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Hammann E.,University of Hohenheim | Behrendt A.,University of Hohenheim | Le Mounier F.,Laboratoire Of Meteorologie Dynamique | Wulfmeyer V.,University of Hohenheim
Atmospheric Chemistry and Physics | Year: 2015

The temperature measurements of the rotational Raman lidar of the University of Hohenheim (UHOH RRL) during the High Definition of Clouds and Precipitation for advancing Climate Prediction (HD(CP)2) Observation Prototype Experiment (HOPE) in April and May 2013 are discussed. The lidar consists of a frequency-tripled Nd:YAG laser at 355 nm with 10 W average power at 50 Hz, a two-mirror scanner, a 40 cm receiving telescope, and a highly efficient polychromator with cascading interference filters for separating four signals: the elastic backscatter signal, two rotational Raman signals with different temperature dependence, and the vibrational Raman signal of water vapor. The main measurement variable of the UHOH RRL is temperature. For the HOPE campaign, the lidar receiver was optimized for high and low background levels, with a novel switch for the passband of the second rotational Raman channel. The instrument delivers atmospheric profiles of water vapor mixing ratio as well as particle backscatter coefficient and particle extinction coefficient as further products. As examples for the measurement performance, measurements of the temperature gradient and water vapor mixing ratio revealing the development of the atmospheric boundary layer within 25 h are presented. As expected from simulations, a reduction of the measurement uncertainty of 70% during nighttime was achieved with the new low-background setting. A two-mirror scanner allows for measurements in different directions. When pointing the scanner to low elevation, measurements close to the ground become possible which are otherwise impossible due to the non-total overlap of laser beam and receiving telescope field of view in the near range. An example of a low-level temperature measurement is presented which resolves the temperature gradient at the top of the stable nighttime boundary layer 100 m above the ground. © Author(s) 2015. CC Attribution 3.0 License.

Alexander M.J.,NorthWest Research Associates, Inc. | Teitelbaum H.,Laboratoire Of Meteorologie Dynamique
Journal of Geophysical Research: Atmospheres | Year: 2011

The southern Andes region has been clearly identified in previous satellite and balloon observations and in global models as a "hot spot" of small-scale gravity wave activity, with monthly mean momentum fluxes exceeding 10 times background values in fall, winter, and spring seasons. This makes this region a focus of interest for global circulation and climate studies. We analyze a case study on 8 May 2006, combining observations from the Atmospheric Infrared Sounder instrument on the Aqua satellite and the High Resolution Dynamics Limb Sounder instrument of the Aura satellite to form a three-dimensional picture of the wave field. The observations show a widespread wave pattern over the southern Andes extending eastward over the south Atlantic. Simulations with the Weather Research Forecasting model clearly identify the waves as orographic in origin, but the observed wave pattern is far from the simple two-dimensional wave field forced by steady flow over a mountain ridge. The morphology of the pattern is consistent with three-dimensional linear theoretical calculations of downstream propagation and latitudinal focusing of mountain waves into the stratospheric jet. The observations confirm the importance of this process in the stratosphere, and we find the process also occurring in the global analysis and forecasts from the European Centre for Medium-Range Weather Forecasting. Our analysis evaluates some strengths and weaknesses of current orographic wave drag parameterizations in global models and the relevance of parameterization assumptions in global models with high resolution. Copyright 2011 by the American Geophysical Union.

Tzella A.,Laboratoire Of Meteorologie Dynamique | Haynes P.H.,University of Cambridge
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

This paper studies the spatial structure of decaying chemical fields generated by a chaotic-advection flow and maintained by a spatially smooth chemical source. Previous work showed that in a regime where diffusion can be neglected (large Péclet number), the structures are filamental or smooth depending on the relative strength of the chemical dynamics and the stirring induced by the flow. The scaling exponent, γq, of the qth -order structure function depends, at leading order, linearly on the ratio of the rate of decay of the chemical processes, α, and the average rate of divergence of neighboring fluid parcel trajectories (Lyapunov exponent), h̄. Under a homogeneous stretching approximation, γq /q=max { α/ h̄, 1 } which implies that a well-defined filamental-smooth transition occurs at α= h̄. This approximation has been improved by using the distribution of finite-time Lyapunov exponents to characterize the inhomogeneous stretching of the flow. However, previous work focused more on the behavior of the exponents as q varies and less on the effects of α and hence the implications for the filamental-smooth transition. Here we set out the precise relation between the stretching rate statistics and the scaling exponents and emphasize that the latter are determined by the distribution of the finite-size (rather than finite-time) Lyapunov exponents. We clarify the relation between the two distributions. We show that the corrected exponents, γ∼ q, depend nonlinearly on α with γ∼ q < γq for γ∼ q hmax, where hmax denotes the maximum finite-time Lyapunov exponent and unambiguously filamental for α< h̄, with an intermediate character for α between these two values. Theoretical predictions are confirmed by numerical results obtained for a linearly decaying chemistry coupled to a renewing type of flow together with careful calculations of the Crámer function. © 2010 The American Physical Society.

Nam C.C.W.,Max Planck Institute for Meteorology | Nam C.C.W.,Laboratoire Of Meteorologie Dynamique | Quaas J.,Max Planck Institute for Meteorology | Quaas J.,University of Leipzig
Journal of Climate | Year: 2012

Observations from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and CloudSat satellites are used to evaluate clouds and precipitation in the ECHAM5 general circulation model. Active lidar and radar instruments on board CALIPSO and CloudSat allow the vertical distribution of clouds and their optical properties to be studied on a global scale. To evaluate the clouds modeled by ECHAM5with CALIPSO and CloudSat, the lidar and radar satellite simulators of the Cloud Feedback Model Intercomparison Project's Observation Simulator Package are used. Comparison of ECHAM5 with CALIPSO and CloudSat found large-scale features resolved by the model, such as the Hadley circulation, are captured well. The lidar simulator demonstrated ECHAM5 overestimates the amount of high-level clouds, particularly optically thin clouds. High-altitude clouds in ECHAM5 consistently produced greater lidar scattering ratios compared with CALIPSO. Consequently, the lidar signal in ECHAM5 frequently attenuated high in the atmosphere. The large scattering ratios were due to an underestimation of effective ice crystal radii in ECHAM5. Doubling the effective ice crystal radii improved the scattering ratios and frequency of attenuation. Additionally, doubling the effective ice crystal radii improved the detection of ECHAM5's highest-level clouds by the radar simulator, in better agreement with CloudSat. ECHAM5 was also shown to significantly underestimate midlevel clouds and (sub)tropical low-level clouds. The low-level clouds produced were consistently perceived by the lidar simulator as too optically thick. The radar simulator demonstrated ECHAM5 overestimates the frequency of precipitation, yet underestimates its intensity compared with CloudSat observations. These findings imply compensating mechanisms inECHAM5 balance out the radiative imbalance caused by incorrect optical properties of clouds and consistently large hydrometeors in the atmosphere. © 2012 American Meteorological Society.

Guimberteau M.,University Paris - Sud | Laval K.,Laboratoire Of Meteorologie Dynamique | Perrier A.,Agro ParisTech | Polcher J.,French National Center for Scientific Research
Climate Dynamics | Year: 2012

In a context of increased demand for food and of climate change, the water consumptions associated with the agricultural practice of irrigation focuses attention. In order to analyze the global influence of irrigation on the water cycle, the land surface model ORCHIDEE is coupled to the GCM LMDZ to simulate the impact of irrigation on climate. A 30-year simulation which takes into account irrigation is compared with a simulation which does not. Differences are usually not significant on average over all land surfaces but hydrological variables are significantly affected by irrigation over some of the main irrigated river basins. Significant impacts over the Mississippi river basin are shown to be contrasted between eastern and western regions. An increase in summer precipitation is simulated over the arid western region in association with enhanced evapotranspiration whereas a decrease in precipitation occurs over the wet eastern part of the basin. Over the Indian peninsula where irrigation is high during winter and spring, a delay of 6 days is found for the mean monsoon onset date when irrigation is activated, leading to a significant decrease in precipitation during May to July. Moreover, the higher decrease occurs in June when the water requirements by crops are maximum, exacerbating water scarcity in this region. A significant cooling of the land surfaces occurs during the period of high irrigation leading to a decrease of the land-sea heat contrast in June, which delays the monsoon onset. © 2011 Springer-Verlag.

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