A unified, long‐term, high‐latitude stratospheric aerosol and cloud database using SAM II, SAGE II, and POAM II/III data: Algorithm description, database definition, and climatology

Abstract

A 22 year, high‐latitude, stratospheric aerosol and cloud database has been formed in a “unified” manner by combining the Stratospheric Aerosol Measurement (SAM) II, Stratospheric Aerosol and Gas Measurement (SAGE) II, Polar Ozone and Aerosol Measurement (POAM) II, and POAM III 1 μm aerosol extinction profiles. The database is “unified” in that it embodies similar aerosol extinction measurements, uses a single meteorological data set, and employs a single algorithm for calculating background extinction and cloud detection thresholds. Latitude is constrained to poleward of 45° in each hemisphere. The Unified cloud detection algorithm and database are designed for the straightforward addition of new data when other compatible data sets (e.g., SAGE III) become available. “Unified” cloud detection is similar to, but a refinement of, earlier attempts to identify polar stratospheric clouds (PSCs) with SAM II and POAM II data. The Unified algorithm is instrument‐independent and circumvents fundamental cloud detection pitfalls. The database contains over 73,000 (36,000) polar vortex‐region profiles in the Antarctic (Arctic) and over 21,000 (2000) PSC observations. An introductory climatology of Unified “background” extinction is presented. It is seen that volcanic effects dominate the evolution of outside‐vortex background extinctions, but perturbations apparently not related to volcanoes are seen as well. Interannual variations of background extinction inside the austral vortex are seen to be nearly decoupled from volcanic effects, while in the Arctic, inside‐vortex extinctions show a considerable volcanic influence. An analysis of long‐term PSC sighting is presented. Midwinter (July and January) PSC and clear‐sky measurements at 20 km, in a fixed temperature range, are used for computing PSC probability. The grand average PSC probability calculated this way is nearly identical between hemispheres. In the Antarctic the interannual PSC probability pattern is distinctly cyclic but is convoluted by volcanic perturbations in background aerosol. In the Arctic the PSC probability has much less temporal coherence than in the Antarctic but is similarly impacted by volcanic background increases. An explanation for the variation in PSC probabilities, in terms of interannual differences in denitrification, is discussed. Finally, a statistical analysis of tropopause height in relation to PSC formation is also presented. PSC observations are seen to be strongly associated with elevated tropopause heights, indicating that tropospheric, synoptic‐scale flow perturbations are the primary forcing mechanism for Arctic PSC formation, as evidenced in this long‐term satellite record.

CALIPSO polar stratospheric cloud observations: second-generation detection algorithm and composition discrimination

“…we set the number of observations in bins to the right of the mode to be equal to the measured number in the analogous bins to the left of the mode. This is similar to the approach used by Fromm et al (2003) to define a baseline aerosol-only extinction PDF for identifying PSCs in solar occultation data. The "residual" PDF is formed by subtracting the STS PDF from the measured PDF, and we can then calculate a confidence function after Liu et al (2004) that quantifies the differential occurrence probability of the STS and residual STS/solid mixture classes.…”

“…The lidar on CALIPSO, CALIOP (Cloud-Aerosol LIdar with Orthogonal Polarization), is an excellent instrument for observing PSCs (Pitts et al, 2007) and has already produced an extensive set of PSC observations covering three Antarctic and two Arctic "seasons." These observations provide unprecedented spatial and temporal resolution that greatly reduces sampling bias characteristic of occultation instruments that were the basis of past space-based studies of PSC characteristics (Poole and Pitts, 1994;Fromm et al, 2003). The dramatic scale of these differences was demonstrated by Pitts et al (2007).…”

“…More recent studies (e.g., Gobbi et al, 1998;Toon et al, 2000;Biele et al, 2001;Hu et al, 2002;Adriani et al, 2004;Massoli et al, 2006) indicate that such distinct "typing" of PSCs is an oversimplification, with the clouds often occurring as mixtures of liquid and solid particles. Hence, we explore the CALIOP measurement space using a theoretical optical model for simple notional mixtures of STS and NAT or ice particles, and then use these calculations in conjunction with statistical analyses of the CALIOP aerosol depolarization data to define new CALIPSO-based PSC composition classes.…”

Characterization of Polar Stratospheric Clouds with spaceborne lidar: CALIPSO and the 2006 Antarctic season

“…In particular, lidar observations have established that there are three primary forms of PSC particles: supercooled liquid ternary (H 2 SO 4 -HNO 3 -H 2 O) solution droplets (STS) that evolve from background aerosol without a phase change; solid nitric acid hydrate crystals, most likely nitric acid trihydrate (NAT); and H 2 O ice (Poole and McCormick, 1988;Browell et al, 1990;Toon et al, 1990). Multi-year ground-based lidar observations have been utilized to study the seasonal chronology of PSC composition (David et al, 1998;Gobbi et al, 1998;Santacesaria et al 2001;Adriani et al, 2004). More recently, satellite limb emission measurements of the infrared spectra of PSCs by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instrument on ENVISAT have provided valuable new insight to PSC composition (e.g.…”

A 3D-CTM with detailed online PSC-microphysics: analysis of the Antarctic winter 2003 by comparison with satellite observations

“…We estimated these parameters on the condition that the initial lognormal distribution should reproduce the Unified POAM reference background 1 µm extinction monthly means (Fromm et al, 2003). This condition leaves a lot of arbitrariness in fixing the lognormal parameters.…”