ALBERT

Description: Stratospheric water vapor is important not only for its greenhouse forcing, but also because it plays a significant role in stratospheric chemistry. Several recent studies have focused on the potential for dehydration due to ice cloud formation in air rising slowly through the tropical tropopause layer (TTL). Holton and Gettelman showed that temperature variations associated with horizontal transport of air in the TTL can drive ice cloud formation and dehydration, and Gettelman et al. recently examined the cloud formation and dehydration along kinematic trajectories using simple assumptions about the cloud properties. In this study, a Lagrangian, one-dimensional cloud model has been used to further investigate cloud formation and dehydration as air is transported horizontally and vertically through the TTL. Time-height curtains of temperature are extracted from meteorological analyses. The model tracks the growth, advection, and sedimentation of individual cloud particles. The regional distribution of clouds simulated in the model is comparable to the subvisible cirrus distribution indicated by SAGE II. The simulated cloud properties and cloud frequencies depend strongly on the assumed supersaturation threshold for ice nucleation. The clouds typically do not dehydrate the air along trajectories down to the temperature minimum saturation mixing ratio. Rather the water vapor mixing ratio crossing the tropopause along trajectories is 10-50% larger than the saturation mixing ratio. I will also discuss the impacts of Kelvin waves and gravity waves on cloud properties and dehydration efficiency. These simulations can be used to determine whether observed lower stratospheric water vapor mixing ratios can be explained by dehydration associated with in situ TTL cloud formation alone.

Description: The light absorption coefficient, Beta(a) of the stratospheric aerosol is an important quantity that determines its radiative effects. When combined with the aerosol scattering coefficient, Beta(a) it becomes possible to evaluate the aerosol single scatter albedo, omega = Beta(s)/(Beta(s) + Beta(a)) which is essential for modeling the overall radiative effects of the stratospheric aerosol. Pollack1 determined that omega = 0.98 is a critical value that separates stratospheric cooling from warming.

Description: In-situ Multi-Angle Spectrometer Probe (MASP) particle measurements have been analyzed to determine the typical behaviour of sulphate particles during the SOLVE campaign. The analysis has focussed on variations in the total particle concentration measured by MASP, which is a diagnostic that is primarily sensitive to the growth of small particles (those which are initially smaller than 0.2 micrometers in radius). At all levels of the stratosphere, the total MASP concentration varies continuously with temperature. This behaviour is well-reproduced by assuming that the sulphate aerosols are liquid ternary solutions, but cannot be reproduced if the aerosol is assumed to be frozen. This liquid-like behaviour is apparent for all measurements made during SOLVE, both inside and outside the vortex, and even at the coldest temperatures sampled during the campaign. The only anomalous measurements were made during the flight of January 14th, 2001; however, this midlatitude flight is very unlikely to contain SAT particles based on the recent warm temperatures experienced by the air. At the levels with the coldest measured temperatures, at least 90% of the particles grow as liquids. Therefore, the freezing that occurred during the 1999-2000 Arctic winter was selective, causing most of the particles to remain liquid even in the presence of a small number of frozen particles.

Description: During the dry season component of the Southern African Regional Science Initiative (SAFARI) in late winter 2000, the net solar spectral irradiance was measured at flight levels throughout biomass burning haze layers. From these measurements, the flux divergence, fractional absorption, instantaneous heating rate, and absorption efficiency were derived. Two cases are examined: on 24 August 2000 off the coast of Mozambique in the vicinity of Inhaca Island and on 6 September 2000 in a very thick continental haze layer over Mongu, Zambia. The measured absolute absorption was substantially higher for the case over Mongu where the measured midvisible optical depth exceeded unity. Instantaneous heating from aerosol absorption was 4 K d(sup -1) over Mongu, Zambia and 1.5 K d(sup -1) near Inhaca Island, Mozambique. However, the spectral absorption efficiency was nearly identical for both cases. Although the observations over Inhaca Island preceded the river of smoke from the southern African continent by nearly 2 weeks, the evidence here suggests a continental influence in the lower tropospheric aerosol far from source regions of burning.

Description: Measurements of aerosol light-absorption coefficients are useful for studies of radiative transfer and heating rates. Ogren appears to have published the first light- absorption coefficients in the stratosphere in 1981, followed by Clarke in 1983 and Pueschel in 1992. Because most stratospheric soot appears to be due to aircraft operations, application of an aircraft soot aerosol emission index to projected fuel consumption suggests a threefold increase of soot loading and light absorption by 2025. Together, those four data sets indicate an increase in mid-visible light extinction at a rate of 6 % per year. This trend is similar to the increase per year of sulfuric acid aerosol and of commercial fleet size. The proportionality between stepped-up aircraft operations above the tropopause and increases in stratospheric soot and sulfuric acid aerosol implicate aircraft as a source of stratospheric pollution. Because the strongly light-absorbing soot and the predominantly light-scattering sulfuric acid aerosol increase at similar rates, however, the mid-visible stratospheric aerosol single scatter albedo is expected to remain constant and not approach a critical value of 0.98 at which stratospheric cooling could change to warming.

Description: We have compared in situ measurements near the leading-edges of wave-clouds observed during the SUCCESS experiment with numerical modeling of ice nucleation. Observations of high supersaturations with respect to ice (\geq 150\%) near the leading edge of a very cold wave cloud (T 〈 --60^\circC) are consistent with recent theoretical and laboratory studies suggesting that large supersaturations are required to homogeneously freeze sulfate aerosols. Also, the peak ice crystal number densities observed in this cloud (about 4cm〈sup〉-3/sup〉) are consistent with the number densities calculated in our model. The consistency between model results (using homogeneous freezing and the observations indicates that the number density of effective heterogeneous ice nuclei must be no more than about 2 cm〈sup〉-3〈/sup〉. In the warmer wave-cloud (T (approx.)eq --37^\circC) relatively large ice number densities were observed (20--40 cm〈sup〉-3〈/sup〉. Our model calculations suggest that these large number densities are only possible if liquid droplets were activated at the cloud leading-edge, followed by subsequent homogeneous freezing. If sulfate aerosols had frozen before liquid droplets could be activated, then the peak ice crystal number density should have been less than 10 cm〈sup〉-3〈/sup〉.

Description: Type Ia PSCs are believed to be composed of nitric acid hydrate particles. Recent results from the SOLVE/THESEO 2000 campaign showed evidence that this type of PSC was composed of a small number of very large particles capable of sedimentary denitrification of regions of the stratosphere. It is unknown whether homogeneous or heterogeneous nucleation is responsible for the formation of these PSCs. Arctic winters are tending to be colder in response to global tropospheric warming. The degree to which this influences ozone depletion will depend on the freezing mechanism of nitric acid hydrate particles. If nucleation is homogeneous it implies that the freezing process is an inherent property of the particle, while heterogeneous freezing means that the extent of PSCs will depend in part on the number of nuclei available. The Polar Ozone and Aerosol Measurement (POAM)II and III satellites have been making observations of stratospheric aerosols and Polar Stratospheric Clouds (PSCs) since 1994. Recently, we have developed a technique that can discriminate between Type Ia and Ib PSCs using these observations. A statistical approach is employed to demonstrate the robustness of this approach and results are compared with lidar measurements. The technique is used to analyze observations from POAM II and II during Northern Hemisphere winters where significant PSC formation occurred with the objective of exploring Type I PSC formation mechanisms. The different PSCs identified using this method exhibit different growth curve as expressed as extinction versus temperature.

Description: Recent studies (Charlock, et al.; Kato, et. al) have indicated a potential discrepancy between measured solar irradiance in the cloud-free atmosphere and model derived downwelling solar irradiance. These conclusions were based primarily on broadband integrated solar flux. Extinction (both absorption and scattering) phenomena, however, typically have spectral characteristics that would be present in moderate resolution (e.g., 10 nm) spectra, indicating the need for such measurements to thoroughly investigate the cause of any discrepancies. The 1996 Department of Energy Atmospheric Radiation Measurement Program (ARM) Intensive Observation Period (IOP), held simultaneously with the NASA Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS) Program, provided an opportunity for two simultaneous but independent measurements of moderate resolution solar spectral downwelling irradiance at the surface. The instruments were the NASA Ames Solar Spectral Flux Radiometer and the Analytical Spectral Devices, Inc., FieldSpecT-FR. Spectral and band integrated quantities from both sets of measurements will be presented, along with estimates of the downwelling solar irradiance from band model and line by line calculations, in an effort to determine the compatibility between measured and calculated solar irradiance in the cloud-free atmosphere.

Description: The evolution of the 1999-2000 Arctic winter has been examined using a microphysical/photochemical model run along diabatic trajectories. A large number of trajectories have been generated, filling the vortex throughout the region of polar stratospheric cloud (PSC) formation, and extending from November until the vortex breakup, in order to provide representative sampling of the evolution of PSCs and their effect on stratospheric chemistry. The 1999-2000 winter was particularly cold, allowing extensive PSC formation. Many trajectories have ten-day periods continuously below the Type I PSC threshold; significant periods of Type II PSCs are also indicated. The model has been used to test the extent and severity of denitrification and dehydration predicted using a range of different microphysical schemes. Scenarios in which freezing only occurs below the ice frost point (causing explicit coupling of denitrification and dehydration) have been tested, as well as scenarios with partial freezing at warmer temperatures (in which denitrification can occur independently of dehydration). The sensitivity to parameters such as aerosol freezing rates and heterogeneous freezing have been explored. Several scenarios cause sufficient denitrification to affect chlorine partitioning, and in turn, model-predicted ozone depletion, demonstrating that an improved understanding of the microphysics responsible for denitrification is necessary for understanding ozone loss rates.

Description: There are nearly 800 water Vapor-lines in the 13,200-16,500/cm region that do not have rovibrational assignments in the HITRAN database. The positions and intensities in the database were determined by Mandin et al., but assignments could not be determined at that time. Polyansky, et al. have now assigned over 600 of the unassigned lines in the 11,200-16,500/cm region. Schwenke has also given rovibrational assignments to many of these unassigned lines throughout the visible and near-infrared. Both articles changed the assignments of some HITRAN lines. Carleer et al. extend assignments to some weaker lines measured by them on new spectra with excellent signal/noise. However, some lines measured by Mandin et al. were omitted by Carleer, et al. because of blends due to lower spectral resolution. The rovibrational assignments of Polyansky et al. completely agree with those in Schwenke's article for only about 200 lines. However, Schwenke's ab initio line list is available on his internet site (http://ccf.arc.nasa.gov/-dschwenke). A detailed comparison of the Polyansky et al.line list, the Carleer et al.line list, and Schwenke's ab initio line list shows a larger number of agreements. In many cases the disagreement is only about the vibrational and/or rotational upper level, while there is agreement on the lower state assignment and energy level, "E", which is of primary importance for atmospheric applications. We will present a line list of "consensus" assignments in the 13,200-16,500/cm region for consideration of inclusion on the HITRAN and GEISA databases. This will substantially reduce the number of unassigned lines on the databases in this spectral region.