Ground based measurements of upper atmospheric CO and H2O using microwave radiometry
Doctoral thesis, 2003
Long series of continuous measurements of middle and upper atmospheric constituents are needed to be able to understand the anthropogenic influence on the atmosphere. Phenomena like ozone depletion and the enhanced greenhouse effect are examples where the role of human activities has to be well investigated in order to make the right decisions of necessary changes in our way of life.
Microwave radiometry has developed into a powerful tool in aeronomy research from both ground- and satellite-based observing platforms. The method has been successfully used to detect a number of middle atmospheric molecules including carbon monoxide, ozone and water vapour and to estimate the vertical profile of these molecules.
This dissertation presents a radiometer system for continuous long-term measurements of carbon monoxide and water vapour in the middle atmosphere. The system consists of two spectral, Dicke-switched, heterodyne receivers one for 22.23 GHz based on uncooled HEMT preamplifier technique and one for 15.27 GHz based on a cooled Schottky mixer.
The instruments are regularly with two absorber targets, a cold load at 77 K and a load at ambient temperature, respectively. In the calibration procedures for the 22 GHz system methods have been developed to use the sky as the calibration cold load. In-between the calibration measurements the receiver noise temperature is assumed to be constant, which is a sufficient assumption to determine the brightness temperature of the atmosphere. This simplification gives a total estimated calibration error of 5%.
The longest continuous series of ground-based measurements of mesospheric CO is presented. Comparison of the observations with simulations by the Whole Atmosphere Community Climate Model (WACCM) indicates that the model can reproduce the observed seasonal cycle as well as much of its variability, although calculated columns amounts are smaller than the largest values seen in the data.
Altitude profiles of CO and H2O are used to trace the vertical movement of mesospheric air caused by the seasonally varying mean meridional circulation. The mesospheric adiabatic temperature changes caused by the vertical movement of air is compared to mesopause temperature as estimated by the radiance of the Meinel band OH airglow.