Gerhard Ehret

Depolarization ratio profiling at several wavelengths in pure Saharan dust during SAMUM 2006

Vertical profiles of the linear particle depolarization ratio of pure dust clouds were measured during the Saharan Mineral Dust Experiment (SAMUM) at Ouarzazate, Morocco (30.9◦N, –6.9◦E), close to source regions in May–June 2006, with four lidar systems at four wavelengths (355, 532, 710 and 1064 nm). The intercomparison of the lidar systems is accompanied by a discussion of the different calibration methods, including a new, advanced method, and a detailed error analysis. Over the whole SAMUM periode pure dust layers show a mean linear particle depolarization ratio at 532 nm of 0.31, in the range between 0.27 and 0.35, with a mean Ångström exponent (AE, 440–870 nm) of 0.18 (range 0.04–0.34) and still high mean linear particle depolarization ratio between 0.21 and 0.25 during periods with aerosol optical thickness less than 0.1, with a mean AE of 0.76 (range 0.65–1.00), which represents a negative correlation of the linear particle depolarization ratio with the AE. A slight decrease of the linear particle depolarization ratio with wavelength was found between 532 and 1064 nm from 0.31 ± 0.03 to 0.27 ± 0.04.

Airborne high spectral resolution lidar for measuring aerosol extinction and backscatter coefficients

An airborne high spectral resolution lidar (HSRL) based on an iodine absorption filter and a high-power frequency-doubled Nd:YAG laser has been developed to measure backscatter and extinction coefficients of aerosols and clouds. The instrument was operated aboard the Falcon 20 research aircraft of the German Aerospace Center (DLR) during the Saharan Mineral Dust Experiment in May-June 2006 to measure optical properties of Saharan dust. A detailed description of the lidar system, the analysis of its data products, and measurements of backscatter and extinction coefficients of Saharan dust are presented. The system errors are discussed and airborne HSRL results are compared to ground-based Raman lidar and sunphotometer measurements.

Role of entrainment in surface-atmosphere interactions over the boreal forest

We present a description of the evolution of the convective boundary layer (CBL) over the boreal forests of Saskatchewan and Manitoba, as observed by the National Center for Atmospheric Research (NCAR) Electra research aircraft during the 1994 Boreal Ecosystem-Atmosphere Study (BOREAS). All observations were made between 1530 and 2230 UT (0930–1630 local solar time (LST)). We show that the CBL flux divergence often led to drying of the CBL over the course of the day, with the greatest drying (approaching 0.5 g kg−1 h−1) observed in the morning, 1000–1200 LST, and decreasing over time to nearly no drying (0–0.1 g kg−1 h−1) by midafternoon (1500–1600 LST). The maximum warming (0.45 K h−1 ) also occurred in the morning and decreased slightly to about 0.4 Kh−1 by midafternoon. The CBL vapor pressure deficit (VPD) increased over the course of the day. A significant portion of this increase can be explained by the vertical flux divergence, though horizontal advection also appears to be important. We suggest a linkage among boundary layer growth, the vertical flux divergences, and boundary layer cloud formation, with cloud activity peaking at midday in response to rapid CBL growth, then decreasing somewhat later in the day in response to CBL warming and decreased growth. We also see evidence of feedback between increasing VPD and stomatal control. We use eddy-covariance flux measurements from the Electra to compute the virtual temperature entrainment ratio Ar. The computed mean value of 0.08±0.12 is somewhat lower than the commonly assumed value of 0.2, as well as with other estimates from BOREAS. This value is very sensitive to the determination of CBL depth. We find that Ar increases with an increasing jump in mean wind across the CBL top. The entrainment flux of water vapor is found to be most dependent on time of day (negative correlation). The ratio of entrainment to surface flux of water vapor is 1.57±0.25. Airborne lidar observations of the CBL top reveal a CBL top “thickness” that is smaller than would be expected from simple theory but consistent with past lidar observations. The normalized thickness is found to have a very consistent value h¯/h0-1/0.116±0.008, where 12 cases were examined. A new method of computing the variability of the CBL top is illustrated, and we show that this variance in the CBL depth also scales with the depth but that the value of this normalized variance differs substantially from the “thickness” defined in past literature.

Airborne remote sensing of tropospheric water vapor with a near–infrared differential absorption lidar system

A near-infrared airborne differential absorption lidar (DIAL) system has become operational. Horizontal and vertical water vapor profiles of the troposphere during summer (nighttime) conditions extending from the top of the planetary boundary layer (PBL) up to near the tropopause are investigated. These measurements have been performed in Southern Bavaria, Germany. The system design, the frequency control units, and an estimation of the laser line profile of the narrow-band dye laser are discussed. Effective absorption cross sections in terms of altitude are calculated. Statistical and systematic errors of the water vapor measurements are evaluated as a function of altitude. The effect of a systematic range-dependent error caused by molecular absorption is investigated by comparing the DIAL data with in situ measurements. Typical horizontal resolutions range from 4 km in the lower troposphere to 11 km in the upper troposphere, with vertical resolutions varying from 0.3 to 1 km, respectively. The lower limit of the sensitivity of the water vapor mixing ratio is calculated to be 0.01 g/kg. The total errors of these measurements range between 8% and 25%. A sine-shaped wave structure with a wavelength of 14 km and an amplitude of 20% of its mean value, detected in the lower troposphere, indicates an atmospheric gravity wave field.

Estimation of boundary layer humidity fluxes and statistics from airborne differential absorption lidar (DIAL)

The water vapor differential absorption lidar (DIAL) of the German Aerospace Research Establishment (DLR) was flown aboard the National Center for Atmospheric Research (NCAR) Electra research aircraft during the Boreal Ecosystem-Atmosphere Study (BOREAS). The downward looking lidar system measured two-dimensional fields of aerosol backscatter and water vapor mixing ratio in the convective boundary layer (CBL) and across the CBL top (zt). We show a case study of DIAL observations of vertical profiles of mean water vapor, water vapor variance, skewness, and integral scale in the CBL. In the entrainment zone (EZ) and down to about 0.3 zi the DIAL observations agree with in situ observations and mixed-layer similarity theory. Below, the water vapor optical depth becomes large and the DIAL signal-to-noise ratio degrades. Knowing the water vapor surface flux and the convective velocity scale w* from in situ aircraft measurements, we derive entrainment fluxes by applying the mixed-layer gradient (MLG) and mixed-layer variance (MLV) methods to DIAL mixing ratio gradient and variance profiles. Entrainment flux estimates are sensitive to our estimate of zt. They are shown to be rather insensitive to the input surface flux and to the DIAL data spatial resolution within the investigated range. The estimates break down above about 0.9 zt as the flux-gradient and flux-variance relationships were developed to describe the large-scale mixing in the mid-CBL. The agreement with in situ entrainment flux estimations is within 30% for the MLV method. On a flight leg with significant mesoscale variability the entrainment flux turns out to be 70% higher than the in situ value. This is in good agreement with the fact that large-eddy simulations (LES) of mean water vapor profiles and variances, upon which the MLG and MLV methods are based, do not include mesoscale variability. The additional water vapor variance from mesoscales may then lead to the overestimate of the flux. Deviations from the in situ observations may also be due to poor LES resolution of small-scale mixing in the EZ, similarly coarse resolution of the DIAL data, or a capping inversion in the LES model (8 K) which is significantly stronger than the observed inversion (3–4 K).

Intercomparison of Water Vapor Data Measured with Lidar during IHOP_2002. Part I: Airborne to Ground-Based Lidar Systems and Comparisons with Chilled-Mirror Hygrometer Radiosondes

Abstract The water vapor data measured with airborne and ground-based lidar systems during the International H2O Project (IHOP_2002), which took place in the Southern Great Plains during 13 May–25 June 2002 were investigated. So far, the data collected during IHOP_2002 provide the largest set of state-of-the-art water vapor lidar data measured in a field campaign. In this first of two companion papers, intercomparisons between the scanning Raman lidar (SRL) of the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) and two airborne systems are discussed. There are 9 intercomparisons possible between SRL and the differential absorption lidar (DIAL) of Deutsches Zentrum fur Luft- und Raumfahrt (DLR), while there are 10 intercomparisons between SRL and the Lidar Atmospheric Sensing Experiment (LASE) of the NASA Langley Research Center. Mean biases of (−0.30 ± 0.25) g kg−1 or −4.3% ± 3.2% for SRL compared to DLR DIAL (DLR DIAL drier) and (0.16 ± 0.31) g kg−1 or 5.3% ± 5.1% ...

Water Vapor Flux Measurements from Ground-Based Vertically Pointed Water Vapor Differential Absorption and Doppler Lidars

Abstract For the first time, two lidar systems were used to measure the vertical water vapor flux in a convective boundary layer by means of eddy correlation. This was achieved by combining a water vapor differential absorption lidar and a heterodyne wind lidar in a ground-based experiment. The results prove that the combined lidar system can determine vertical flux profiles with a height resolution of approximately 100 m. Vertical averaging over a greater height interval reduces the error sufficiently that the changes in flux occurring throughout the day as a result of solar heating can be resolved. Horizontal and, for the first time, vertical integral scales were calculated from the lidar signals. The error analysis based on these results indicates that instrumental white noise and sampling error are the main sources of the statistical error in the flux measurement. Since the lidars measure simultaneously at many levels throughout the boundary layer, these errors can be reduced by vertical averaging to ...

Mixing of mineral dust with urban pollution aerosol over Dakar (Senegal): impact on dust physico-chemical and radiative properties

In the framework of the Saharan Mineral Dust Experiment (SAMUM) in 2008, the mixing of the urban pollution plume of Dakar (Senegal) with mineral dust was studied in detail using the German research aircraft Falcon which was equipped with a nadir-looking high spectral resolution lidar (HSRL) and extensive aerosol in situ instrumentation. The mineral dust layer as well as the urban pollution plume were probed remotely by the HSRL and in situ. Back trajectory analyses were used to attribute aerosol samples to source regions.We found that the emission from the region of Dakar increased the aerosol optical depth (532 nm) from approximately 0.30 over sea and over land east of Dakar to 0.35 in the city outflow. In the urban area, local black carbon (BC) emissions, or soot respectively, contributed more than 75% to aerosol absorption at 530 nm. In the dust layer, the single-scattering albedo at 530 nm was 0.96 − 0.99, whereas we found a value of 0.908 ± 0.018 for the aerosol dominated by urban pollution. After 6 h of transport over the North Atlantic, the externally mixed mode of secondary aerosol particles had almost completely vanished, whereas the BC agglomerates (soot) were still externally mixed with mineral dust particles.

Latent heat flux profiles from collocated airborne water vapor and wind lidars during IHOP_2002

Abstract Latent heat flux profiles in the convective boundary layer (CBL) are obtained for the first time with the combination of the Deutsches Zentrum fur Luft- und Raumfahrt (DLR) water vapor differential absorption lidar (DIAL) and the NOAA high resolution Doppler wind lidar (HRDL). Both instruments were integrated nadir viewing on board the DLR Falcon research aircraft during the International H2O Project (IHOP_2002) over the U.S. Southern Great Plains. Flux profiles from 300 to 2500 m AGL are computed from high spatial resolution (150 m horizontal and vertical) two-dimensional water vapor and vertical velocity lidar cross sections using the eddy covariance technique. Three flight segments on 7 June 2002 between 1000 and 1300 LT over western Oklahoma and southwestern Kansas are analyzed. On two segments with strong convection, the latent heat flux peaks at (700 ± 200) W m−2 in the entrainment zone and decreases linearly to (200 ± 100) W m−2 in the lower CBL. A water vapor budget analysis reveals that ...

MERLIN: a space-based methane monitor

Methane is a powerful greenhouse gas. The radiative forcing caused by methane contributes significantly to the warming of the atmosphere. To better understand the complex global Methane Cycle, it is necessary to apply space-based measurements techniques in order to obtain global coverage at high precision The Methane Remote Sensing Lidar Mission (MERLIN) is a joint French-German cooperation on a micro satellite mission for space-based measurement of spatial and temporal gradients of atmospheric methane columns on a global scale. MERLIN will be the first Integrated Path Differential Absorption LIDAR for methane monitoring from space. In contrast to passive methane missions, the LIDAR instrument allows to retrieve methane fluxes at all-latitudes, allseasons and during night as it is not relying on sunlight. First scientific studies show a substantial reduction of the prior methane flux uncertainties in key observational regions when using synthetic MERLIN observations in the flux inversion experiments. Furthermore, MERLIN observations can help to quantify and verify in scientific credible way national emission reduction scenarios as formulated in the Kyoto protocol. This paper reports on the present status of MERLIN and gives an overview on the joint mission concept with the German LIDAR on the French satellite platform MYRIADE.