A11D-01 08:00h
Fluctuations of Cloud, Humidity, and Thermal Structure Neare the Tropical Tropopause
Thermal and humidity structure near the tropical tropopause are studied in microwave satellite retrievals of water vapor from MLS, along with contemporaneous dynamical structure in ECMWF analyses and cold cloud in high resolution Global Cloud Imagery. These fields all vary coherently with the outflow from convective centers -- in the upper troposphere as well as in the lowermost stratosphere. The outbreak of deep convection is accompanied by diabatic heating below a level between 250 and 150 mb, but by diabatic cooling at higher levels. The reversal from heating to cooling is broadly consistent with cumulus detrainment. Through irreversible mixing, that process serves as a heat source for the environment below the Level of Neutral Buoyancy (LNB), but as a heat sink at higher levels. Calculations, inclusive of entrainment, place the LNB very near the observed reversal from heating to cooling. The outbreak of convection is also accompanied humidification below 125 mb, but by dehydration at higher levels. The reversal from humidification to dehydration coincides with levels where environmental conditions approach saturation. Those conditions suggest the efficient removal of total water from cumulus updrafts, leaving dessicated air to ventilate higher levels. Cumulus detrainment then acts to humidify the environment beneath the zone of nearly-saturated environmental conditions, but to dehydrate it at higher levels. Dry air emerges from the region of coldest cloud. It then extends into the winter hemisphere, along streamlines that characterize the Hadley circulation. Coinciding with diabatic cooling are stratospheric convergence and downwelling. These features of stratospheric motion amplify simultaneously with divergence at tropospheric levels, which represents the major outflow from deep convection. The deepest convection, found over the equatorial Pacific, coincides with the highest moist static energy. The latter yields an LNB that is some 3 km higher over the equatorial Pacific than elsewhere, in agreement with the observed reversal from heating to cooling. Observed brightness temperatures place the level at which cumulus anvils are most extensive very near the cold point over the equatorial Pacific. This, in turn, lies near the tropical tropopause throughout the tropics. Collectively, these features suggest that the coldest cloud, found over the equatorial Pacific, plays a key role in maintaining temperature and humidity near the tropical tropopause.
A11D-02 08:15h
Insights into Stratospheric Dehydration Using Isotopes of Water
An analytic model of transport and microphysics in the tropical tropopause layer (TTL) is extended to include stable isotopes of water. The model, running along trajectories, is tested against in-situ and satellite observations of HDO and H$_2$$^{18}$O. The model is able to reproduce the range of isotopic depletions observed in the data, and reproduce individual episodes that mirror or depart from Rayleigh fractionation processes. The results indicate that water substance in the upper troposphere does not follow a Rayleigh distillation model due to the presence of detrained ice in the TTL. The results are also used to illustrate that stratospheric abundances of stable isotopes of water can be understood based on known isotopic physics, convective detrainment of ice and gradual dehydration.
A11D-03 08:30h
Thermal structure of the TTL and its relation to stratospheric-tropospheric exchange of water.
The annual cycle of the TTL fine scale thermal structure is described as captured by GPS radio occultation and the pressure levels of the ECMWF weather analysis. This annual cycle is compared to the annual cycle in water concentrations at the upper troposphere/lower stratosphere measured by HALOE. It is found that the saturation mixing ratios at the Cold Point Tropopause temperatures are consistent and sligthly below HALOE values with some temporal lag. This suggests that if dehydration mechanisms other than those associated with slow vertical asscent are working effectively, they must be counterbalanced by other hydration mechanisms. A comparison between saturation mixing ratios at the temperatures captured by GPS radio occultation and HALOE concentrations of water vapor show an annual cycle dominated by supersaturation in the boreal winter months, when the upward mass fluxes are larger, and subsaturation in the summer. The longitudinal dependence of these cycles is discussed and so is its possible implication for the seasonality of statospheric-tropospheric exchange of water.
A11D-04 INVITED 08:45h
Control of Stratospheric Water Vapor
We use trajectory calculations based on European Center for Medium-range Weather Forecast reanalysis data (ERA-40) to analyse tropical troposphere to stratosphere transport (TST) for the period 1979-2001. Based on the minimum saturation mixing ratio of tropical TST we calculate estimates for the humidity of air entering the stratosphere ([H2O]e). We show that this estimate, which considers the planetary scale dynamics of tropical TST and the synoptic-scale temperature field, but greatly simplifies cloud microphysics and largely neglects mesoscale temperature perturbations, yields an excellent first order estimate for the annual mean, seasonal cycle and even interannual variability of [H2O]e. Having established a tight relationship between [H2O]e and the Lagrangian mean cold point temperature in the tropics, we argue that it is the synoptic scale temperature field in combination with planetary-scale dynamics at the tropical tropopause that controls [H2O]e, and mesoscale processes such as deep, overshooting convection may play only a secondary role. Finally, we discuss the implications of this hypothesis for the cause of the observed decadal-scale positive trend of stratospheric moisture, and the apparently paradoxical negative trend of tropopause temperatures in the tropics.
A11D-05 INVITED 09:00h
Anomalously Low Water Vapor, Temperature and Ozone Near the Tropical Tropopause Since 2001
Observations of stratospheric water vapor from HALOE (launched in 1991) show anomalously low values (by ~0.5 ppmv) beginning in early 2001, and continuing to present. The anomalously dry air originates near the tropical tropopause, and propagates globally; persistent low values since 2001 are also observed from balloon measurements at Boulder, Colorado (40 N), and in the Arctic stratosphere by the POAM satellite. The anomalous tropical water vapor values are consistent with cold temperature anomalies that have occurred near the tropical tropopause since 2001. Analysis of ozone in the tropical lower stratosphere, from SHADOZ ozonesondes and SAGE II satellite data, also show persistent low values since 2001. Together, these observations suggest a possible intensification of the upward Brewer-Dobson circulation in the tropical lower stratosphere, beginning in 2001.
A11D-06 INVITED 09:15h
Assessing the Accuracy of Water Vapor Measurements in the UT/LS:Problems, Paradigms, and Prescriptions
Because accurate water vapor measurements are critically important for assessing the atmosphere's response to global climate change, the assessment of water vapor instrument accuracy as presented in the widely disseminated 2000 SPARC report requires careful attention. Rather than judging any instrument on its own merit, the report uses in-flight instrument intercomparison as the only tool with which to evaluate instrument performance. Figure 1 of the report that provides a summary of intercomparison results in the troposphere and stratosphere, plots the % difference of water vapor measurements relative to the satellite-borne Halogen Occultation Experiment (HALOE). While the report does not explicitly draw conclusions from the figure about instrument accuracy, the figure invites inferences regarding the accuracy of those instruments. As with other publications that similarly have utilized intercomparisons alone to judge instrument accuracy, the effect of the SPARC evaluation is to ignore other criteria that can be, should be, or have already been used to validate some of those same instruments. The purpose of this talk is to: 1. Critically evaluate the implications of the SPARC intercomparison figure. 2. Provide a list of criteria for judging instrument performance and accuracy that include laboratory, airborne, and on-orbit strategies for establishing absolute accuracy. 3. Use those criteria to evaluate the accuracies of representative instruments included in the intercomparison figure. 4. Suggest potential guidelines that enable independent and impartial instrument evaluation to take place. 5. Suggest other measurements that require the same care and attention now being focused on water vapor. 6. Enlist community support to establish the framework necessary to evaluate instrument accuracy on an ongoing basis.
A11D-07 INVITED 09:30h
Effect of Convection on the Tropical Tropopause Layer in a Microphysical Model
The Tropical Tropopause Layer (TTL), a region that surrounds the thermal tropical tropopause and extends from about 14 to 18 km, controls the input of water vapor into the lower tropical stratosphere. Recent observational work has shown that most convection does not penetrate into the TTL, but that the TTL nevertheless contains large sheets of subvisible cirrus clouds. This has led to the conceptual model where TTL control of stratospheric water vapor is achieved largely by in situ formation of clouds and subsequent sedimentation, which arises by horizontal air motion through cold regions. In fact, trajectory-based microphysical models including detailed microphysics and temperature perturbations at all scales, but excluding convective inputs, have successfully simulated the distribution of water vapor near the top of the TTL. But even though most convection does not penetrate into the TTL, convection is probably the TTL's only source of air. Also, water isotope measurements indicate that air cannot be dehydrated by in situ cloud formation alone. The present work includes convective inputs in a trajectory-based microphysical model by using global geostationary satellite imagery. The model calculates ice particle nucleation, growth, evaporation, and sedimentation on a column of air moving along calculated back trajectories. Realistic temperature variations are included, as well as a mean radiatively induced uplift. As the column encounters convection, the air is saturated up to the cloud top as determined by relating satellite brightness temperature to the vertical temperature profile. Sensitivity of the simulated TTL water vapor and cirrus cloud distributions to convective ice crystal inputs and the degree of cloud penetration into the stratosphere are investigated.
A11D-08 09:45h
Trimodal distribution of ozone and water vapor in the UT/LS during boreal summer
The relation of ozone and water vapor in the upper troposphere and lower stratosphere (UT/LS) is strongly influenced by the off-equatorial Asian and North American monsoons in boreal summer. Both regions experience hydration, presumably as a result of deep convection. This behavior contrasts sharply with the apparent dehydrating influence of near-equatorial deep convection in boreal winter. There is also a striking difference in ozone between Asia and North America in boreal summer. Over Asia, ozone concentrations are low, evidently a result of ubiquitous deep convection and the vertical transport of ozone-poor air, while over North America, ozone concentrations are much higher. Since deep convection also occurs in the North American monsoon, it appears that the difference in ozone concentration between Asia and North America in boreal summer reflects a differing influence of the large-scale circulation in the two regions: specifically, (i) isolation of the Tibetan anticyclone versus (ii) the intrusion of filaments of ozone-rich air from the stratosphere over North America. During boreal summer, as in winter, near-equatorial concentrations of ozone and water vapor are low near the equator. The result of these geographical variations is a trimodal distribution of ozone and water-vapor correlation. Our talk reviews the observational evidence of this trimodal distribution and possible dynamical and microphysical causes, focusing primarily on the quality and possible sampling bias of satellite and aircraft measurements. A key issue is the ability of HALOE to sample areas of ubiquitous deep convection. Other issues include the vertical structure of tracer anomalies, isentropic stirring in the UT/LS, horizontal transport of biomass burning products lofted by deep convection, and connections to the moist phase of the tropical `tape recorder' signal in water vapor.