Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 111

tethered balloon and aircraft data. Inspection of the profile suggests that the top
of the boundary layer is around 1200-1400 m above ground where the specific
humidity drops drastically with height.
For many purposes the knowledge of relative humidity is more important than
specific humidity or mixing ratio. Relative humidity controls, e.g. cloud forma-
tion and aerosol optical properties and thus visibility. Mattis et al. ( 2002 )have,
therefore, tried to directly determine relative humidity profiles of the air from
Raman LIDAR measurements. For this purpose they combined in one instrument
the Raman-LIDAR techniques that are used for the profiling of water vapour and
temperature. Close agreement of the remotely measured profiles of relative humid-
ity and those measured with radiosonde demonstrates the potential of this approach.
A vertical resolution of 100 m in the boundary layer and a relative error of less than
10% seem achievable from this study.

4.3.3.3 Radiometer Measurements

The radiometer HATPRO (Pospichal and Crewell 2007 ), which is able to retrieve
temperature profiles (see Section 4.3.2.3 ), can be used to obtain boundary layer
humidity profiles, too, from an analysis of the signals from the seven channels
between 22.24 and 31.4 GHz. In contrast to the temperature soundings, no boundary
layer scans are performed for obtaining humidity profiles. Therefore, the verti-
cal resolution is several hundreds of meters only. Profiles shown in Rose et al.
( 2005 ) demonstrate that the error in absolute humidity measurements is about
10%. Like the Raman LIDAR described just before, HATPRO is able to simulta-
neously observe temperature and humidity profiles as well. This again allows for
the determination of relative humidity profiles in the boundary layer and the whole
troposphere.

4.3.3.4 Atmospheric Emitted Radiance Interferometer (AERI) and FTIR

An atmospheric emitted radiance interferometer (AERI) can measure vertical tem-
perature and humidity profiles with 10 min temporal and 100-200 m vertical
resolution. Figure 4.25 shows the passage of a dry line as an example. The accu-
racy of such humidity retrievals is approximately 5% of the absolute value when
compared with well-calibrated radiosonde data. The parallel retrievals of tempera-
ture and moisture profiles with this instrument allow for the assessment of the onset
of deep convection and the formation of thunderstorms (Feltz et al. 2003 ). AERI
retrievals show high potential, especially for retrieving humidity in the boundary
layer, where accuracies are on the order of 0.25-0.5 g m -3 for a central European

climate (Löhnert et al. 2009 ).
Due to a large vertical gradient and strong variability of water vapour, algorithms
that are effectively applied for ground-based remote sensing of many different atmo-
spheric trace gases (see below) can be insufficient for the retrieval of tropospheric
water vapour profiles. A review of the most important features of the retrieval and
of the radiative transfer modelling required for accurate monitoring of tropospheric