89 research outputs found
Recommended from our members
Horizontal transport affecting trace gas seasonality in the Tropical Tropopause Layer (TTL)
We analyze horizontal transport from midlatitudes into the tropics (in-mixing) and its impact on seasonal variations of ozone, carbon monoxide and water vapor in the Tropical Tropopause Layer (TTL). For this purpose, we use three-dimensional backward trajectories, driven by ECMWF ERA-Interim winds, and a conceptual one-dimensional model of the chemical composition of the TTL. We find that the fraction of in-mixed midlatitude air shows an annual cycle with maximum during NH summer, resulting from the superposition of two inversely phased annual cycles for in-mixing from the NH and SH, respectively. In-mixing is driven by the monsoonal upper-level anticyclonic circulations. This circulation pattern is dominated by the Southeast Asian summer monsoon and, correspondingly, in-mixing shows an annual cycle. The impact of in-mixing on TTL mixing ratios depends on the in-mixed fraction of midlatitude air and on the meridional gradient of the particular species. For CO the meridional gradient and consequently the effect of in-mixing is weak. For water vapor, in-mixing effects are negligible. For ozone, the meridional gradient is large and the contribution of in-mixing to the ozone maximum during NH summer is about 50%. This in-mixing contribution is not sensitive to the tropical ascent velocity, which is about 40% too fast in ERA-Interim. As photochemically produced ozone in the TTL shows no distinct summer maximum, the ozone annual anomaly in the upper TTL turns out to be mainly forced by in-mixing of ozone-rich extratropical air during NH summer
Trend in ice moistening the stratosphere â constraints from isotope data of water and methane
Water plays a major role in the chemistry and radiative budget of the stratosphere. Air enters the stratosphere predominantly in the tropics, where the very low temperatures around the tropopause constrain water vapour mixing ratios to a few parts per million. Observations of stratospheric water vapour show a large positive long-term trend, which can not be explained by change in tropopause temperatures. Trends in the partitioning between vapour and ice of water entering the stratosphere have been suggested to resolve this conundrum. We present measurements of stratospheric H_(2)O, HDO, CH_4 and CH_(3)D in the period 1991â2007 to evaluate this hypothesis. Because of fractionation processes during phase changes, the hydrogen isotopic composition of H_(2)O is a sensitive indicator of changes in the partitioning of vapour and ice. We find that the seasonal variations of H_(2)O are mirrored in the variation of the ratio of HDO to H_(2)O with a slope of the correlation consistent with water entering the stratosphere mainly as vapour. The variability in the fractionation over the entire observation period is well explained by variations in H_(2)O. The isotopic data allow concluding that the trend in ice arising from particulate water is no more than (0.01±0.13) ppmv/decade in the observation period. Our observations suggest that between 1991 and 2007 the contribution from changes in particulate water transported through the tropopause plays only a minor role in altering in the amount of water entering the stratosphere
Recommended from our members
Variability and trends in dynamical forcing of tropical lower stratospheric temperatures
The contribution of dynamical forcing to variations and trends in tropical
lower stratospheric
70 hPa temperature for the period 1980â2011 is estimated based on ERA-Interim
and Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis data. The dynamical forcing is estimated from the
tropical mean residual upwelling calculated with the momentum balance equation,
and with a simple proxy based on eddy heat fluxes averaged between
25° and 75° in both hemispheres. The thermodynamic energy equation
with Newtonian cooling is used to relate the dynamical forcing to temperature.
The deseasonalised, monthly mean time series of all four calculations are
highly correlated (~ 0.85) with temperature for the period 1995â2011
when variations in radiatively active tracers are small.
All four calculations provide additional support to previously noted
prominent aspects of the
temperature evolution 1980â2011:
an anomalously strong dynamical cooling (~ â1 to â2 K)
following the Pinatubo eruption that partially offsets the warming
from enhanced aerosol, and
a few years of enhanced dynamical cooling
(~ â0.4 K) after October 2000 that contributes to
the prominent drop in water entering the stratosphere at that time.
The time series of dynamically forced temperature calculated with the same
method are more highly correlated and have more
similar trends than those from the same reanalysis but with different methods.
For 1980â2011 (without volcanic periods), the eddy heat flux calculations give a
dynamical cooling of
~ â0.1 to ~ â0.25 K decade−1
(magnitude sensitive to latitude belt considered and reanalysis),
largely due to increasing high latitude eddy heat flux trends in September
and DecemberâJanuary. The eddy heat flux trends also explain the seasonality
of temperature trends very well, with maximum cooling in JanuaryâFebruary.
Trends derived from momentum balance calculations show near-zero annual mean
dynamical cooling, with weaker seasonal trends especially in DecemberâJanuary.
These contradictory results arising from uncertainties in data and methods are
discussed and put in context to previous analyses
What causes the irregular cycle of the atmospheric tape recorder signal in HCN?
Variations in the mixing ratio of long-lived trace gases entering the stratosphere in the tropics are carried upward with the rising air with the signal being observable throughout the tropical lower stratosphere. This phenomenon, referred to as "atmospheric tape recorder" has previously been observed for water vapor, CO2, and CO which exhibit an annual cycle. Recently, based on Microwave Limb Sounder (MLS) and the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS) satellite measurements, the tape recorder signal has been observed for hydrogen cyanide (HCN) but with an approximately two-year period. Here we report on a model simulation of the HCN tape recorder for the time period 2002-2008 using the Chemical Lagrangian Model of the Stratosphere (CLaMS). The model can reproduce the observed pattern of the HCN tape recorder signal if time-resolved emissions from fires in Indonesia are used as lower boundary condition. This finding indicates that inter-annual variations in biomass burning in Indonesia, which are strongly influenced by El Nino events, control the HCN tape recorder signal. A longer time series of tropical HCN data will probably exhibit an irregular cycle rather than a regular biannual cycle. Citation: Pommrich, R., R. Muller, J.-U. Grooss, G. Gunther, P. Konopka, M. Riese, A. Heil, M. Schultz, H.-C. Pumphrey, and K. A. Walker (2010), What causes the irregular cycle of the atmospheric tape recorder signal in HCN?, Geophys. Res. Lett., 37, L16805, doi:10.1029/2010GL044056
Technical Note: Chemistry-climate model SOCOL: version 2.0 with improved transport and chemistry/microphysics schemes
International audienceWe describe version 2.0 of the chemistry-climate model (CCM) SOCOL. The new version includes fundamental changes of the transport scheme such as transporting all chemical species of the model individually and applying a family-based correction scheme for mass conservation for species of the nitrogen, chlorine and bromine groups, a revised transport scheme for ozone, furthermore more detailed halogen reaction and deposition schemes, and a new cirrus parameterisation in the tropical tropopause region. By means of these changes the model manages to overcome or considerably reduce deficiencies recently identified in SOCOL version 1.1 within the CCM Validation activity of SPARC (CCMVal). In particular, as a consequence of these changes, regional mass loss or accumulation artificially caused by the semi-Lagrangian transport scheme can be significantly reduced, leading to much more realistic distributions of the modelled chemical species, most notably of the halogens and ozone
The SCOUT-O3 Darwin Aircraft Campaign: rationale and mateorology
An aircraft measurement campaign involving the Russian high-altitude aircraft M55 Geophysica and the German DLR Falcon was conducted in Darwin, Australia in November and December 2005 as part of the European integrated
project SCOUT-O3. The overall objectives of the campaign were to study the transport of trace gases through the tropical tropopause layer (TTL), mechanisms of dehydration close to the tropopause, and the role of deep convection in these processes. In this paper a detailed roadmap of the
campaign is presented, including rationales for each flight, and an analysis of the local and large-scale meteorological context in which they were embedded. The campaign took place during the pre-monsoon season which is characterized by a pronounced diurnal evolution of deep convection including a mesoscale system over the Tiwi Islands north of Darwin known as ïżœ\x83ÂąĂïżœĂïżœHectorĂÂąĂïżœĂïżœ. This allowed studying in detail the role of deep convection in structuring the tropical tropopause region, in situ sampling convective overshoots above storm anvils, and probing the structure of anvils and cirrus clouds by Lidar and a suite of in situ instruments onboard the two aircraft. The large-scale flow during the first half of the campaign was such that local flights, away from convection, sampled air masses downstream of the ĂÂąĂïżœĂïżœcold trapĂÂąĂïżœĂïżœ region over Indonesia.
Abundant cirrus clouds enabled the study of active dehydration, in particular during two TTL survey flights. The campaign period also encompassed a Rossby wave breaking event transporting stratospheric air to the tropical middle
troposphere and an equatorial Kelvin wave modulating tropopause temperatures and hence the conditions for dehydration
Tropopause and hygropause variability over the equatorial Indian Ocean during February and March 1999.
Measurements of temperature, water vapor, total water, ozone, and cloud properties were made above the western equatorial Indian Ocean in February and March 1999. The cold-point tropopause was at a mean pressure-altitude of 17 km, equivalent to a potential temperature of 380 K, and had a mean temperature of 190 K. Total water mixing ratios at the hygropause varied between 1.4 and 4.1 ppmv. The mean saturation water vapor mixing ratio at the cold point was 3.0 ppmv. This does not accurately represent the mean of the measured total water mixing ratios because the air was unsaturated at the cold point for about 40% of the measurements. As well as unsaturation at the cold point, saturation was observed above the cold point on almost 30% of the profiles. In such profiles the air was saturated with respect to water ice but was free of clouds (i.e., backscatter ratio <2) at potential temperatures more than 5 K above the tropopause and hygropause. Individual profiles show a great deal of variability in the potential temperatures of the cold point and hygropause. We attribute this to short timescale and space-scale perturbations superimposed on the seasonal cycle. There is neither a clear and consistent âsettingâ of the tropopause and hygropause to the same altitude by dehydration processes nor a clear and consistent separation of tropopause and hygropause by the Brewer-Dobson circulation. Similarly, neither the tropopause nor the hygropause provides a location where conditions consistently approach those implied by a simple âtropopause freeze dryingâ or âstratospheric fountainâ hypothesis
Recommended from our members
Multitimescale variations in modeled stratospheric water vapor derived from three modern reanalysis products
Stratospheric water vapor (SWV) plays important roles in the radiation budget
and ozone chemistry and is a valuable tracer for understanding stratospheric
transport. Meteorological reanalyses provide variables necessary for
simulating this transport; however, even recent reanalyses are subject to
substantial uncertainties, especially in the stratosphere. It is therefore
necessary to evaluate the consistency among SWV distributions simulated using
different input reanalysis products. In this study, we evaluate the
representation of SWV and its variations on multiple timescales using
simulations over the period 1980â2013. Our simulations are based on the
Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by horizontal
winds and diabatic heating rates from three recent reanalyses: ERA-Interim,
JRA-55 and MERRA-2. We present an intercomparison among these model results
and observationally based estimates using a multiple linear regression method
to study the annual cycle (AC), the quasi-biennial oscillation (QBO), and
longer-term variability in monthly zonal-mean H2O mixing ratios
forced by variations in the El NiñoâSouthern Oscillation (ENSO) and the
volcanic aerosol burden. We find reasonable consistency among simulations of
the distribution and variability in SWV with respect to the AC and QBO.
However, the amplitudes of both signals are systematically weaker in the
lower and middle stratosphere when CLaMS is driven by MERRA-2 than when it is
driven by ERA-Interim or JRA-55. This difference is primarily attributable to
relatively slow tropical upwelling in the lower stratosphere in simulations
based on MERRA-2. Two possible contributors to the slow tropical upwelling in
the lower stratosphere are suggested to be the large long-wave cloud
radiative effect and the unique assimilation process in MERRA-2. The impacts
of ENSO and volcanic aerosol on H2O entry variability are
qualitatively consistent among the three simulations despite differences of
50 %â100 % in the magnitudes. Trends show larger discrepancies among the
three simulations. CLaMS driven by ERA-Interim produces a neutral to slightly
positive trend in H2O entry values over 1980â2013
(+0.01 ppmv decadeâ1), while both CLaMS driven by JRA-55 and CLaMS
driven by MERRA-2 produce negative trends but with significantly different
magnitudes (â0.22 and â0.08 ppmv decadeâ1, respectively).</p
- âŠ