Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 108

4.3.2.4 Atmospheric Emitted Radiance Interferometer (AERI)

The very limited vertical resolution of passive remote sensing can be improved
by obtaining spectra instead of measuring irradiances in certain spectral channels.
An atmospheric emitted radiance interferometer (AERI) that delivers spectra can
measure vertical temperature and humidity profiles with 10 min temporal and 100-
200 m vertical resolution. Figure 4.22 shows an example presenting the passage
of a cold front around noon and subsequent cooling especially in higher layers.
The accuracy of the temperature retrievals has been analyzed to be better than 1 K
(Feltz et al. 2003 ). Wind data from windprofiler measurements have been overlaid
in Fig. 4.22 to show the wind direction shift towards North with the passage of the
cold front. Above about 500 m above ground, instruments such as AERI are superior
to microwave radiometers (Löhnert et al. 2009 ).
The German Weather Service operates an experimental version of a FTIR spec-
trometer at the Richard-Assmann observatory Lindenberg called EISAR (Emission
Infrared Spectrometer for Atmospheric Research, Spänkuch et al. 1996 , Engelbart
2005 ). This passive instrument records radiation from the frequency band between
3.3 and 20 μ m.

4.3.3 Humidity, Water, and Ice

This section deals with active measurements of gaseous humidity as well as of pre-
cipitation elements. Atmospheric water vapour is the most abundant trace gas in the
Earth's atmosphere. Due to the possible latent heat release during phase changes
of water vapour into liquid and solid states, atmospheric humidity has influence
on atmospheric thermal stability. Due to this and its influence on the atmospheric
energy budgets, humidity is treated here separately from other trace gases. Gaseous
humidity can actively be detected by DIAL and Raman LIDAR observations and
passively by radiometer and interferometer measurements. All other trace gases are
addressed in the subsequent section.

Fig. 4.22 Time-height cross section of potential temperature from AERI measurements ( colours
and lines with labels). Horizontal wind arrows are from additional 404 MHz wind profiler
soundings (from Feltz et al. 2003 )