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Bernd Kärcher - One of the best experts on this subject based on the ideXlab platform.

  • Dust ice nuclei effects on Cirrus Clouds in ECHAM5-HAM
    2013
    Co-Authors: Ulrike Lohmann, Miriam Kübbeler, Johannes Hendricks, Bernd Kärcher
    Abstract:

    Aerosol-cloud interactions are one of the main uncertainties in climate research. Up to now a lot of research has been conducted on aerosol-cloud interactions in warm Clouds. The impact of aerosols on ice or mixed-phase Clouds is much less understood. Cirrus Clouds in an unpolluted environment are assumed to form mainly via homogeneous freezing. The presence of heterogeneous ice nuclei can lead to earlier ice crystal formation and change the microphysical properties of Cirrus Clouds. Recent box model studies even suggest that heterogeneous freezing can suppress homogeneous freezing, if several conditions are fulfilled. Most likely this would lead to Cirrus Clouds containing fewer and larger ice crystals. If homogeneous and heterogeneous freezing compete either freezing mechanism may dominate depending mainly on vertical velocity and number density of ice nuclei. Thus, it is not clear yet how number and size of ice crystals are affected.

  • dust ice nuclei effects on Cirrus Clouds
    Atmospheric Chemistry and Physics, 2013
    Co-Authors: M Kuebbeler, Johannes Hendricks, Ulrike Lohmann, Bernd Kärcher
    Abstract:

    Abstract. In order to study aerosol–cloud interactions in Cirrus Clouds, we apply a new multiple-mode ice microphysical scheme to the general circulation model ECHAM5-HAM. The multiple-mode ice microphysical scheme allows for analysis of the competition between homogeneous freezing of solution droplets, deposition nucleation of pure dust particles, and immersion freezing of coated dust particles and pre-existing ice. We base the freezing efficiencies of coated and pure dust particles on the most recent laboratory data. The effect of pre-existing ice, which has been neglected in previous ice nucleation parameterizations, is to deplete water vapour by depositional growth and thus prevent homogeneous and heterogeneous freezing from occurring. As a first step, we extensively tested the model and validated the results against in situ measurements from various aircraft campaigns. The results compare well with observations; properties such as ice crystal size and number concentration as well as supersaturation are predicted within the observational spread. We find that heterogeneous nucleation on mineral dust particles and the consideration of pre-existing ice in the nucleation process may lead to significant effects: globally, ice crystal number and mass are reduced by 10 and 5%, whereas the ice crystals' size is increased by 3%. The reductions in ice crystal number are most pronounced in the tropics and mid-latitudes in the Northern Hemisphere. While changes in the microphysical and radiative properties of Cirrus Clouds in the tropics are mostly driven by considering pre-existing ice, changes in the northern hemispheric mid-latitudes mainly result from heterogeneous nucleation. The so-called negative Twomey effect in Cirrus Clouds is represented in ECHAM5-HAM. The net change in the radiation budget is −0.94 W m−2, implying that both heterogeneous nucleation on dust and pre-existing ice have the potential to modulate Cirrus properties in climate simulations and thus should be considered in future studies.

  • Nitric acid in Cirrus Clouds
    Geophysical Research Letters, 2006
    Co-Authors: Christiane Voigt, Bernd Kärcher, Beiping Luo, Hans Schlager, Martina Krämer, Cornelius Schiller, Helmut Ziereis, P. J. Popp, Hitoshi Irie, Yoshiko Kondo
    Abstract:

    [1] Uptake of nitric acid (HNO3) in Arctic Cirrus ice crystals was observed on 11 February 2003 by in-situ instruments onboard the M55 Geophysica aircraft. The Cirrus cloud with a mean ice water content of 5.4 mg m−3 covered northern Scandinavia for several hours and extended up to the thermal tropopause at 12.3 km. Within the Cirrus region, on average 9% of the total HNO3 measured as reactive nitrogen (NOy) is present in ice particles, increasing to 19% at temperatures below 205 K. In contrast to previous studies, we discuss the HNO3 uptake in ice in terms of HNO3/H2O molar ratios in ice crystals. The HNO3 content of the ice increases with increasing gas phase HNO3 concentrations and decreasing temperatures. Enhanced uptake of HNO3 in ice and heterogeneous chemistry on cold Cirrus Clouds may disturb the upper tropospheric ozone budget.

  • Cirrus Clouds in the tropical tropopause layer role of heterogeneous ice nuclei
    Geophysical Research Letters, 2004
    Co-Authors: Bernd Kärcher
    Abstract:

    [1] The role of heterogeneous ice nuclei in controlling the occurrence and properties of Cirrus Clouds in the tropical tropopause layer is examined with the help of a Lagrangian microphysical Cirrus model that includes competition between insoluble and volatile aerosol particles during ice nucleation and small-scale temperature perturbations. The potential of ice nuclei to influence the moisture budget of air at entry to the stratosphere appears to be limited. Additional dehydration of up to 0.3 ppmv may occur, relative to 1.3 ppmv caused by pure homogeneous freeze-drying. In contrast, ice nuclei significantly enhance the frequency of occurrence of subvisible Cirrus Clouds, even when present at concentrations as low as 0.01 L−1. This is likely to be the largest effect of ice nuclei on Cirrus near the tropical tropopause. Ice nuclei may also alter cloud radiative forcing by changing the ice water content, increasing the effective crystal radius, and decreasing the cloud lifetime.

  • On the Transition of Contrails into Cirrus Clouds
    Journal of the Atmospheric Sciences, 2000
    Co-Authors: F. Schröder, Bernd Kärcher, C. Duroure, Johan Ström, Andreas Petzold, Jean-françois Gayet, B. Strauss, P. Wendling, Stephan Borrmann
    Abstract:

    In situ observations of the microphysical properties of upper-tropospheric contrails and Cirrus Clouds have been performed during more than 15 airborne missions over central Europe. Experimental and technical aspects concerning in situ characterization of ice Clouds with the help of optical and nonoptical detection methods (preferably FSSP-300 and Hallet-type replicator) are addressed. The development of contrails into Cirrus Clouds on the timescale of 1 h is discussed in terms of a representative set of number densities, and size distributions and surface area distributions of aerosols and cloud elements, with special emphasis on small ice crystals (diameter ,20 mm). Contrails are dominated by high concentrations (.100 cm23) of nearly spherical ice crystals with mean diameters in the range 1‐10 mm. Young Cirrus Clouds, which mostly contain small regularly shaped ice crystals in the range 10‐20-mm diameter and typical concentrations 2‐5 cm23, have been observed. Measurement results are compared to simple parcel model calculations to identify parameters relevant for the contrail‐Cirrus transition. Observations and model estimates suggest that contrail growth is only weakly, if at all, affected by preexisting Cirrus Clouds.

Lunche Wang - One of the best experts on this subject based on the ideXlab platform.

  • Revisiting global satellite observations of stratospheric Cirrus Clouds
    Atmospheric Chemistry and Physics, 2020
    Co-Authors: Ling Zou, Sabine Griessbach, Lars Hoffmann, Bing Gong, Lunche Wang
    Abstract:

    Abstract. As knowledge about the Cirrus Clouds in the lower stratosphere is limited, reliable long-term measurements are needed to assess their characteristics, radiative impact and important role in upper troposphere and lower stratosphere (UTLS) chemistry. We used 6 years (2006–2012) of Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) measurements to investigate the global and seasonal distribution of stratospheric Cirrus Clouds and compared the MIPAS results with results derived from the latest version (V4.x) of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data. For the identification of stratospheric Cirrus Clouds, precise information on both the cloud top height (CTH) and the tropopause height is crucial. Here, we used lapse rate tropopause heights estimated from the ERA-Interim global reanalysis. Considering the uncertainties of the tropopause heights and the vertical sampling grid, we define CTHs more than 0.5 km above the tropopause as stratospheric for CALIPSO data. For MIPAS data, we took into account the coarser vertical sampling grid and the broad field of view so that we considered Cirrus CTHs detected more than 0.75 km above the tropopause as stratospheric. Further sensitivity tests were conducted to rule out sampling artefacts in MIPAS data. The global distribution of stratospheric Cirrus Clouds was derived from night-time measurements because of the higher detection sensitivity of CALIPSO. In both data sets, MIPAS and CALIPSO, the stratospheric Cirrus cloud occurrence frequencies are significantly higher in the tropics than in the extra-tropics. Tropical hotspots of stratospheric Cirrus Clouds associated with deep convection are located over equatorial Africa, South and Southeast Asia, the western Pacific, and South America. Stratospheric Cirrus Clouds were more often detected in December–February (15 %) than June–August (8 %) in the tropics ( ±20 ∘ ). At northern and southern middle latitudes (40–60 ∘ ), MIPAS observed about twice as many stratospheric Cirrus Clouds (occurrence frequencies of 4 %–5 % for MIPAS rather than about 2 % for CALIPSO). We attribute more frequent observations of stratospheric Cirrus Clouds with MIPAS to the higher detection sensitivity of the instrument to optically thin Clouds. In contrast to the difference between daytime and night-time occurrence frequencies of stratospheric Cirrus Clouds by a factor of about 2 in zonal means in the tropics (4 % and 10 %, respectively) and at middle latitudes for CALIPSO data, there is little diurnal cycle in MIPAS data, in which the difference of occurrence frequencies in the tropics is about 1 percentage point in zonal mean and about 0.5 percentage point at middle latitudes. The difference between CALIPSO day and night measurements can also be attributed to their differences in detection sensitivity. Future work should focus on better understanding the origin of the stratospheric Cirrus Clouds and their impact on radiative forcing and climate.

  • Revisiting global satellite observations of stratospheric Cirrus Clouds
    2020
    Co-Authors: Ling Zou, Sabine Griessbach, Lars Hoffmann, Bing Gong, Lunche Wang
    Abstract:

    Abstract. As knowledge about the Cirrus Clouds in the lower stratosphere is limited, reliable long-term measurements are needed to assess their characteristics, radiative impact and important role in upper troposphere and lower stratosphere (UTLS) chemistry. To investigate the global and seasonal distribution of stratospheric Cirrus Clouds, we used the latest version (V4.x) of the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data. For the identification of stratospheric Cirrus Clouds, precise information on both, the cloud top height (CTH) and the tropopause height is crucial. Here, we used lapse rate tropopause heights estimated from the ERA-Interim global reanalysis. Considering the uncertainties of the tropopause heights and the vertical sampling grid of the CALIPSO data, we considered Cirrus Clouds with CTHs more than 0.5 km above the tropopause as being stratospheric. We focused on nighttime CALIPSO measurements, because of their higher detection sensitivity. A six-year mean (2006–2012) global distribution of stratospheric Cirrus cloud from CALIPSO showed that higher CTH occurrence frequencies are observed in the tropics than in the extra-tropics. Tropical hotspots of stratospheric Cirrus Clouds associated with deep convection are located over Equatorial Africa, South and Southeast Asia, the western Pacific and South America. Stratospheric Cirrus Clouds were more often detected in December–February (15 %) than June–August (8 %) in the tropics (± 20°). At middle (40–60°) and higher latitudes (> 60°), CALIPSO observed on average about 2 % stratospheric Cirrus Clouds. Observations of stratospheric Cirrus cloud with MIPAS are presented here for the first time. Taking into account the MIPAS vertical sampling and broad field of view, we considered Cirrus CTHs detected not less than 0.75 km above the tropopause as being stratospheric. Compared to CALIPSO, MIPAS observed twice as many stratospheric Cirrus Clouds at northern and southern middle latitudes (occurrence frequencies of 4–5 % for MIPAS rather than about 2 % for CALIPSO). We attribute more frequent observations of stratospheric Cirrus Clouds with MIPAS to higher detection sensitivity of the instrument to optically thin Clouds. Sensitivity tests on MIPAS stratospheric cloud detections have been conducted to rule out sampling artefacts. Future work should focus on better understanding the origin of the stratospheric Cirrus Clouds and their impact on radiative forcing and climate.

  • Satellite observations of Cirrus Clouds in the lower stratosphere
    2020
    Co-Authors: Ling Zou, Sabine Griessbach, Lars Hoffmann, Bing Gong, Lunche Wang
    Abstract:

    <div> <p>While Cirrus cloud are frequently observed by ground-based lidars in the lowermost stratosphere, evidence from satellite observations is less conclusive. Following previous studies, we extracted information on stratospheric Cirrus Clouds from the latest version of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) data (V4) and the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) data to investigate their global distribution and occurrence frequencies. The detection of stratospheric Cirrus with MIPAS is particularly challenging because of the broad field-of-view of the instrument and presented here for the first time.</p> <p>For the identification of stratospheric Cirrus Clouds, precise information on both, the cloud top height (CTH) and the tropopause height are crucial. The tropopause heights we derived from ERA-Interim using the WMO criterion for the first thermal tropopause. As tropopause from ERA-Interim show ~0.3km bias with GPS data and CALIPSO data are reported on a ~0.2 km vertical grid, we considered Cirrus Clouds with CTHs 0.5 km above the tropopause as being stratospheric. We focus on nighttime CALIPSO measurements because of their higher sensitivity. MIPAS measurements are known to overestimate CTHs of optically thick Clouds and underestimate the CTHs of optically thin Clouds. For the detection of stratospheric Cirrus, we started 0.75 km above the tropopause, which is the average CTH overestimation by MIPAS found in previous studies. The comparison with the CALIPSO statistics showed that in the tropics the MIPAS stratospheric Cirrus cloud occurrence frequency were slightly larger than for CALIPSO. Assuming that this is due to MIPAS overestimating the CTH, for MIPAS we increased the minimum distance of the CTH to the tropopause until the occurrence frequencies of both measurements agreed.</p> <p>In the tropics, a four-year mean global analysis of stratospheric Cirrus Clouds from CALIPSO showed high occurrence frequencies (max. >32%) over the western Pacific Ocean, South Africa, and South America. Stratospheric Cirrus Clouds were more often detected in December-February than June-August in the tropics. At middle (40-60°) and higher latitudes (>60°), CALIPSO observed about 2% stratospheric Cirrus Clouds. MIPAS observed about twice as many stratospheric Cirrus Clouds at northern middle latitudes (>3%) and southern middle latitudes (4%). The maximum differences of nighttime stratospheric Cirrus Clouds between MIPAS and CALIPSO data were 4-6% over the northern Pacific and 6-8% over the Drake Passage.</p> <p>Further sensitivity tests with higher average distance to the tropopause for MIPAS resulted in lower occurrence frequencies at middle latitudes. However, they were still larger than the occurrence frequencies derived from CALIPSO data. Hence, we consider the finding of higher stratospheric Cirrus cloud occurrence frequencies at middle latitudes by MIPAS as robust. One possible explanation for MIPAS finding more stratospheric Cirrus Clouds at middle latitudes is that MIPAS is more sensitive towards thin Cirrus Clouds than CALIPSO (nighttime measurements), because of the satellite limb measurement geometry.</p> </div>

P. H. Flamant - One of the best experts on this subject based on the ideXlab platform.

  • Remote sensing of Cirrus Clouds and aerosols by a sun photometer in Tunisia
    Atmospheric Chemistry and Physics Discussions, 2006
    Co-Authors: H. Chtioui, F. B. Mansour, S. Elouragini, P. H. Flamant
    Abstract:

    Some ground based measurements of solar radiation by using a sun photometer, have been conducted in Tunisia during the period of November 2000–February 2002. Five key measurement sites were selected: Three Sites (Tunis, Sousse, Gabes) are located on the Mediterranean coast and Two sites (Gafsa, Tozeur) on the boarder of Sahara. Over a total of 149 measurement days, 21 days are identified as clear sky, 114 days as Cirrus Clouds and 14 days as aerosols. Aerosols and Cirrus Clouds Optical Thickness (AOT) are derived from photometric measurements at 532 nm wavelength. Spatial and temporal variabilities of AOT are presented and discussed in this paper. Cirrus Clouds were frequently observed at Gafsa and Tozeur where saharan aerosol events are expected to be more frequent than Cirrus Clouds. The mediterranean sea and saharan aerosols are suspected to have the main role in Cirrus Clouds formation, by providing water vapor and high concentrations of cloud condensation and ice forming nuclei.

  • Ice crystal shapes in Cirrus Clouds derived from POLDER/ADEOS-1
    Journal of Geophysical Research, 2001
    Co-Authors: Hélène Chepfer, Philippe Goloub, J Riedi, J F De Haan, J W Hovenier, P. H. Flamant
    Abstract:

    This paper discusses the retrieval of ice crystal shapes of Cirrus Clouds on a global scale using observations collected with POLDER-1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS-1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for Cirrus Clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the Cirrus Clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick Cirrus Clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes.

Pierre H. Flamant - One of the best experts on this subject based on the ideXlab platform.

  • Classification of Particle Effective Shape Ratios in Cirrus Clouds Based on the Lidar Depolarization Ratio
    Applied optics, 2002
    Co-Authors: Vincent Noël, Guy Ledanois, Arnaud Delaval, Hélène Chepfer, Pierre H. Flamant
    Abstract:

    A shape classification technique for Cirrus Clouds that could be applied to future spaceborne lidars is presented. A ray-tracing code has been developed to simulate backscattered and depolarized lidar signals from Cirrus Clouds made of hexagonal-based crystals with various compositions and optical depth, taking into account multiple scattering. This code was used first to study the sensitivity of the linear depolarization rate to cloud optical and microphysical properties, then to classify particle shapes in Cirrus Clouds based on depolarization ratio measurements. As an example this technique has been applied to lidar measurements from 15 mid-latitude Cirrus cloud cases taken in Palaiseau, France. Results show a majority of near-unity shape ratios as well as a strong correlation between shape ratios and temperature: The lowest temperatures lead to high shape ratios. The application of this technique to spaceborne measurements would allow a large-scale classification of shape ratios in Cirrus Clouds, leading to better knowledge of the vertical variability of shapes, their dependence on temperature, and the formation processes of Clouds.

  • ice crystal shapes in Cirrus Clouds derived from polder adeos 1
    Journal of Geophysical Research, 2001
    Co-Authors: Hélène Chepfer, Philippe Goloub, J Riedi, J F De Haan, J W Hovenier, Pierre H. Flamant
    Abstract:

    This paper discusses the retrieval of ice crystal shapes of Cirrus Clouds on a global scale using observations collected with POLDER-1 (POLarization and Directionality of the Earth Reflectance) onboard the ADEOS-1 platform. The retrieval is based on polarized bidirectional observations made by POLDER. First, normalized polarized radiances are simulated for Cirrus Clouds composed of ice crystals that differ in shape and are randomly oriented in space. Different values of cloud optical depths, viewing geometries and solar zenith angles are used in the simulations. This sensitivity study shows that the normalized polarized radiance is highly sensitive to the shape of the scatterers for specific viewing geometries, and that it saturates after a few scattering events, which makes it rapidly independent of the optical depth of the Cirrus Clouds. Next, normalized polarized radiance observations obtained by POLDER have been selected, based on suitable viewing geometries and on the occurrence of thick Cirrus Clouds composed of particles randomly oriented in space. For various ice crystal shapes these observations are compared with calculated values pertaining to the same geometry, in order to determine the shape that best reproduces the measurements. The method is tested fully for the POLDER data collected on January 12, 1997. Thereafter, it is applied to six periods of 6 days of observations obtained in January, February, March, April, May, and June 1997. This study shows that the particle shape is highly variable with location and season, and that polycrystals and hexagonal columns are dominant at low latitudes, whereas hexagonal plates occur more frequently at high latitudes.

Steven C Sherwood - One of the best experts on this subject based on the ideXlab platform.

  • rapidly evolving Cirrus Clouds modulated by convectively generated gravity waves
    Journal of Geophysical Research, 2019
    Co-Authors: Abhnil Amtesh Prasad, Steven C Sherwood, Michael J Reeder, Todd P Lane
    Abstract:

    Cirrus Clouds can strongly affect Earth's radiation balance, but questions remain about their growth mechanisms and rates. Here we show that gravity (buoyancy) waves generated by a storm in Northern Australia on 13 November 2015 caused an observable rippling effect on Cirrus Clouds up to 1,000 km away, as seen by the recently launched Himawari-8/9 geostationary satellite. Regional model simulations reproduce the propagation speed of the wave, which agrees with theoretical predictions, and show that the wave amplitude and timing near the tropopause can account for the Cirrus modulation. The observed Cirrus reach peak optical depths of order 0.3–1.0 and appear roughly in phase with the arrival of the relative humidity maximum, providing new evidence that Cirrus Clouds can respond rapidly (<30 min) to environmental lifting. Moreover, the edge of a thick anvil cloud attached to the storm itself is observed to expand at the same speed as the wave, showing that the lifting mechanism can also apply to optically thicker ice Clouds close to convective centers.

  • On moistening of the tropical troposphere by Cirrus Clouds
    Journal of Geophysical Research: Atmospheres, 1999
    Co-Authors: Steven C Sherwood
    Abstract:

    Sublimation of ice crystals in Cirrus Clouds is often invoked as a moistening mechanism for the free troposphere, particularly in the tropics. An alternative mechanism is investigated here: “pumping” of water vapor into the cloudy atmospheric column from surrounding regions by diabatic transports resulting from the local radiative effects of the cloud. The effectiveness of this mechanism is computed for a variety of Cirrus cloud types, using a simple model. For nonprecipitating Cirrus Clouds it is found that the pumping mechanism can import much more vapor mass into the cloudy column than the Clouds contain as ice, especially as the Clouds become optically thin. The advected vapor per unit ice mass (or “pumping ratio”) also varies with cloud height, is proportional to cloud lifetime, and (for thin Clouds) is approximately inversely proportional to effective particle radius. Thin Cirrus above 300 hPa can pump vapor rapidly enough to sustain itself against dessication. The overall results are supported by the limited information available from observations and indicate that at sufficient distances from active convection, direct moistening of the environment by the sublimation of ice has significantly less impact on vapor distributions than cloud-radiative-dynamic effects.