Climate models predict that tropical lower stratospheric humidity will increase as the climate warms. We examine this trend in two state‐of‐the‐art chemistry‐climate models. Under high greenhouse gas ...emissions scenarios, the stratospheric entry value of water vapor increases by ~1 ppmv over the 21st century in both models. We show with trajectory runs driven by model meteorological fields that the warming tropical tropopause layer (TTL) explains 50–80% of this increase. The remainder is a consequence of trends in evaporation of ice convectively lofted into the TTL and lower stratosphere. Our results further show that within the models we examined, ice lofting is primarily important on long time scales; on interannual time scales, TTL temperature variations explain most of the variations in lower stratospheric humidity. Assessing the ability of models to realistically represent ice lofting processes should be a high priority in the modeling community.
Key Points
Models all predict water in the stratosphere will increase
Much of this is due to warming of the tropopause
A significant fraction, however, is due to increases in evaporation of convective ice
A single-particle soot photometer (SP2) was flown on a NASA WB-57F high-altitude research aircraft in November 2004 from Houston, Texas. The SP2 uses laser-induced incandescence to detect individual ...black carbon (BC) particles in an air sample in the mass range of approx.3-300 fg (approx.0.15-0.7 microns volume equivalent diameter). Scattered light is used to size the remaining non-BC aerosols in the range of approx.0.17-0.7 microns diameter. We present profiles of both aerosol types from the boundary layer to the lower stratosphere from two midlatitude flights. Results for total aerosol amounts in the size range detected by the SP2 are in good agreement with typical particle spectrometer measurements in the same region. All ambient incandescing particles were identified as BC because their incandescence properties matched those of laboratory-generated BC aerosol. Approximately 40% of these BC particles showed evidence of internal mixing (e.g., coating). Throughout profiles between 5 and 18.7 km, BC particles were less than a few percent of total aerosol number, and black carbon aerosol (BCA) mass mixing ratio showed a constant gradient with altitude above 5 km. SP2 data was compared to results from the ECHAM4/MADE and LmDzT-INCA global aerosol models. The comparison will help resolve the important systematic differences in model aerosol processes that determine BCA loadings. Further intercomparisons of models and measurements as presented here will improve the accuracy of the radiative forcing contribution from BCA.
This study investigates the potential contribution of observed changes in lower stratospheric water vapour to stratospheric temperature variations over the past three decades using a comprehensive ...global climate model (GCM). Three case studies are considered. In the first, the net increase in stratospheric water vapour (SWV) from 1980–2010 (derived from the Boulder frost‐point hygrometer record using the gross assumption that this is globally representative) is estimated to have cooled the lower stratosphere by up to ∼0.2 K decade−1 in the global and annual mean; this is ∼40% of the observed cooling trend over this period. In the Arctic winter stratosphere there is a dynamical response to the increase in SWV, with enhanced polar cooling of 0.6 K decade−1 at 50 hPa and warming of 0.5 K decade−1 at 1 hPa. In the second case study, the observed decrease in tropical lower stratospheric water vapour after the year 2000 (imposed in the GCM as a simplified representation of the observed changes derived from satellite data) is estimated to have caused a relative increase in tropical lower stratospheric temperatures by ∼0.3 K at 50 hPa. In the third case study, the wintertime dehydration in the Antarctic stratospheric polar vortex (again using a simplified representation of the changes seen in a satellite dataset) is estimated to cause a relative warming of the Southern Hemisphere polar stratosphere by up to 1 K at 100 hPa from July–October. This is accompanied by a weakening of the westerly winds on the poleward flank of the stratospheric jet by up to 1.5 m s−1 in the GCM. The results show that, if the measurements are representative of global variations, SWV should be considered as important a driver of transient and long‐term variations in lower stratospheric temperature over the past 30 years as increases in long‐lived greenhouse gases and stratospheric ozone depletion.
We examine variations in water vapor in air entering the stratosphere through the tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory model. Most of the ...variance can be explained by three processes that affect the TTL: the quasi‐biennial oscillation, the strength of the Brewer‐Dobson circulation, and the temperature of the tropical troposphere. When these factors act in phase, significant variations in water entering the stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the TTL, affect the amount of water entering the stratosphere. While there is clear decadal variability in the data and models, we find little evidence for a long‐term trend in water entering the stratosphere through the TTL over the past 3 decades.
Key Points
Examination of water vapor entering the stratosphereWe show that it is controlled by four factorsWe see no trends in water entering the stratosphere
The ozone hole is an important driver of recent Southern Hemisphere (SH) climate change, and capturing these changes is a goal of climate modeling. Most climate models are driven by off‐line ozone ...data sets. Previous studies have shown that there is a substantial range in estimates of SH ozone depletion, but the implications of this range have not been examined systematically. We use a climate model to evaluate the difference between using the ozone forcing (Stratospheric Processes and their Role in Climate (SPARC)) used by many Intergovernmental Panel on Climate Change Fifth Assessment Report (Coupled Model Intercomparison Project) models and one at the upper end of the observed depletion estimates (Binary Database of Profiles (BDBP)). In the stratosphere, we find that austral spring/summer polar cap cooling, geopotential height decreases, and zonal wind increases in the BDBP simulations are all doubled compared to the SPARC simulations, while tropospheric responses are 20–100% larger. These results are important for studies attempting to diagnose the climate fingerprints of ozone depletion.
Key Points
CMIP5 ozone data set is at the lower end of observed depletion for ozone hole regionUsing upper estimate of depletion doubles stratospheric signal of ozone hole and increases tropospheric signal of the ozone hole by 20–100%
Acquiring accurate measurements of water vapor at the low mixing ratios (< 10 ppm) encountered in the upper troposphere and lower stratosphere (UT/LS) has proven to be a significant analytical ...challenge evidenced by persistent disagreements between high‐precision hygrometers. These disagreements have caused uncertainties in the description of the physical processes controlling dehydration of air in the tropical tropopause layer and entry of water into the stratosphere and have hindered validation of satellite water vapor retrievals. A 2011 airborne intercomparison of a large group of in situ hygrometers onboard the NASA WB‐57F high‐altitude research aircraft and balloons has provided an excellent opportunity to evaluate progress in the scientific community toward improved measurement agreement. In this work we intercompare the measurements from the Midlatitude Airborne Cirrus Properties Experiment (MACPEX) and discuss the quality of agreement. Differences between values reported by the instruments were reduced in comparison to some prior campaigns but were nonnegligible and on the order of 20% (0.8 ppm). Our analysis suggests that unrecognized errors in the quantification of instrumental background for some or all of the hygrometers are a likely cause. Until these errors are understood, differences at this level will continue to somewhat limit our understanding of cirrus microphysical processes and dehydration in the tropical tropopause layer.
Key Points
Agreement among in situ measurements of UT/LS water vapor is improved
Scientifically significant disagreements between instruments still exist
We present a new approach for calculating the fraction of very‐short lived substances (VSLS) emitted at the surface (and their degradation products) that reach the stratosphere (β) using the FLEXPART ...Lagrangian model. The values of β play a key role in determining the efficiency of these compounds for depleting stratospheric ozone, and are used to estimate ozone depletion potentials (ODPs) of several short‐lived compounds. Calculated β and ODPs of VSLSs show large regional and seasonal variability owing to the importance of convective transport. For instance, β and ODPs associated with emissions from the Indian subcontinent is an order of magnitude larger than that from Europe, mid‐latitude North America, or East Asia. The seasonal cycle of β is mainly driven by transport efficiency from the boundary layer into the tropical stratosphere; β has a minimum in winter and a maximum in summer.
Stratospheric water vapor concentrations measured at two midlatitude locations in the northern hemisphere show water vapor amounts have increased at a rate of 1–1.5% yr−1 (0.05–0.07 ppmv yr−1) for ...the past 35 years. At Washington, D.C., measurements were made from 1964–1976, and at Boulder, Colorado, observations began in 1980 and continue to the present. While these two data sets do not comprise a single time series, they individually show increases over their respective measurement periods. At Boulder the trends do not show strong seasonal differences; significant increases are found throughout the year in the altitude range 16–28 km. In winter these trends are significant down to about 13 km.
In situ measurements of OH, HO
2
(square brackets denote species concentrations), and other chemical species were made in the tropical upper troposphere (TUT). OH showed a robust correlation with ...solar zenith angle. Beyond this dependence, however, OH did not correlate to its primary source, the product of O
3
and H
2
O (O
3
•H
2
O), or its sink NO
y
. This suggests that OH is heavily buffered in the TUT. One important exception to this result is found in regions with very low O
3
, NO, and NO
y
. Under these conditions, OH is highly suppressed, pointing to the critical role of NO in sustaining OH in the TUT and the possibility of low OH over the western Pacific warm pool due to strong marine convections bringing NO-poor air to the TUT. In contrast to OH, HO
x
(OH + HO
2
) correlated reasonably well with O
3
•H
2
O/NO
y
, suggesting that O
3
•H
2
O and NO
y
are the significant source and sink, respectively, of HO
x
.
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