Publishers version - DTU Orbit

14 Ground based passive microwave radiometry
and temperature profiles
Fabio Scanzani 1,2
1 Institute of Atmospheric Sciences and Climate CNR, Roma, Italy
2 Geoinformation Doctorate - Tor Vergata University Rome, Rome, Italy
14.1 Introduction
The Earth’s atmosphere is a layer of negligible thickness if compared to the Earth’s diameter
and is also, to a first approximation, transparent to electromagnetic radiation. Deviations
from complete transparency are essential for life on Earth (e.g. importance of absorption
of the ultraviolet radiation by ozone layer) and are the focus in the radiative physics of the
atmospherestudies.Atfrequenciesabove10GHz,atmospherecontinuouslyabsorbsandemits
electromagnetic radiation. The spectrum of this emitted radiation depends on a variety of
atmosphericvariablesincludingtemperature,watervaporconcentration,andliquidwater(e.g.
clouds, rain and fog). Detecting and measuring the power of the thermal radiation emitted
at certain frequencies allows to estimate some atmospheric parameters with very high spatial
and temporal resolution.
The importance of the information that can be obtained from microwave observations
of the atmosphere, is given in an excellent review article of the Environmental Research
Laboratories of NOAA (Hogg et al., 1983): “... Atmospheric observations form the essential
base for almost all atmospheric research and services. Since the atmosphere is a variable
three-dimensional fluid, these observations must be obtained in all three spatial dimensions.
Ideally, such data sets should be continuous in both space and time; in practice, this has not
been possible. Because existing observational systems use in situ instruments carried aloft by
balloons,the NationalWeatherService hashad to acceptacompromisein whichthe data sets
are neither continuous in time, nor continuous in space, but instead are taken once every 12
hours at stations spaced roughly 350 km apart across the United States. This system provides
observations of upper air conditions that are suitable for identification and forecasting of
synoptic scale phenomena such as cyclones and anti-cyclones (which have lifetimes of days
and dimensions of 1000 km or more), but is not adequate for the observation and prediction
of smaller scale, shorter lived phenomena such as thunderstorms, flash floods, etc. Other
disadvantages of the existing system are that the profiles obtained are not usually vertical,
and that significant man-hours per profile is required. ...”
Hogg et al. (1983) also proposed an alternative observational networkconsistingof groundbased
microwave systems upward-looking and satellite-borne, downward-looking radiometer
systems. This network is envisioned to provide “profiles of wind, temperature, and to a lesser
extent, humidity, continuously in time, in an unattended mode”. In essence, the proposal
integrates the capabilities of two complementary remote-sensing approaches to monitoring
the atmosphere, one from below and the other from above. The retrieval of atmospheric
parameters by passive remote sensing of thermal electromagnetic radiation at determinate
frequencies is generally referred to as radiometry. A review of several radiometric techniques
and the basics of the inversion algorithms to estimate the atmospheric temperature’s profiles
are here given.
14.2 Microwave radiometry fundamentals
Ground basedradiometricmeasurementsoflowertroposphericlayersthermalemission,known
as ground-based passive radiometric sensing, has been successfully used in a variety of ap-
260 DTU Wind Energy-E-Report-0029(EN)