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Abstract. Scattering codes are used to study the optical properties of polar stratospheric clouds (PSCs). Particle backscattering and depolarization coefficients can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of supercooled ternary solution (STS) droplets, solid nitric acid trihydrate (NAT) and possibly ice particles, making the assumption of a single refractive index and a single morphology to model the scatterers questionable.Here we consider a set of 15 coincident measurements of PSCs above McMurdo Station, Antarctica, using ground-based lidar, a balloon-borne optical particle counter (OPC) and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows us to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present. We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if it is above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical. Scattering from NAT particles, considered spheroids of different aspect ratio (AR), is treated with T-matrix results where applicable. The geometric-optics–integral-equation approach is used whenever the particle size parameter is too large to allow for a convergence of the T-matrix method.The parameters Rth and AR of our model have been varied between 0.1 and 2 µm and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 µm and for AR less than 0.55 and greater than 1.5.To further constrain the variability of AR within the identified intervals, we have sought an agreement with the experimental data by varying AR on a case-by-case basis and further optimizing the agreement by a proper choice of AR smaller than 0.55 and greater than 1.5 and Rth within the interval 0.5 and 0.8 µm. The ARs identified in this way cluster around the values 0.5 and 2.5.The comparison of the calculations with the measurements is presented and discussed. The results of this work help to set limits to the variability of the dimensions and asphericity of PSC solid particles, within the limits of applicability of our model based on the T-matrix theory of scattering and on assumptions on a common particle shape in a PSD and a common threshold radius for all the PSDs.more » « less
Abstract. In situ measurements in the climatically important upper troposphere–lower stratosphere (UTLS) are critical for understanding controls on cloud formation, the entry of water into the stratosphere, and hydration–dehydration of the tropical tropopause layer.Accurate in situ measurement of water vapor in the UTLS however is difficult because of low water vapor concentrations (<5 ppmv) and a challenging low temperature–pressure environment.The StratoClim campaign out of Kathmandu, Nepal, in July and August 2017, which made the first high-altitude aircraft measurements in the Asian Summer Monsoon (ASM), also provided an opportunity to intercompare three in situ hygrometers mounted on the M-55 Geophysica: ChiWIS (Chicago Water Isotope Spectrometer), FISH (Fast In situ Stratospheric Hygrometer), and FLASH (Fluorescent Lyman-α Stratospheric Hygrometer).Instrument agreement was very good, suggesting no intrinsic technique-dependent biases: ChiWIS measures by mid-infrared laser absorption spectroscopy and FISH and FLASH by Lyman-α induced fluorescence.In clear-sky UTLS conditions (H2O<10 ppmv), mean and standard deviations of differences in paired observations between ChiWIS and FLASH were only (-1.4±5.9) % and those between FISH and FLASH only (-1.5±8.0) %.Agreement between ChiWIS and FLASH for in-cloud conditions is even tighter, at (+0.7±7.6) %.Estimated realized instrumental precision in UTLS conditions was 0.05, 0.2, and 0.1 ppmv for ChiWIS, FLASH, and FISH, respectively.This level of accuracy and precision allows the confident detection of fine-scale spatial structures in UTLS water vapor required for understanding the role of convection and the ASM in the stratospheric water vapor budget.more » « less
Abstract. The Asian monsoon anticyclone (AMA) represents one of thewettest regions in the lower stratosphere (LS) and is a key contributor tothe global annual maximum in LS water vapour. While the AMA wet pool islinked with persistent convection in the region and horizontal confinementof the anticyclone, there remain ambiguities regarding the role oftropopause-overshooting convection in maintaining the regional LS watervapour maximum. This study tackles this issue using a unique set ofobservations from aboard the high-altitude M55-Geophysica aircraft deployedin Nepal in summer 2017 within the EU StratoClim project. We use acombination of airborne measurements (water vapour, ice water, waterisotopes, cloud backscatter) together with ensemble trajectory modellingcoupled with satellite observations to characterize the processescontrolling water vapour and clouds in the confined lower stratosphere (CLS)of the AMA. Our analysis puts in evidence the dual role of overshootingconvection, which may lead to hydration or dehydration depending on thesynoptic-scale tropopause temperatures in the AMA. We show that all of theobserved CLS water vapour enhancements are traceable to convective eventswithin the AMA and furthermore bear an isotopic signature of the overshootingprocess. A surprising result is that the plumes of moist air with mixingratios nearly twice the background level can persist for weeks whilstrecirculating within the anticyclone, without being subject to irreversibledehydration through ice settling. Our findings highlight the importance ofconvection and recirculation within the AMA for the transport of water into thestratosphere.more » « less
Abstract. A comparison of polar stratospheric cloud (PSC) occurrence from 2006 to2010 is presented, as observed from the ground-based lidar station at McMurdo(Antarctica) and by the satellite-borne CALIOP lidar (Cloud-Aerosol Lidarwith Orthogonal Polarization) measuring over McMurdo. McMurdo (Antarctica) isone of the primary lidar stations for aerosol measurements of the NDACC (Network forDetection of Atmospheric Climate Change). The ground-based observations havebeen classified with an algorithm derived from the recent v2 detection andclassification scheme, used to classify PSCs observed by CALIOP.
A statistical approach has been used to compare ground-based and satellite-based observations, since point-to-point comparison is often troublesome dueto the intrinsic differences in the observation geometries and the imperfectoverlap of the observed areas.
A comparison of space-borne lidar observations and a selection of simulationsobtained from chemistry–climate models (CCMs) has been made by using a series ofquantitative diagnostics based on the statistical occurrence of different PSCtypes. The distribution of PSCs over Antarctica, calculated by severalCCMVal-2 and CCMI chemistry–climate models has been compared with the PSCcoverage observed by the satellite-borne CALIOP lidar. The use of severaldiagnostic tools, including the temperature dependence of the PSCoccurrences, evidences the merits and flaws of the different models. Thediagnostic methods have been defined to overcome (at least partially) thepossible differences due to the resolution of the models and to identifydifferences due to microphysics (e.g., the dependence of PSC occurrence onT−TNAT).
A significant temperature bias of most models has been observed, as well as alimited ability to reproduce the longitudinal variations in PSC occurrencesobserved by CALIOP. In particular, a strong temperature bias has been observedin CCMVal-2 models with a strong impact on PSC formation. The WACCM-CCMI(Whole Atmosphere Community Climate Model – Chemistry-Climate ModelInitiative) model compares rather well with the CALIOP observations, althougha temperature bias is still present.
Macroscopic stratospheric aerosol properties such as surface area density (SAD) and volume density (VD) are required by modern chemistry climate models. These quantities are in continuous need of validation by observations. Direct observation of these parameters is not possible, but they can be derived from optical particle counters (OPCs) which provide concentration (number density) and size distributions of aerosol particles, and possibly from ground‐based and satellite‐borne lidar observations of particle backscatter coefficients and aerosol type. When such measurements are obtained simultaneously by OPCs and lidars, they can be used to calculate backscatter and extinction coefficients, as well as SAD and VD. Empirical relations can thus be derived between particle backscatter coefficient, extinction coefficient, and SAD and VD for a variety of aerosols (desert dust, maritime aerosols, stratospheric aerosols) and be used to approximate SAD and VD from lidar measurements. Here we apply this scheme to coincident measurements of polar stratospheric clouds above McMurdo Station, Antarctica, by ground‐based lidar and balloon‐borne OPCs. The relationships derived from these measurements will provide a means to obtain values of SAD and VD for supercooled ternary solutions (STS) and nitric acid trihydrate (NAT) PSCs from the backscatter coefficients measured by lidar. Coincident lidar and OPC measurements provided 15 profile comparisons. Empirical expressions of SAD and VD as a function of particle backscatter coefficient,
β, were calculated from fits of the form log(SAD/VD) = A+ Blog( β) using βfrom the lidar and SAD/VD from the OPC. The PSCs were classified as STS and NAT mixtures, ice being absent.