An improved algorithm for polar cloud-base detection by ceilometer over the ice sheets

Optically thin ice and mixed-phase clouds play an important role in polar regions due to their effect on cloud radiative impact and precipitation. Cloud-base heights can be detected by ceilometers, low-power backscatter lidars that run continuously and therefore have the potential to provide basic cloud statistics including cloud frequency, base height and vertical structure. The standard cloud-base detection algorithms of ceilometers are designed to detect optically thick liquid-containing clouds, while the detection of thin ice clouds requires an alternative approach. This paper presents the polar threshold (PT) algorithm that was developed to be sensitive to optically thin hydrometeor layers (minimum optical depth τ ≥ 0.01). The PT algorithm detects the first hydrometeor layer in a vertical attenuated backscatter profile exceeding a predefined threshold in combination with noise reduction and averaging procedures. The optimal backscatter threshold of 3 × 10^−4 km⁻¹ sr⁻¹ for cloud-base detection near the surface was derived based on a sensitivity analysis using data from Princess Elisabeth, Antarctica and Summit, Greenland. At higher altitudes where the average noise level is higher than the backscatter threshold, the PT algorithm becomes signal-to-noise ratio driven. The algorithm defines cloudy conditions as any atmospheric profile containing a hydrometeor layer at least 90 m thick. A comparison with relative humidity measurements from radiosondes at Summit illustrates the algorithm's ability to significantly discriminate between clear-sky and cloudy conditions. Analysis of the cloud statistics derived from the PT algorithm indicates a year-round monthly mean cloud cover fraction of 72% (±10%) at Summit without a seasonal cycle. The occurrence of optically thick layers, indicating the presence of supercooled liquid water droplets, shows a seasonal cycle at Summit with a monthly mean summer peak of 40 % (±4%). The monthly mean cloud occurrence frequency in summer at Princess Elisabeth is 46% (±5%), which reduces to 12% (±2.5%) for supercooled liquid cloud layers. Our analyses furthermore illustrate the importance of optically thin hydrometeor layers located near the surface for both sites, with 87% of all detections below 500 m for Summit and 80% below 2 km for Princess Elisabeth. These results have implications for using satellite-based remotely sensed cloud observations, like CloudSat that may be insensitive for hydrometeors near the surface. The decrease of sensitivity with height, which is an inherent limitation of the ceilometer, does not have a significant impact on our results. This study highlights the potential of the PT algorithm to extract information in polar regions from various hydrometeor layers using measurements by the robust and relatively low-cost ceilometer instrument

JOYCE: Jülich Observatory for cloud evolution

The Jülich Observatory for Cloud Evolution (JOYCE), located at Forschungszentrum Jülich in the most western part of Germany, is a recently established platform for cloud research. The main objective of JOYCE is to provide observations, which improve our understanding of the cloudy boundary layer in a midlatitude environment. Continuous and temporally highly resolved measurements that are specifically suited to characterize the diurnal cycle of water vapor, stability, and turbulence in the lower troposphere are performed with a special focus on atmosphere–surface interaction. In addition, instruments are set up to measure the micro- and macrophysical properties of clouds in detail and how they interact with different boundary layer processes and the large-scale synoptic situation. For this, JOYCE is equipped with an array of state-of-the-art active and passive remote sensing and in situ instruments, which are briefly described in this scientific overview. As an example, a 24-h time series of the evolution of a typical cumulus cloud-topped boundary layer is analyzed with respect to stability, turbulence, and cloud properties. Additionally, we present longer-term statistics, which can be used to elucidate the diurnal cycle of water vapor, drizzle formation through autoconversion, and warm versus cold rain precipitation formation. Both case studies and long-term observations are important for improving the representation of clouds in climate and numerical weather prediction models

Water vapor turbulence profiles in stationary continental convective mixed layers

The U. S. Department of Energy Atmospheric Radiation Measurement (ARM) program's Raman lidar at the ARM Southern Great Plains site in north central Oklahoma has collected water vapor mixing ratio (q) profile data more than 90% of the time since October 2004. Three hundred (300) cases were identified where the convective boundary layer was quasi-stationary and well mixed for a 2 h period, and q mean, variance, third-order moment, and skewness profiles were derived from the 10 s, 75 m resolution data. These cases span the entire calendar year, and demonstrate that the q variance profiles at the mixed layer (ML) top changes seasonally and is strongly related to the gradient of q across the interfacial layer. The q variance at the top of the ML shows onlyweak correlations (r < 0.3) with sensible heat flux, Deardorff convective velocity scale, and turbulence kinetic energy measured at the surface. The median q skewness profile is most negative at 0.85 z(i), zero at approximately z(i), and positive above z(i), where z(i) is the depth of the convective ML. The spread in the q skewness profiles is smallest between 0.95 z(i) and z(i). The q skewness at altitudes between 0.6 z(i) and 1.2 z(i) is correlated with the magnitude of the q variance at z(i), with increasingly negative values of skewness observed lower down in the ML as the variance at z(i) increases, suggesting that in cases with larger variance at z(i) there is deeper penetration of the warm, dry free tropospheric air into the ML

Entstehung, Verhalten und Ablagerung von gas-, partikel und tropfenfoermigen Reaktionsprodukten Abschlussbericht

Es wurden vergleichende Studien zur Deposition und Emission sowie zur Stoffumwandlung im Gebiet des Nationalparks Bayerischer Wald durchgefuehrt. Die Ergebnisse zeigen, dass die Stoffeintraege durch trockene Deposition diejenigen durch nasse Deposition uebertreffen. Bei dem Saeureeintrag sowie dem Eintrag von Sulfat kommt den Prozessen der Codeposition (d.h. der reaktiven Ablagerung von SO_2) eine dominierende Rolle zu. Da die verschiedenen Vorgaenge recht komplex miteinander verknuepft sind, wurde vorerst nur eine Berechnung des minimalen Eintrags durch trockene Deposition durchgefuehrt. Diese ergibt, dass der gesamte Nitrateintrag mindestens 9,0 kgNha&quot;-&quot;1a&quot;0&quot;1, der gesamte Ammoniumeintrag mindestens etwa 16 kgNha&quot;-&quot;1a&quot;-&quot;1, der H&quot;+-Eintrag etwa 2,0 kgH&quot;+ha&quot;-&quot;1a&quot;-&quot;1 und der Sulfateintrag etwa 66 kgSO_4&quot;2&quot;-ha&quot;-&quot;1a&quot;-&quot;1 betraegt. Wird die trockene NO_2-Deposition als Nitrateintrag bewertet, so erhoeht sich dieser auf ca. 14 kgNha&quot;-&quot;1a&quot;-&quot;1. Wichtig erscheint dabei, dass wesentliche Beitraege zur trockenen H&quot;+ und SO_4&quot;2&quot;--Deposition durch Prozesse der Codeposition erfolgen. Diese Vorgaenge werden offensichtlich i.a. deutlich unterschaetzt. In Regionen, in denen nur eine H_2O_2Produktion von 0,1 ppbh&quot;-&quot;1 stattfindet, kann durch Codeposition mit H_2O_2 ein jaehrlicher Sulfatfluss von ca. 20 kgha&quot;-&quot;1a&quot;-&quot;1 erzeugt werden. Durch Codeposition von SO_2 mit O_3 werden ca. 17-20 kgha&quot;-&quot;1a&quot;-&quot;1 gebildet. Es konnte ferner gezeigt werden, dass chemische Reaktionen in der Gas- und Partikelphase signifikant die Stoffbilanzen und damit auch die Stoffdeposition beeinflussen koennen. Die NHO_3-Bildung vermag sogar tagsueber durch die Radikalreaktion von NO_3+NO_2 beeinflusst zu werden, da die Photolyseraten im Bestand gegenueber den Werten in der Atmosphaehre stark reduziert sind. (orig.)Various studies were performed in the Nationalpark 'Bavarian Forest' at a spruce/beech forest-site to determine the fluxes of N- and S-compounds by dry and wet deposition. The minimum deposition is determined for nitrate 9.0 kgNha&quot;-&quot;1yr&quot;-&quot;1, for ammonium (NH_4&quot;++NH_3) 16 kgNha&quot;-&quot;1yr&quot;-&quot;1, for H&quot;+ 2.0 kgH&quot;+ha&quot;-&quot;1yr&quot;-&quot;1 and for sulfate 66 kgSO_2&quot;2&quot;-ha&quot;-&quot;1yr&quot;-&quot;1. If the dry deposition of NO_2 is considered to contribute to the nitrate-deposition, the total 'nitrate' deposition increases up to about 14 kgNha&quot;-&quot;1yr&quot;-&quot;1. It is important to mention that the processes of co-deposition (e.g. reactive deposition of SO_2 with O_3 or/and H_2O_2) at the needles/leaves and at the soil surface considerable contribute to the total deposition of H&quot;+ and sulfate. Even in regions with H_2O_2-production rates of 0.1 ppb h&quot;-&quot;1 (Dlugi, 1993) a minimum flux of sulfate of 0.07 #mu#gm&quot;-&quot;2s&quot;-&quot;1 can be produced by the process of codeposition resulting in an effective reduction of surface resistance of SO_2 by the surface reaction SO_2+H_2O_2. This is equivalent to a sulfate flux of about 10 kgha&quot;-&quot;1yr&quot;-&quot;1. In addition, by the surface reaction of SO_2 and O_3 again the effective surface resistance is lowered and this contributes to sulfate deposition by about 17-20 kgha&quot;-&quot;1yr&quot;-&quot;1. Also fluxes of NH_3 from the forest to the atmosphere are observed which show a contribution of direct emission from trees below-10-15% of the total emission. The remaining contribution (e.g. 90-85%) at this site mainly results from dissociation of ammonium nitrate in the crown region during daytime. The HNO_3-formation inside the canopy from the reaction NO_3+NO_2 is possible as the photodissociation of NO_3 inside the forste with LAI&gt;5 (Leave Area Index) is small compared to the atmosphere even during daytime. (orig.)SIGLEAvailable from TIB Hannover: RR 8418(95-S1)+a / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekBundesministerium fuer Bildung, Wissenschaft, Forschung und Technologie, Bonn (Germany)DEGerman