A21I-01 INVITED
Ice Nucleation in Cold Cirrus Near the Tropical Tropopause Layer
Recent in situ measurements of thin cirrus near the tropical tropopause from the CRAVE and TC4 field experiments indicate ice number concentrations less than 100/L that are far lower than homogeneous- freezing nucleation theory predicts. In addition, the measured size distributions are much broader than theoretical models predict. Consistent with the apparent overestimate of ice concentrations by numerical models, we find that simulated extinctions are substantially larger than those indicated by CALIPSO. All of this evidence points to a fundamental problem with our theoretical understanding of ice cloud formation processes at low temperature with important implications for cloud radiative properties and dehydration of air entering the stratosphere. Various hypotheses for why ice concentrations are so low (including heterogeneous nuclei and glassy states of organic-containing aerosols) will be presented.
A21I-02
Convective and Wave Signatures in Ozone Profiles in Equatorial Americas: Views from TC4 and SHADOZ
Trace gas observations from NATIVE (Nittany Atmospheric Trailer and Integrated Validation Experiment)
operations in coastal Panama (Las Tablas, 8N, 80W) during the TC4 campaign, 17 July to 8 August 2007,
along with daily ozonesondes and NASA DC-8 measurements in the equatorial Americas, are used to assess
convective influence in the tropical UT/LS during this period. A climatological perspective is gained through
laminar analyses of the Panama soundings using the method of wave identification reported by Grant et al.
[1998] and Thompson et al. [2007]. Wave activity, most prominent in the TTL and LS, occurred in 40-60%
of the soundings over Las Tablas and was associated with vertical displacements, classified as a mixture of
Gravity and Kelvin Waves ("G/KW"). Wave activity also occurred just above the mixed layer in ~25%
of the Las Tablas soundings. Concurrent soundings at San Jose, Costa Rica (10N, 84W, a SHADOZ site),
also indicate G/KW. A larger view is provided by laminar analysis of San Cristobal (1S, 90W)and Paramaribo
(5.8N, 55W) soundings since 1999. Using ancillary TC4 observations from aircraft and satellite, day-to-day
variability in UT and LS O3 over Panama can be attributed to convection, possibly lightning, pollution, or
stratospheric influence.
http://ozone.met.psu.edu
A21I-03
Equatorial Waves from Three Years of HIRDLS Satellite Observations: Annual and Interannual Variations
Equatorial waves forced by tropical convection affect cirrus cloud formation in the upper troposphere and drive global circulations in the middle atmosphere. We will present an analysis of equatorial waves in temperature measurements of the High Resolution Dynamics Limb Sounder (HIRDLS) instrument on the Aura satellite from 2005-2007. A rich variety of wave modes are revealed in the space-time Fourier spectral analysis of these data, including Kelvin waves, mixed-Rossby-gravity waves, equatorial Rossby waves, and equatorial inertia-gravity waves. Vertical wavelengths as short as 4 km are well resolved. Kelvin waves with horizontal wavenumber 1 through 5 dominate the spectrum. Their amplitudes between 20-35-km altitude show clear interannual variation that is locked to the descent of the easterly phase of the quasibiennial oscillation in tropical zonal winds. Below 20 km, Kelvin wave amplitudes show an annual cycle that is related to the annual cycle in upper troposphere winds. From the results of the analysis we also compute wave momentum fluxes, and estimate the strength of the wave-mean flow interaction in the tropical stratosphere.
A21I-04
Inorganic Iodine and Bromine in the Tropical Upper Troposphere/Lower Stratosphere Derived From Balloon Borne Observations
Due to the ozone destroying capabilities of bromine and iodine bearing compounds, the stratospheric budget of inorganic bromine and iodine is of major interest for modeling ozone depletion and assessing the future evolution of the ozone layer. In particular the contribution of very short lived substances (VSLS) to the bromine budget has recently been shown to enhance ozone depletion in mid-latitudes and polar regions. So far, iodine species have not been unambiguously detected in the stratosphere with upper limits for total inorganic iodine (Iy) of about 0.1 ppt. However, observations are sparse and mainly restricted to mid- and high-latitudes. Here, we assess the budget of iodine and bromine in the tropical Upper Troposphere/ Lower Stratosphere (UT/LS) where the halogen source gases enter the stratosphere and supply the stratosphere with halogen species. We report on two stratospheric balloon flights of the LPMA/DOAS (Limb Profile Monitor of the Atmosphere/Differential Optical Absorption Spectrometer) payload from a tropical station in northern Brazil (5°S, 43°W) in June 2005 and June 2008. There, the LPMA/DOAS payload conducted spectroscopic direct sun measurements in the UV/visible and infrared spectral range during balloon ascent and in solar occultation geometry. The LPMA/DOAS observations allow for the retrieval of IO and OIO from their absorption features in the visible spectral range. Neither species could be detected unambiguously with detection limits ranging between 0.01 and 0.2 ppt in the UT/LS. Constraining a stratospheric chemistry model by the inferred detection limits for IO and OIO, yields an upper limit for Iy of 0.1 to 0.3 ppt. Implications for stratospheric ozone are discussed on the basis of model studies. BrO is inferred from absorption bands in the UV spectral range yielding the first BrO vertical profile in the tropical UT/LS. For the balloon flight in June 2005, total inorganic bromine (Bry) is estimated to (21.5 ± 2.5) ppt in 4.5-year-old air using a stratospheric model constrained by measured BrO. We derive a total contribution of (5.2 ± 2.5) ppt from brominated VSLS and inorganic product gases to Bry. Tropospheric BrO was found to be < 1 ppt. Our results are compared to 3-D CTM SLIMCAT model runs.
A21I-05
The seasonality of the composition of air in the tropical tropopause layer
Multi-annual simulations with the Chemical Model of the Stratosphere (CLaMS) are used to study transport of air and the seasonality of its composition within the tropical tropopause layer (TTL). In agreement with satellite and in-situ observations, CLaMS simulations show a pronounced seasonal cycle in CO and ozone and, in addition, in the mean age. Below the zero clear sky heating rate level (Q=0) around 360 K potential temperature, the semi-annual cycle of convection, with strongest upwelling around April and November, determines the composition of the TTL. Although above this level, the contribution of photochemistry modulated by the annual cycle of the Brewer-Dobson circulation increases with altitude, the seasonality of ozone and CO is overlaid by a clear annual and a weak semi-annual cycle of horizontal in-mixing from the stratosphere into the TTL. The strongest in-mixing occurs from the northern hemisphere during the boreal summer. Both, CLaMS simulations and pure trajectory calculations show that this equatorward transport is mainly driven by the Asian monsoon anticyclone.
A21I-06
Mean Ages and Age Spectra for the Tropical Tropopause Layer From Observations of CO2: Implications for Air Transport and Distributions of Short-Lived Chemical Species
We present extensive CO2 observations of the TTL as related to the characteristics of TTL structure and transport. The results from the CR-AVE and TC4 missions show that air entering the upper TTL above ~360K retains imprint of the global seasonal cycle derived from the well-measured CO2 at the surface, suggesting that data for CO2 can be used as a principal tool to infer the mean age and age spectrum for the TTL, as in the lower stratosphere (Andrews et al., 2001a, 2001b; Boering et al., 1996; Park et al., 2007). The transport rates in the upper TTL inferred from CO2 are applied to generate vertical distributions in the TTL of short-lived organic halogens, non-methane hydrocarbons and alkyl nitrates, which have potentially major impacts on stratospheric ozone and humidity. New data from the Whole Air Sampler obtained over Central America in August, 2007 demonstrate that direct inputs of these transient tracers are largely limited to altitudes ~2 km below the tropical tropopause, and reactive species are efficiently removed with local lifetimes less than or comparable to the mean age of air in the upper TTL (i.e., ranging from a few days to several months). Interestingly, the observations in the subtropical lowermost stratosphere reveal higher concentrations of these species than at the same isentropic level of the TTL, indicating direct loading by summertime midlatitude convection as one of input pathways of these short-lived species into the stratosphere.
A21I-07
Sub-micron Aerosol Size Distributions in the Upper Troposphere and Lower Stratosphere
Dry aerosol size distributions in the diameter range from 4 to 1000 nm have been measured to altitudes of approximately 18.3 km in the non-volcanic lower stratosphere and upper troposphere. Aerosol size distributions reflect aerosol processes such as new particle formation and sedimentation and respond to atmospheric transport and mixing. We present distributions from various regions of the atmosphere and discuss the processes that impact them. We show that distributions vary with region in the stratosphere and with season in the tropics. The relationship between the size distributions and trace gas concentrations reflects the impact of aging (aerosol coagulation), transport and mixing. These evolving size distributions reflect processes key to understanding the chemistry and physics of the upper troposphere and lower stratosphere. Stratospheric aerosol particles provide surface for heterogeneous chemistry important in determining the abundance of HOx, NOx and Clx in the stratosphere. These observations contribute to our understanding of stratospheric aerosol abundance.
A21I-08
Convective Troposphere-Stratosphere Transport in the Tropics and Hydration by ice Crystals Geysers
Twenty-five years ago the suggestion was made by Danielsen of direct fast convective penetration of tropospheric air in the stratosphere over land convective systems. Although the existence of the mechanism is accepted, it was thought to be rare and thus its contribution to Troposphere-Stratosphere Transport (TST) of chemical species and water vapour at global scale unimportant at global scale. In contrast to this assumption, observations of temperature, water vapour, ice particles, long-lived tropospheric species during HIBISCUS, TROCCINOX and SCOUT-O3 over Brazil, Australia and Africa and more recently CALIPSO aerosols observations suggest that it is a general feature of tropical land convective regions in the summer. Particularly relevant to stratospheric water vapour is the observation of geyser like ice crystals in the TTL over overshooting events which may result in the moistening of the stratosphere. Although such events successfully captured by small scale Cloud-Resolving Models may have a significant impact on stratospheric ozone chemistry and climate, they are currently totally ignored by NWPs, CTMs and CCMs. Several recent balloon and aircraft observations of overshoots and CRM simulations will be shown illustrating the mechanism, as well as observations from a variety of satellites suggesting a significant impact at global scale.