Showing papers by "Kevin J. Noone published in 2003"

Ultrathin Tropical Tropopause Clouds (UTTCs): I. Cloud morphology and occurrence

Abstract: Subvisible cirrus clouds (SVCs) may contribute to dehydration close to the tropical tropopause. The higher and colder SVCs and the larger their ice crystals, the more likely they represent the last efficient point of contact of the gas phase with the ice phase and, hence, the last dehydrating step, before the air enters the stratosphere. The first simultaneous in situ and remote sensing measurements of SVCs were taken during the APE-THESEO campaign in the western Indian ocean in February/March 1999. The observed clouds, termed Ultrathin Tropical Tropopause Clouds (UTTCs), belong to the geometrically and optically thinnest large-scale clouds in the Earth's atmosphere. Individual UTTCs may exist for many hours as an only 200--300 m thick cloud layer just a few hundred meters below the tropical cold point tropopause, covering up to 105 km2. With temperatures as low as 181 K these clouds are prime representatives for defining the water mixing ratio of air entering the lower stratosphere.

Dehydration potential of ultrathin clouds at the tropical tropopause

Abstract: [1] We report on the first simultaneous in situ and remote measurements of subvisible cirrus in the uppermost tropical troposphere. The observed cirrus, called UTTCs (ultrathin tropical tropopause clouds), are the geometrically (200–300 m) and optically (τ ≈ 10−4) thinnest large-scale clouds ever sampled (≈105 km2). UTTCs consist of only a few ice particles per liter with mean radius ≈5 μm, containing only 1–5 % of the total water. Yet, brief adiabatic cooling events only 1–2 K below mean ambient temperature destabilize UTTCs, leading to large sedimenting particles (r ≈ 25 μm). Due to their extreme altitude above 17 km and low particle number density, UTTCs may efficiently dehydrate air during its last encounter with the ice phase before entering the stratosphere.

Ultrathin Tropical Tropopause Clouds (UTTCs) : II. Stabilization mechanisms

Abstract: Mechanisms by which subvisible cirrus clouds (SVCs) might contribute to dehydration close to the tropical tropopause are not well understood. Recently Ultrathin Tropical Tropopause Clouds (UTTCs) with optical depths around 10 -4 have been detected in the western Indian ocean. These clouds cover thousands of square kilometers as 200-300 m thick distinct and homogeneous layer just below the tropical tropopause. In their condensed phase UTTCs contain only 1-5% of the total water, and essentially no nitric acid. A new cloud stabilization mechanism is required to explain this small fraction of the condensed water content in the clouds and their small vertical thickness. This work suggests a mechanism, which forces the particles into a thin layer, based on upwelling of the air of some mm/s to balance the ice particles, supersaturation with respect to ice above and subsaturation below the UTTC. In situ measurements suggest that these requirements are fulfilled. The basic physical properties of this mechanism are explored by means of a single particle model. Comprehensive 1-D cloud simulations demonstrate this stabilization mechanism to be robust against rapid temperature fluctuations of +/- 0.5 K. However, rapid warming (\Delta T > 2 K) leads to evaporation of the UTTC, while rapid cooling (\Delta T < -2 K) leads to destabilization of the particles with the potential for significant dehydration below the cloud

Scavenging efficiencies of aerosol particles in marine stratocumulus and cumulus clouds

Abstract: Scavenging efficiencies of aerosol particles in marine stratocumulus and cumulus clouds obtained using Counterflow Virtual Impactors mounted on aircraft are presented as a function of pollution level. The greatest accumulation-mode particle number and mass-scavenging fractions were found for clean conditions, and are consistent with (but slightly lower than) previous studies. Vertical gradients in the concentrations of coarse-mode particles observed in shallow and decoupled marine boundary layers reduced the influence of sea-salt particles, in terms of broadening the cloud droplet spectra and as a possible mechanism to oxidize SO2 to non-sea-salt sulphate in the marine stratiform cloud layer. For supersaturations typical for marine stratocumulus clouds, particles smaller than 0.1µm in diameter dominated the formation of cloud droplets for several of the cases analysed here. For larger supersaturations, in stratocumulus clouds inside an area of cumulus penetration, the concentration of residual particles smaller than 0.1µm in diameter was nearly a factor of three higher than the accumulation-mode number. Poor correlation was found between accumulation-mode residual particle number and mass with cloud droplet number for the clean-condition category. This suggests that the particles determining the cloud droplet number populations in clean conditions and for polluted situations with relatively large updraughts were below the accumulation-mode size range. Additionally, it seems that the droplet numbers were insensitive to changes in the accumulation-mode mass for these conditions. © Royal Meteorological Society, 2003. S. R. Osborne's contribution is Crown copyright

Comparisons of airborne CVI and FSSP measurements of cloud droplet number concentrations in marine stratocumulus clouds

Abstract: Comparisons of counterflow virtual impactor (CVI) and forward scattering spectrometer probe (FSSP) measurements of cloud droplet number concentrations obtained by two different aircraft in marine boundary layer stratocumulus clouds are presented. The cut sizes of the CVIs have been calculated using a mathematical model (D L H 50) and using Stokes' theory (D S 50). For most of the cases the agreement between D L H 50 and D S 50 is within 3%. Based on the results obtained with the mathematical model the agreement between CVI and FSSP concentrations is within 25% for most of the cases analyzed in this study and also within 10% for several of the cases. The reasonable agreement between CVI and FSSP concentrations supports the assumption that a droplet releases a single residual particle upon evaporation. Furthermore, in this study it is shown that droplet shattering can qualitatively be identified and it occurs often when there are elevated concentrations of drizzle droplets in the clouds. Poor agreement between the CVI and FSSP was found in a few instances. For one of these cases the discrepancy obtained between the CVI and FSSP seems to have been caused by losses in the CVI mainly by cloud droplets with larger sizes.

UAVs for atmospheric research in the north of Sweden

Abstract: The Eagle UAV, built by IAI and marketed in Europe by EADS, can be considered a mature UAV. It is approximately 17 m in wingspan, weighs 1100 kg and can fl y up to 10 000 m altitude. The max speed is around 200 km/h and it has 24 hour endurance. The aircraft has a large cargo-bay in the middle of the fuselage around CG, which makes it easy to use payloads of different kinds as the different payload weight does not affect the CG. In other UAVs lighter payloads might have to be balanced with extra weights. The available volume is around 1 cubic metre and maximum payload weight around 250 kg. In the future, hardpoints will also be installed on the wings. These will be able to hold 50 kg each, making it possible to hang science pods on the vehicle.