20 research outputs found
The influence of cloud top variability from radar measurements on 3-D radiative transfer
International audienceIn radiative transfer simulations the simplification of cloud top structure by homogeneous assumptions can cause mistakes in comparison to realistic heterogeneous cloud top structures. This paper examines the influence of cloud top heterogeneity on the radiation at the top of the atmosphere. The use of cloud top measurements with a high temporal resolution allows the analysis of small spatial cloud top heterogeneities by using the frozen turbulence assumption for the time ? space conversion. Radiative observations are often based on satellite measurements, whereas small spatial structures are not considered in such treatments. A spectral analysis of the cloud top measurements showed slopes of power spectra between ?1.8 and ?2.0, these values are larger than the spectra of ?5/3 which is often applied to generate cloud field variability. The comparison of 3-D radiative transfer results from cloud fields with homogeneous and heterogeneous tops has been done for a single wavelength of 0.6 ?m. The radiative transfer calculations result in lower albedos for heterogeneous cloud tops. The differences of albedos between heterogeneous and homogeneous cloud top decrease with increasing solar zenith angle. The influence of cloud top variability on radiances is shown. The reflectances for heterogeneous tops are explicitly larger in forward direction, in backward direction lower. The largest difference of the mean reflectances (mean over cloud field) between homogeneous and heterogeneous cloud top is approximately 0.3, which is 30% of illumination
Combined vertical-velocity observations with Doppler lidar, cloud radar and wind profiler
Case studies of combined vertical-velocity measurements of Doppler lidar, cloud radar and wind profiler are presented. The measurements were taken at the Meteorological Observatory, Lindenberg, Germany. Synergistic products are presented that are derived from the vertical-velocity measurements of the three instruments: a comprehensive classification mask of vertically moving atmospheric targets and the terminal fall velocity of water droplets and ice crystals corrected for vertical air motion. It is shown that this combination of instruments can up-value the measurement values of each single instrument and may allow the simultaneous sensing of atmospheric targets and the motion of clear air
Combining cloud radar and radar wind profiler for a value added estimate of vertical air motion and particle terminal velocity within clouds
Vertical-stare observations from a 482âMHz radar wind profiler and a
35âGHz cloud radar are combined on the level of individual Doppler
spectra to measure vertical air motions in clear air, clouds and
precipitation. For this purpose, a separation algorithm is proposed to remove
the influence of falling particles from the wind profiler Doppler spectra and
to calculate the terminal fall velocity of hydrometeors. The remaining error
of both vertical air motion and terminal fall velocity is estimated to be
better than 0.1âmâsâ1 using numerical simulations. This
combination of instruments allows direct measurements of in-cloud vertical
air velocity and particle terminal fall velocity by means of ground-based
remote sensing. The possibility of providing a profile every 10âs
with a height resolution of â<â100âm allows further insight into the
process scale of in-cloud dynamics. The results of the separation algorithm
are illustrated by two case studies, the first covering a deep frontal cloud
and the second featuring a shallow mixed-phase cloud.</p
Aerosol backscatter profiles from ceilometers: validation of water vapor correction in the framework of CeiLinEx2015
With the rapidly growing number of automated single-wavelength backscatter
lidars (ceilometers), their potential benefit for aerosol remote sensing
received considerable scientific attention. When studying the accuracy of
retrieved particle backscatter coefficients, it must be considered that most
of the ceilometers are influenced by water vapor absorption in the spectral
range around 910 nm. In the literature methodologies have been proposed to correct for this
effect; however, a validation was not yet performed. In
the framework of the ceilometer intercomparison campaign CeiLinEx2015 in
Lindenberg, Germany, hosted by the German Weather Service, it was possible to
tackle this open issue. Ceilometers from Lufft (CHM15k and CHM15kx, operating
at 1064 nm), from Vaisala (CL51 and CL31) and from Campbell Scientific
(CS135), all operating at a wavelength of approximately 910 nm, were
deployed together with a multi-wavelength research lidar (RALPH) that served
as a reference. In this paper the validation of the water vapor correction is
performed by comparing ceilometer backscatter signals with measurements of
the reference system extrapolated to the water vapor regime. One inherent
problem of the validation is the spectral extrapolation of particle optical
properties. For this purpose AERONET measurements and inversions of RALPH
signals were used. Another issue is that the vertical range where validation
is possible is limited to the upper part of the mixing layer due to incomplete
overlap and the generally low signal-to-noise ratio and signal artifacts
above that layer. Our intercomparisons show that the water vapor correction
leads to quite a good agreement between the extrapolated reference signal and
the measurements in the case of CL51 ceilometers at one or more wavelengths
in the specified range of the laser diode's emission. This ambiguity is due
to the similar effective water vapor transmission at several wavelengths. In
the case of CL31 and CS135 ceilometers the validation was not always
successful. That suggests that error sources beyond the water vapor
absorption might be dominant. For future applications we recommend monitoring
the emitted wavelength and providing âdarkâ measurements on a regular
basis.</p
The infuence of cloud top variability from radar measurements on 3-D radiative transfer
International audienceIn radiative transfer simulations the simplification of cloud top structure by homogeneous assumptions can cause mistakes in comparison to realistic heterogeneous cloud top structures. This paper examines the influence of cloud top heterogeneity on the radiation at the top of the atmosphere. The use of cloud top measurements with a high temporal resolution allows the analysis of small spatial cloud top heterogeneities by using the frozen turbulence assumption for the time ? space conversion. Radiative observations are often based on satellite measurements, whereas small spatial structures are not considered in such treatments. A spectral analysis of the cloud top measurements showed slopes of power spectra between ?1.8 and ?2.0, these values are larger than the spectra of ?5/3 which is often applied to generate cloud field variability. The comparison of 3-D radiative transfer results from cloud fields with homogeneous and heterogeneous tops has been done for a single wavelength of 0.6 ?m. The radiative transfer calculations result in lower albedos for heterogeneous cloud tops. The differences of albedos between heterogeneous and homogeneous cloud top decrease with increasing solar zenith angle. The influence of cloud top variability on radiances is shown. The reflectances for heterogeneous tops are explicitly larger in forward direction, in backward direction lower. The largest difference of the mean reflectances (mean over cloud field) between homogeneous and heterogeneous cloud top is approximately 0.3, which is 30% of illumination
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Combining cloud radar and radar wind profiler for a value added estimate of vertical air motion and particle terminal velocity within clouds
Vertical-stare observations from a 482MHz radar wind profiler and a 35GHz cloud radar are combined on the level of individual Doppler spectra to measure vertical air motions in clear air, clouds and precipitation. For this purpose, a separation algorithm is proposed to remove the influence of falling particles from the wind profiler Doppler spectra and to calculate the terminal fall velocity of hydrometeors. The remaining error of both vertical air motion and terminal fall velocity is estimated to be better than 0.1ms-1 using numerical simulations. This combination of instruments allows direct measurements of in-cloud vertical air velocity and particle terminal fall velocity by means of ground-based remote sensing. The possibility of providing a profile every 10s with a height resolution of < 100m allows further insight into the process scale of in-cloud dynamics. The results of the separation algorithm are illustrated by two case studies, the first covering a deep frontal cloud and the second featuring a shallow mixed-phase cloud
Combined vertical-velocity observations with Doppler lidar, cloud radar and wind profiler
Case studies of combined vertical-velocity measurements of Doppler lidar,
cloud radar and wind profiler are presented. The measurements were taken at
the Meteorological Observatory, Lindenberg, Germany. Synergistic products are
presented that are derived from the vertical-velocity measurements of the
three instruments: a comprehensive classification mask of vertically moving
atmospheric targets and the terminal fall velocity of water droplets and ice
crystals corrected for vertical air motion. It is shown that this combination
of instruments can up-value the measurement values of each single instrument
and may allow the simultaneous sensing of atmospheric targets and the motion
of clear air