Oleg A. Gusev

Solution of the non-LTE problem for molecular gas in planetary atmospheres: superiority of accelerated lambda iteration

Abstract A general formulation is given of the multi-level rotation–vibrational non-LTE problem for a mixture of radiating molecular gases in a planetary atmosphere, treating explicitly the coupling of molecular energy levels by collisionally induced energy transfer processes and by band overlap. Various limiting cases of non-LTE effects are discussed. Three techniques—lambda iteration, matrix and accelerated lambda iteration—which are used to solve these problem are discussed and compared. In the case of the CO 2 non-LTE problem in the Earths atmosphere, it is demonstrated that accelerated lambda iteration is far superior to the other algorithms in minimizing computer time and storage and in converging much more rapidly; moreover the convergence rate is insensitive to the initial population estimates and to wide range of variation in the model input parameters. Accelerated lambda iteration therefore makes possible the calculation of much larger and more physically complete atmospheric and molecular models.

PMCs and the water frost point in the Arctic summer mesosphere

In August, 1997 the Middle Atmosphere High Resolution Spectrograph Investigation (MAHRSI) obtained vertical profiles of OH number density and polar mesospheric cloud (PMC) brightness by scanning the limb up to 71° N while the Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) obtained co-located vertical profiles of temperature. MAHRSI OH densities are converted to water vapor using a one-dimensional model that assumes photochemical equilibrium. By combining water vapor profiles with CRISTA temperatures we map the frost point both vertically and horizontally in the Arctic summer mesosphere. Our data show that supersaturation can exist between 80-87 km suggesting that growth of ice particles is limited to these altitudes. Additionally, we find that not only is supersaturation an insufficient condition for a PMC but also that PMCs can exist in apparently unsaturated air.

Zonal asymmetries in middle atmosphere temperatures

Zonal asymmetries are frequently seen in stratospheric temperature or trace gas fields as surf zones, streamers, filaments etc. They are also seen as very small-scale fluctuations, the intensity of which varies with longitude. Similar structures might be expected in the mesosphere as well, and several examples have recently been found. CRISTA 1 large-scale data are presented that indicate a surf zone in the middle mesosphere at the beginning of winter. Very small-scale data are shown from the CRISTA 2 mission. Mesospheric variability is found to be high at all altitudes, latitudes, and longitudes. There are considerable non-zonal structures in these fluctuations. (The duration of the Crista missions was about one week each.) Zonal asymmetries have been known for a long time from comparisons of ground stations measuring the same parameter in the mesosphere/lower thermosphere. As an example, upper mesosphere temperatures derived from OH∗ emissions are compared here for Wuppertal and Moscow (Zvenigorod), which are about 2000 km apart. A systematic and substantial difference in temperature is obtained, with higher temperatures at Wuppertal than at Moscow. The difference appears to follow the solar cycle: it is small at solar maximum and large (up to 28 K) at solar minimum. The reason for this surprising behavior is as yet unknown. The Moscow and Wuppertal temperatures have also been analyzed for long-term trends: a trend discrepancy between the two stations is not seen in the data interval common to the two stations.

A global view of tidal temperature perturbations above the mesopause: Preliminary model/observation intercomparison

Abstract Migrating and nonmigrating diurnal tides in the temperature data from the satellite borne, Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere (CRISTA) experiment are compared to model predictions from the Thermosphere Ionosphere Mesosphere Electrodynamics General Circulation Model (TIME-GCM) and from the Global Scale Wave Model (GSWM). The comparative model/observation analysis is performed between 75–120 km altitude and at 7.5°N. The results suggest that interactions between quasi-stationary planetary waves and radiative and latent heat forced tidal components may play an important role in generating the thermal tidal structure above the mesopause though more extended model studies are necessary.

New results from CRISTA

Trace gas distributions and temperatures in the mesosphere and lower thermosphere were derived from infrared spectra measured by the two CRISTA experiments flown in November 1994 and in August 1997. CRISTA (CRyogenic Infrared Spectrometers and Telescopes for the Atmosphere) is a triple telescope cryogenically cooled infrared spectrometer which senses the Earth limb from a Shuttle orbit. The geographical coverage was -57°/+68° and -74°/+74° during the two missions, respectively. Each mission lasted slightly more than one week. The mesospheric set of trace gases include ozone, carbon dioxide and carbon monoxide, methane, water vapor, and atomic oxygen. In addition temperatures and pressures are obtained from the CO2 15 μm band. The temperature/pressure results are used to derive geostrophic wind fields. Most of the data reduction required non-LTE modelling of the radiation properties of the species. Practically all data exhibit considerable large scale structures in both latitude and longitude due to planetary waves or interhemispheric transport.