Middle Atmospheric Water Vapor Research Articles

Latitude-altitude cross-sections of middle atmospheric water vapour obtained by airbourne millimeter-wave spectroscopy

The observation of rotational transitions from minor constituents in the mm-wave range permits the retrieval of the corresponding molecule abundance in the altitude range of 15 to 70 km. This work describes the latitude-altitude cross-sections of water vapour which were derived from measurements of the 183 GHz emission line and were are obtained during different seasons with the Learjet of the Swiss Air Force. Several flights from Spitzbergen (78N) to the Cap Verde islands (15N) have been performed in the last three years. A maximum VMR of 7 ppm was found around 60 km altitude at low latitudes during October 1995 and March 1996. At high winter-latitudes up to 6 ppm VMR in the lower stratosphere associated with a decreased mesospheric water vapour abundance has been measured, both indicating downward transport. The measurements obtained in August 1994 were compared with 2-D models showing the typical seasonal and latitudinal features as expected from the current understanding of the large scale transport processes. However, particular dynamical features of our measurements in October and March such as the confinement and descent of air within the polar vortex cannot be resolved by a 2-D model. A persistent local maximum around 10 mb was observed by AMSOS but was not found in current models.

Ground‐based measurements of middle atmospheric water vapor at 183 GHz

We present here the first ground‐based observations of stratospheric/mesospheric H2O emission at the 31,3 → 22,0 resonance (183.3101 GHz of central frequency). The spectra were obtained in January 1991 and January 1994 using the Institut de Radioastronomie Millimétrique (IRAM) 30‐m radiotelescope located on Pico Veleta, Spain, (altitude 2870 m, latitude 37°N), during very dry tropospheric conditions (total precipitable water within the range 0.2–0.3 mm above the observation site). Microwave studies of stratospheric and mesospheric H2O profile have been carried out so far mostly by means of observations of the weak 61,6 → 52,3 resonance at 22.2351 GHz. Some measurements at 183 GHz were previously obtained from airborne observations. The water vapor abundance in the troposphere is, in most cases, so high that the atmosphere appears optically thick at 183 GHz. However, we show that emission from the stratosphere and mesosphere can be sometimes detected from the ground (allowing upper stratospheric and mesospheric water vapor profiles to be obtained). We use a generalized non linear radiative transfer code in order to perform a least squares fit of the observed spectra. The derived H2O abundances are consistent with most measurements at 22 GHz. We suggest the possibility of carrying out this kind of observation in some selected sites during a significant fraction of the whole winter season. The required integration times to get a given signal‐to‐noise ratio are much shorter at 183 GHz than at 22 GHz.

Measurements of water vapor in the middle atmosphere and implications for mesospheric transport

We present data obtained during more than 3 years of nearly continuous measurements of middle atmospheric water vapor. The data are obtained from ground‐based measurements at 22 GHz taken at two sites, one in each hemisphere, using the Naval Research Laboratory water vapor millimeter‐wave spectrometer (WVMS). With the construction of a second instrument, it has been possible to maintain continuous monitoring from both sites since January 1994. The measurements from both instruments show significant seasonal variability. There is a clear annual cycle, with the water vapor above ∼60 km increasing in summer and decreasing in winter. The observed amplitude of the annual oscillation is larger at 45.0°S than at 34.4°N, a result which is qualitatively consistent with the higher latitude of the southern hemisphere site. There is also an indication of a semiannual cycle, particularly at altitudes near 80 km. The annual cycle is consistent with transport due primarily to advection, while the weaker semiannual cycle may be indicative of the effect of gravity waves on diffusive transport.

Ground‐based measurements of water vapor in the middle atmosphere

We present measurements of the middle atmospheric water vapor mixing ratio profile obtained using the ground‐based Naval Research Laboratory water vapor millimeter‐wave spectrometer (WVMS) instrument at the Jet Propulsion Laboratory Table Mountain Observatory. The measurements cover a period of 262 days from January 23, 1992, to October 13, 1992. During this campaign it was possible to retrieve useful daily mixing ratio profiles for 186 days. We thus have a nearly continuous record of water vapor mixing ratios for altitudes from ≈35 to 75 km. The retrievals are obtained using the optimal estimation method. Details of the error analysis are presented, and a technique is introduced that reduces baseline effects and helps to estimate the baseline error. The high‐altitude (≳65 km) data show a sharp rise prior to the expected maximum near the summer solstice and a gradual decline in the following months. The mixing ratios generally peak between 55 and 65 km, at which point the mixing ratios are 6–7 parts per million by volume. The highest peaks occur in January, May, and October.

Relation between increasing methane and the presence of ice clouds at the mesopause

TRENDS of increasing atmospheric methane, carbon dioxide and other species have now been identified1. It is well-known that water vapour is an important product of methane oxidation in the stratosphere2,3 and here we investigate the possibility that a substantial change has occurred in middle-atmospheric water vapour as a result of the increase in methane over the past century and a half. We show from modelling of mesopheric ice-particle formation that noctilucent cloud brightness should be a sensitive indicator of the water content at the high-latitude summertime mesopause (at a height of 85 km). Blake and Rowland4 have recently suggested that the occurrence of polar stratospheric clouds may be increasing because of increasing methane. We look at the record of noctilucent cloud occurrence for which the historical record is more complete. We find that noctilucent clouds are absent from the historical record before 1885, which is consistent with our hypothesis.

Middle atmospheric water vapor observed by the Spacelab One grille spectrometer

Two water vapor atmospheric concentration profiles have been obtained, one at 33°N, 59°E and the other at 68°S, 124°W, during the Spacelab One flight respectively on 2 and 1 December 1983. These profiles extend from the middle stratosphere up to mesopause and show significant differences above the altitude of 70 km, the Antarctic profile showing then higher concentrations. This result correlates with Spacelab One carbon monoxide observations and SM E ozone results as far as the hydroxyl radical chemistry is concerned.