An observational study of water vapor in the mid‐latitude mesosphere using ground‐based microwave techniques
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Abstract:
Ground‐based spectral line measurements of the 22.2‐GHz water vapor atmospheric emission line are used to deduce the mesospheric water vapor profile. The results generally indicate that the water vapor mixing ratio is independent of height or increases slowly in the 50‐ to 60‐km range with typical values of between 5 and 8 parts per million by volume (ppmv). (Thacker et al. (1981) give an analysis of the 1980 data sets obtained in this experiment, which indicated the existence of a pronounced layer of water vapor near 65 km. The reanalysis of these data contained in this paper, which includes the estimation and removal of spectral baselines, results in marked smoothing of the water vapor mixing ratio profile in the lower mesosphere, and the peak is much less pronounced.) Above 65 km the mixing ratio is seen to decrease rather rapidly to values of near 1 ppmv at 85 km. This decrease is far steeper than that expected from photochemical considerations. There has also been a large amount of variability observed in the water vapor profiles, especially on the day‐to‐day time scale. The water vapor retrievals have been used to estimate the vertical component of the upper mesospheric eddy diffusion coefficient ( K z ). This analysis has indicated either that vertical transport time scales in the mesosphere are perhaps an order of magnitude longer than previous studies have shown or that present understanding of the factors important in controlling the vertical distribution of water vapor in the mesosphere is inadequate.Keywords:
Mixing ratio
Microwave Limb Sounder
Mesopause
Aeronomy
We have used the improved NCAR interactive 2‐D model (SOCRATES) to investigate the chemical and thermal response of the mesosphere to composition changes from the preindustrial era (∼1850) to the present, to doubling the CO 2 concentration, and to the 11‐year solar flux variability. The calculations show that all regions in the model mesosphere have cooled relative to the preindustrial times. The mesopause region has cooled by ∼5 K and the winter pole by up to 9 K near 60 km. Ozone mixing ratio has decreased by about 5% in the lower mesosphere and by about 30% near the summer mesopause region (caused by a dramatic increase in [OH]). Doubling the CO 2 abundance cools the whole mesosphere by about 4–16 K and has a complicated effect on O 3 , which exhibits an alternating increase/decrease behavior from the lower mesosphere up to the mesopause region. Similar results are obtained, in both magnitude and structure, for the O 3 response to a decrease in solar UV flux. Similarities are also found in the response of T, OH, and H to these two perturbations. These results lead to the conclusion that the long‐term increase in the well‐mixed greenhouse gases, in particular CO 2 , alters the thermal structure and chemical composition of the mesosphere significantly and that these anthropogenic effects are of the same magnitude as the effects associated with the 11‐year solar cycle. Thus, the difference in the timescales involved suggests that the anthropogenic signal over periods of typically 10 years is smaller than the signal generated by the 11‐year solar variability. Finally, analysis of the results from a simulation of the combined perturbations (2 × CO 2 + 11‐year solar variability) shows that, for the most part, the solar variability does not interact with increasing CO 2 and vice versa; that is, the two effects are additive.
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Water vapor measured by the Solar Occultation for Ice Experiment (SOFIE) instrument on the Aeronomy of Ice in the Mesosphere satellite has been validated in the vertical range 45–95 km. Precision estimates for SOFIE v1.022 H 2 O are ∼0.2%–2.5% up to 80 km and degrade to ∼20% at ∼90 km. The SOFIE total systematic error from the retrieval analysis remains at ∼3%–4% throughout the lower to middle mesosphere and increases from ∼9% at 85 km to ∼16% at 95 km. Comparisons with Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer (ACE‐FTS) and Microwave Limb Sounder (MLS) H 2 O show excellent agreement (0%–2%) up to 80 km in the Northern Hemisphere with rare exceptions. Percentage differences above ∼85 km increase to ∼20% or worse due largely to the low H 2 O volume mixing ratios in the upper mesosphere. For the Southern Hemisphere SOFIE is consistently biased low by 10%–20% relative to both ACE‐FTS and MLS H 2 O. Slopes of SOFIE daily mean H 2 O isopleths on an altitude versus time cross section are used as an indicator of upwelling air motion. In the lower to middle mesosphere, the slope is the largest from mid‐May to mid‐June (maximum of ∼1.5 cm/s), and then in July and August, it is reduced significantly. Both SOFIE and MLS daily mean H 2 O volume mixing ratios at the polar mesospheric cloud height increase rapidly from ∼2.0 to ∼5.0 ppmv prior to the solstice and then approach a near‐constant but slightly increasing level (6.0–6.5 ppmv) throughout the season.
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Microwave Limb Sounder
Occultation
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Mixing ratio
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Ground‐based spectral line measurements of the 22.2‐GHz water vapor atmospheric emission line are used to deduce the mesospheric water vapor profile. The results generally indicate that the water vapor mixing ratio is independent of height or increases slowly in the 50‐ to 60‐km range with typical values of between 5 and 8 parts per million by volume (ppmv). (Thacker et al. (1981) give an analysis of the 1980 data sets obtained in this experiment, which indicated the existence of a pronounced layer of water vapor near 65 km. The reanalysis of these data contained in this paper, which includes the estimation and removal of spectral baselines, results in marked smoothing of the water vapor mixing ratio profile in the lower mesosphere, and the peak is much less pronounced.) Above 65 km the mixing ratio is seen to decrease rather rapidly to values of near 1 ppmv at 85 km. This decrease is far steeper than that expected from photochemical considerations. There has also been a large amount of variability observed in the water vapor profiles, especially on the day‐to‐day time scale. The water vapor retrievals have been used to estimate the vertical component of the upper mesospheric eddy diffusion coefficient ( K z ). This analysis has indicated either that vertical transport time scales in the mesosphere are perhaps an order of magnitude longer than previous studies have shown or that present understanding of the factors important in controlling the vertical distribution of water vapor in the mesosphere is inadequate.
Mixing ratio
Microwave Limb Sounder
Mesopause
Aeronomy
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We have used the interactive two‐dimensional model SOCRATES to investigate the thermal and the chemical response of the mesosphere to the changes in greenhouse gas concentrations observed in the past 50 years (CO 2 , CH 4 , water vapor, N 2 O, CFCs), and to specified changes in gravity wave drag and diffusion in the upper mesosphere. When considering the observed increase in the abundances of greenhouse gases for the past 50 years, a cooling of 3–7 K is calculated in the mesopause region together with a cooling of 4–6 K in the middle mesosphere. Changes in the meridional circulation of the mesosphere damp the pure radiative thermal effect of the greenhouse gases. The largest cooling in the winter upper mesosphere‐mesopause region occurs when the observed increase in concentrations of greenhouse gases and the strengthening of the gravity wave drag and diffusion are considered simultaneously. Depending on the adopted strengthening of the gravity wave drag and diffusion, a cooling varying from typically 6–10 K to 10–20 K over the past 50 years is predicted in the extratropical upper mesosphere during wintertime. In summer, however, consistently with observations, the thermal response calculated by the model is insignificant in the vicinity of the mesopause. Although the calculated cooling of the winter mesopause is still less than suggested by some observations, these results lead to the conclusion that the increase in the abundances of greenhouse gases alone may not entirely explain the observed temperature trends in the mesosphere. Long‐term changes in the dynamics of the middle atmosphere (and the troposphere), including changes in gravity wave activity may have contributed significantly to the observed long‐term changes in thermal structure and chemical composition of the mesosphere.
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Wave drag
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Layered phenomena in the mesopause region is the subject of an international working group whose main focus concerns the physics and chemistry of the summertime mesosphere and the processes involved in forming polar mesospheric summertime echoes (PMSEs) and polar mesospheric clouds (PMCs)—the global equivalent to ground observer's noctilucent clouds (NLCs). PMCs are ice clouds that occur in the summer mesosphere at altitudes typically between 81 and 86 km, and poleward of 50 degrees latitude. PMSEs are strong backscattered signals from the summer mesosphere region, recorded largely, although not exclusively, by VHF radars, that occur most often poleward of 50 degrees latitude and at altitudes typically between 82 and 88 km. The major areas of research within this working group involve understanding and numerically modeling the dynamical, thermal, and chemical processes related to the summer mesosphere; microphysical modeling of MCs; determining the scattering, composition, and shape of mesospheric particles; resolving sources of condensation nuclei; performing laboratory experiments of relevant reactions and related constituents involved in PMC and PMSE behavior, and determining the properties and conditions of PMC, PMSE, and the summer mesosphere by making rocket, satellite, and ground‐based radar and lidar measurements.
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Two VHF atmospheric radars operating in Antarctica during austral summer 2007/2008 found the Polar Mesosphere Summer Echo (PMSE) layer at 3–5 km higher altitude during the early season, compared to the late season, and to earlier seasons. Temperatures from the microwave limb sounder on the Aura satellite show that the height of the cold summer mesopause was ∼3 km higher than usual at the same time. The winter polar vortex over Antarctica did not break up until late December, so that eastward winds in the lower stratosphere were as strong as westward winds in the upper stratosphere during the early part of the austral summer. We find that a combination of limited gravity wave forcing from below in the same hemisphere and interhemisphere coupling between the winter stratosphere/mesosphere and the summer mesopause may explain the observations, and suggest a need for reappraisal of the formation mechanisms for the summer mesopause.
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Microwave Limb Sounder
Forcing (mathematics)
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The polar mesospheric clouds (PMCs) obtained from Aeronomy of Ice in the Mesosphere (AIM)/Cloud Imaging and Particle Size (CIPS) and Himawari-8/Advanced Himawari Imager (AHI) observations are analyzed for the multi-year climatology and interannual variations. The PMCs dependence on mesospheric temperature and water vapor (H2O) are further investigated with data from Microwave Limb Sounder (MLS). Our analysis shows that PMCs onset date and occurrence rate are strongly dependent on the atmospheric environment, i.e. underlying seasonal behavior of temperature and water vapor. Upper-mesospheric dehydration by PMCs is evident in MLS water vapor observations, The spatial patterns of the depleted water vapor resemble the PMCs distribution over the Arctic and Antarctic region during the days after summer solstice. Year-to-year variabilities of the PMCs occurrence rate and onset date are highly correlated with the mesospheric temperature and H2O variations, particularly in the southern hemisphere (SH). The global increase of mesospheric H2O during the last decade may explain the increased PMCs occurrence in the northern hemisphere (NH). Although mesospheric temperature and H2O exhibits a strong 11-year variation, little solar cycle signature is found in the PMCs occurrence during 2005-2021.
Microwave Limb Sounder
Aeronomy
Solstice
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