In recent years attention was increasingly paid to backscatter profiles of
ceilometers as a new source of aerosol information. Several case studies have
shown that – although originally intended for cloud detection only – ceilometers
can provide the planetary boundary layer height and even
quantitative information such as the aerosol backscatter coefficient

In the last few years a large number of autonomous single-wavelength
backscatter lidars, so called ceilometers, has been installed. Several
ceilometer networks are operated by national weather services on a 24/7 basis

Being aware of the significance of aerosols for radiation, cloud physics and
air quality, the discussion recently came up if optical properties can be
derived quantitatively from ceilometers. If so, ceilometer networks might
help to fill observational gaps; this information could be of benefit
particularly taking into account that aerosols are highly variable in time and
space. As a consequence, recently first attempts were made to retrieve
aerosol optical properties.

Ceilometers typically are working in the near IR, either at 1064 nm
(e.g., Jenoptik, Lufft) or at 905–910 nm (e.g., Vaisala, Eliasson, Campbell). Both
wavelengths are in a spectral range where aerosol scattering is clearly
dominating scattering by air molecules (Rayleigh scattering). This
facilitates the detection of aerosol layers but raises problems of the signal
calibration that is required to derive

To our knowledge,

In the following section we give a short outline of different solutions of
the lidar equation applicable in case of water vapor absorption. The
sensitivity to atmospheric and instrumental parameters is investigated in a
general way by means of high spectral resolution calculations of absorption
on the basis of standard atmospheres. This constitutes the basis to develop a
methodology (henceforward referred to as WAPL) for routine evaluation of
ceilometer data; a detailed discussion is provided in Sect. 4. Section 5
includes a case study based on simulations and real measurements to
demonstrate the basic features of WAPL, and a robust estimate of the order of
magnitude of the systematic error of

For aerosol remote sensing by means of a backscatter lidar elastic scattering
is considered, i.e., the emitted and received wavelengths

The lidar equation with water vapor absorption (Eq.

If the range integration is done towards the lidar, slightly different
equations are found: for

Two additional approaches are based on a simple re-arrangement of the lidar Eq. (

Wavelength-dependence of optical properties: extinction coefficients
are given in km

Which of the four formulations is applied, mainly depends on the ceilometer signal and the meteorological conditions, e.g., a backward solution typically can only be applied to nighttime measurements and long temporal averages. The remaining choice between option 1 vs. option 3 or option 2 vs. option 4 is more or less a matter of personal preference.

The solution for

If the absolute calibration is applied, the lidar equation is solved for

As already mentioned the lidar equation is expressed as a monochromatic
formulation. If this approach should be used for ceilometers with an emitted
spectrum of a few nanometers width within a water vapor absorption band, the
wavelength dependence of the optical properties must be investigated. In
particular,

For such small wavelength intervals the spectral dependence of

The spectral variation of

As a consequence only the wavelength-dependence of the water vapor absorption
must be considered. If we understand the ceilometer measurement as a series
of pulses at

For the determination of the effective absorption coefficients of water vapor

The maximum height of the model atmosphere was set to 10 km which is certainly sufficient in view of the measurement range of ceilometers.

To investigate the influence of different water vapor distributions we use
six standard atmospheric models (US-standard, mid-latitude summer and winter,
subarctic summer and winter, tropical;

Squared water vapor transmission

It shows

For the analysis of actual ceilometer data a better vertical resolution than
that of the standard atmospheres is desired resulting in a significant
increase of computing time and mass storage. For compensation a reduction of
the spectral resolution of ARTS could be an option. As a consequence, we have
repeated all calculations with different spectral resolutions using

We conclude that for correcting ceilometer data influenced by water vapor absorption the following information – ordered by decreasing relevance – is essential: the water vapor concentration profile, the central wavelength of the diode laser and the width of its spectrum.

Effective water vapor absorption coefficient

As already stated in Sect. 2, profiles of

As input for our water vapor absorption calculations we choose data from all
radiosonde ascents of the year 2012 at Oberschleißheim (8 km northwest of
the ceilometer site in Munich, WMO station identifier 10868), that reached at
least 10 km above the ground. This results in a total number of 647 profiles
of

Two extreme cases are shown in Fig.

Thus, it can be expected that it is sufficient for the water vapor correction
to consider at each wavelength one profile of

On the basis of these findings a water vapor correction scheme for routine
evaluation of ceilometer data, named “WAPL”, has been developed: from
radiosonde profiles or numerical models the water vapor number density

Vertical profile of the water vapor absorption cross section

To characterize the accuracy of the effective transmission we define a
function

An example of the performance of WAPL, expressed in terms of

Flow chart of WAPL: correction for water vapor absorption in the
determination of the aerosol backscatter coefficient

This conclusion remains valid if we reduce the vertical resolution of the
stored

Deviation of the approximative squared transmissions

To investigate the validity of WAPL and to test whether one archived set of
annual averages of water vapor absorption cross sections

To illustrate the results in comparison to the mid-latitude station at
Oberschleißheim, Fig.

Same as Fig.

The accuracy of WAPL is again expressed in terms of

It has been shown that annual mean absorption cross sections of water vapor
can be used for routine applications. We want to briefly discuss whether it
is necessary to provide

It can be seen (left panel) that using the mid-latitude absorption cross
sections for the tropics results in an overestimation of the effective
transmission. As a result of the previous discussion only the spectral
resolution of

Deviation of the approximative squared transmissions

For the sake of completeness we have also investigated the inverse effect,
i.e., the

We conclude that

The main objective of this section is to show the magnitude of the error of
the particle backscatter coefficient

To get realistic conclusions, it is reasonable to use measurements of a CL51
ceilometer. Measurements were available for a limited period of a few months
in 2012 and took place at the roof platform of the Meteorological Institute
of the Ludwig-Maximilians-Universität in Munich (48.148

Left panel: range corrected signal

For illustration, measurements of 17 March 2012 are shown in
Fig.

From Fig.

Retrieved particle backscatter coefficient at

As it is beyond the scope of this paper to discuss the quality of ceilometer
data – according to our experience from other studies the signal-quality may
change even between instruments of the same model – and how measurement
errors possibly might be corrected, we decided to model realistic ceilometer
signals (influenced by water vapor) based on the CHM15kx-measurements. So we
can consider profiles of aerosol properties that are realistic for the site
(note the very good qualitative agreement of the two profiles in the boundary
layer), we can test the forward and the backward solution (in particular as
we can set

For the assessment of the influence of the water vapor absorption we at first
retrieve

Two representative results of the retrieved

Ratio of the retrieved

Additional insight in the effect of water vapor absorption on the retrieval
is given in Fig.

Same as Fig.

If the same investigations are carried out for different aerosol loads – for
this purpose we have multiplied the particle backscatter coefficient by a
factor of 0.5 and 2.0, respectively – our results in general are confirmed.
Under clear conditions (

By using a “simulated measurement” we can also apply the backward integration
with

Same as Fig.

Corresponding to the above discussion we also show ratios of the two
retrievals

Analogously to the previous discussion we have included in
Fig.

The case study discussed in the previous section – based on 17 March 2012 – was
carried out for a relatively low water vapor content. However, as the
water vapor distribution shows a large spatial and temporal variability, it
is not justified to understand the results from a single case as a generally
applicable number for the magnitude of the

Ratio

These results confirm that it is not possible to find one generally
applicable value for the

If ceilometers, emitting radiation in the spectral range around 905–910 nm, shall be used to derive aerosol optical properties in a quantitative way, the signal must be corrected for water vapor. In this paper a methodology named WAPL is introduced that does not require time-consuming absorption calculations with high spectral resolution but relies on tabulated water vapor absorption cross sections. It has been shown that the inherent error of this approximation is in the order of only 0.3 % and thus virtually negligible. Furthermore, it has been demonstrated that these tabulated values need not necessarily be determined for the site of the ceilometer; this might allow to use one data base for a group of ceilometers in similar environments (climate zone, altitude). The corresponding error is expected to be below 1 %.

To apply WAPL, the emitted spectrum of the laser diode should be known or at
least a reasonable estimate must be available. It has been shown that the
water vapor correction is more accurate at wavelengths around 910 nm than
around 905 nm. The uncertainties of the water vapor correction are, however,
significantly smaller than the error if water vapor is totally ignored as has
usually been the case in the past. To benefit from WAPL as much as possible
the laser spectrum should be provided by the manufacturer; if the laser
source is not temperature-stabilized the emitted wavelengths should be
specified as a function of temperature. In this paper we have assumed a
Gaussian curve with a FWHM

Same as Fig.

Due to the large temporal and spatial variability of the water vapor
distribution it is not possible to give a unique number of the error when
water vapor absorption is not taken into account. To get an indication of its
magnitude we have studied a number of simulations based on a realistic
aerosol distribution in Munich and water vapor distributions over 1 year in
Munich and a tropical site. It was found that the systematic error of the
aerosol backscatter coefficient

Finally we again want to emphasize that ceilometers were primarily designed for the determination of cloud heights, and that attempts to retrieve aerosol properties only came up very recently. We expect that in the near future the number of unattended and automated ceilometers and backscatter lidars will further grow, and that the hardware and proprietary software might be adapted “towards aerosols”. As a consequence it can be anticipated that a significant data set of signals at wavelengths around 910 nm will be available for aerosol remote sensing. Then, water vapor correction must be an indispensable part of any data evaluation scheme.

We gratefully acknowledge Klaus Schäfer and Carsten Jahn (Karlsruher
Institut für Technologie (KIT) – IMK-IFU) for the provision of the CL51
ceilometer during several months in 2012. The radiosonde data were downloaded
from