds are derived and distributions of cloud base height (CBH), top height (CTH), physical cloud thickness (CT), and optical thickness (COT) from local statistics are compared. The goal of this study is (1) to establish a climatology of macrophysical and optical properties for all levels of clouds observed over the ARM SGP site and (2) to estimate the discrepancies between the two remote sensing systems (pulse energy, sampling, resolution, etc.). Our first results tend to show that the MPL, which are the primary ARM lidars, have a distinctly limited range within which all of these cloud properties are detectable, especially cloud top and cloud thickness, but this can include cloud base particularly during summer daytime period. According to the comparisons between RL and MPL, almost 50% of situations show a signal to noise ratio too low (smaller than 3) for the MPL in order to detect clouds higher than 7km during daytime period in summer. Consequently, the MPL-derived annual cycle of cirrus cloud base (top) altitude is biased low, especially for daylight periods, compared with those derived from the RL data, which detects cloud base ranging from 7.5 km in winter to 9.5 km in summer (and tops ranging from 8.6 to 10.5 km). The optically thickest cirrus clouds (COT > 0.3) reach 50% of the total population for the Raman lidar and only 20% for the Micropulse lidar due to the difference of pulse energy and the effect of solar irradiance contamination. A complementary study using the cloud fraction derived from the Micropulse lidar for clouds below 5 km and from the Raman lidar for cloud above 5 km allows for better estimation of the total cloud fraction between the ground and the top of the atmosphere. This study presents the diurnal cycle of cloud fraction for each season in comparisons with Long et al.s (2006) cloud fraction calculation derived from radiative flux analysis. Copyright

Preliminary results of the PreViBOSS project: description of the fog life cycle by ground-based and satellite observation

The instrument set-up designed by the PreViBOSS project for the ParisFog field campaign is suitable to sound microphysical properties of droplets and interstitial aerosols during developed fog in a semi-urban environment. Developed fog is defined as LWC < 7 mg m-3 and the temperature vertical gradient over 30 m, ΔT, smaller than 0.04 K/m. Visibility averaged over November 2011 is 385±340 m (with rare values larger than 1000 m), and month average of LWC is 60±60 mg m-3. The droplet effective radius decreases from 14 to 4 μm when the number concentration increases from less than 10 to 220 cm-3. Particle extinction coefficient is computed by Mie theory applied on size distribution observed during developed fog in ambient conditions by both PALAS WELAS and DMT FM100. Comparison with particle extinction coefficient directly measured by the Degreanne DF20 visibilimeter demonstrates satisfying agreement, within combined uncertainties. Ratio of computed over measured particle extinction coefficient is 1.15±0.35. Visibility smaller than 1000 m at 3 m above ground level is observed not only during developed fog but also during shallow fog, which presents a significant vertical gradient, as ΔT > 0.4 K/m. In this case, LWC is highly variable and may be observed below 7 mg m-3. The consequent month average of LWC is 30±80 mg m-3. The optical counters miss large droplets significantly contributing to extinction in shallow fogs. Consequently, it is not possible to reproduce with satisfaction the particle extinction coefficient in shallow fog. Fog type may be distinguished by association of groundbased visibilimeter and MSG/SEVIRI. When clear-sky is given by EUMETSAT/NWCSAF cloud type product while visibility is observed smaller than 1000 m at SIRTA, in 75% cases a shallow fog occurs, and in other cases, horizontal heterogeneity characterises the developed fog within the SIRTA pixel, as during the dissipation phase. Moreover, consistently, low and very low clouds are mostly detected by the satellite product when developed fog is observed by ground-based instrumentation.

SIRTA, a multi-sensor platform for clouds and aerosols characterization in the atmosphere: infrastructure, objective and prospective

The SIRTA (Site instrumental de Recherche par Télédétection Atmosphérique) is a ground-based platform located 25km south of Paris in France. The SIRTA observatory was created in 1999 by the French research institute IPSL (Institut Pierre Simon Laplace) to conduct research programs in order to improve the knowledge of radiative and dynamic processes in the atmosphere as well as complex interactions between clouds and aerosols. The objective is to better comprehend climate changes and evolution of environment using a suite a state-of-art active and passive remote sensing instruments. Two ground platforms, a wooden tower, a roof platform and a building (where the lidar operates) are the main facilities of SIRTA. The project team is composed of six persons to ensure the station operations from instrument deployment, maintenance, data transfer and preliminary data analysis. The SIRTA infrastructure enables to conduct many research activities that involve the cloud and aerosol lidar. Some of them will be discussed: the development of the STRAT (Structure of the Atmosphere) algorithm dedicated to automatically discriminate atmospheric layers and retrieve geophysical parameters from lidar profiles, and the CALIPSO validation using the dual-channel backscatter lidar deployed at SIRTA.