An overview of multi-instrumental in-situ cirrus clouds observations from the M55 during recent tropical campaigns. Cairo, F.(1); Di Donfrancesco, G.(2); Snels, M. (1); Fierli, F.(1); Viterbini, M.(1); Cardillo, F.(1); Borrmann, S.(3); de Reus, M.(3) ;Voessing, H.(3); C. Schiller (4), M. Kramer (4) : V. Yushkov (5) (1) CNR-Istituto di Scienze dell’Atmosfera e del Clima, Italy; (2) ENEA-Clim, Italy; (3) Institute for Atmospheric Physics, Johannes-Gutenberg-University, Germany; (4) FZJ, Julich, Germany; (5) CAO, Moscow, Russia. / Fax: / Phone: The Instruments: Backscattersonde: The Multiwavelenght Aerosol Backscattersonde (MAS) performs backscattering and depolarization measurements at 532 nm, and at 1064 nm, in close proximity of the mounting platform, ranging from 3 to 10 metres from it. Sampling volume is approx m3. Time resolution is 10s, corresponding to 2 Km horizontal resolution at the average aircraft speed. FSSP100: The FSSP optically detects and sizes particles in a given sampling volume of approx m3. Can be operated to detect in different size ranges. The data used in this study were taken when operating in the µm particle diameter size range. The particle whose dimensions lies within these two cutoffs are grouped into seven size bins, thus creating a binned size distribution nb(r). From that, particle number, surface and volume density can be retrieved, as well as mean particle effective radius. The method: No attempt has been made to reconstruct the size distribution n(r) in terms of analytic functions (lognormals or gamma function), from the binned size distribution nb(r) Instead the volume backscattering coefficient B, defined in terms of the size distribution n(r) and scattering efficiency Q(r) as in (1), was approximated as in (2). The size distributions was stepwise constant over each of the seven FSSP size bins, and the scattering efficiency was computed on the basis of Mie theory. Summary An Optical Particle Counter FSSP-100 able to count and size particles in the micron range, and a backscattersonde to measure in-situ optical properties as color, backscatter and depolarization ratio, are part of the payload of the high altitude research aircraft M55 Geophysica. This aircraft was deployed in tropical field campaigns in Bauru, Brasil (TROCCINOX, 2004) and Darwin, Australia (SCOUT-O3, 2005) There, measurements of cirrus cloud particle size distributions and optical properties were performed. Scope of the work is to assess and discuss the consistency between the particle volume backscatter coefficient B observed by the backscattersonde and the same parameter retrieved by MIE optical scattering theory computations applied to particle size distributions as measured by the FSSP-100. Time serie of backscattering coefficient, as observed by MAS and as retrieved from FSSP-100 by means of (2). Uncertainties on the retrieval come from uncertainties in the FSSP100 counts per bin. A general overestimation of B RETRIEVED is observed. Example of a particle size distribution as measured by FSSP-100. Particles have been counted and binned every 10s time window. Uncertainties on the counts can be estimeated as square root of the counts. ON THE LEFT: Scatterplot of OBSERVED vs RETRIEVED B coefficients. A good 1-1 correlation is observed over 5 orders of magnitude, spanning from subvisible to optically dense clouds., The scattering of the points extends over almost one order of magnitude. Discussion: Observations occurred in cirrus clouds, whose particles are aspherical. The asphericity of the scatterers lead to a decrease in the backward scattering, compared to the spherical particle case. The use of MIE theory thus lead to an overestimation of B RETRIEVED, that may range from 10% to 50% according to particle sizes and shapes. Approximations in (2) may induce significant biases if particles outside the FSSP detection windows are actually present in the real atmospheric size distributions. Sensitivity studies showed that simulating the presence of particles with radii below the FSSP100 low cutoff, in order to double the total particle surface, or total particle volume, leads to respectively a two-fold and a tenfold increase of B. Similarly, an addition of particles with radii above the FSSP100 high cutoff leads to respectively a five fold and a twofold increase of B. Thus, the comparison method is very sensitive to presences of high particle volume densities below the FSSP100 low threshold, and particle surface densities above the FSSP100 high threshold. Finally it should be noted that the scattering between observed and retrieved parameters could be due to different sampling volumes, and imperfect timing between the two instruments, may finally lead to uncorrect comparisons. This work was supported by the SCOUT-O3 Integrated Project of the European Commission under the contract GOCE-CT-2004, and the TROCCINOX project of the European Comission under the contract EVK2-CT LOWER PANES: Rightmost panel: particle number density vs the observed Backscattering, middle, same for the total surface; Rightmost, same for the total volume; In the particle range encountered in cirrus clouds, the strongest dependency of B is from the particle number density, and to al lesser extent, to the total surface detected by the FSSP. Conclusions: Computations show that within the limits of our methods, observed FSSP100 size distributions between 2.7 and 31 um diameter size range well reproduce the backscattering observed on tropical cirrus clouds, i.e. the FSSP100 is effective in catching the most optically active particles of the size distributions. Particles above that range, although highly affectiong the cloud IWC, have a negligible effect on the cloud optical properties. Sensitivity to undetected particles: 1) Undetected particles whose diameters are below the FSSP lower detection limit would have a large effect on the overall optical properties. In terms of volumes, an equal ratio of detected to non detected volumes would be mirrored in tenfold variations in the scattering coefficients. 2) Undetected particles whose diameters are above the FSSP higher detection limit would have a smaller effect on the overall optical properties: an equal ratio of detected to non detected volumes would be mirrored in a twofold variation of the scattering coefficients. 3) In our sensitivity study, we added particles in the um diameter range, the closest to the upper FSSP detection limit. Shifting their diameter to bigger values would only decrease their optical effectiveness. Thus our choice represent an upper limit to the assessment of the effect of undetected particles. LEFT PANELS: Leftmost: Increase of backscatter coefficient induced by doubling of particle total surface area, due toparticles below (purple line) and above (blue line) the FSSP lower and upper detection limits, respectively. Rightmost: Increase of backscatter coefficient induced by doubling of particle total volume, due to particles below (purple line) and above (blue line) the FSSP lower and upper detection limits, respectively. 30 Nov 2005 Case study: LOWER PANEL: scatterplot between the backscatter coefficient observed by MAS, and the one reconstructed from the FSSP size distributions. UPPER PANEL: Scatterplot of IWC as derived by FSSP volumes, and from FISH and FLASH measurements. FSSP Undetcted particles seem to cause a tenfold increase in IWC with respet to what detected by FSSP. Still the backscattering coefficient comparison is quite satisfatory.