A configuration of the High-Altitude Long-Range Research Aircraft (HALO) as a remote sensing cloud observatory is described, and its use is illustrated with results from the first and second ...Next-Generation Aircraft Remote Sensing for Validation (NARVAL) field studies. Measurements from the second NARVAL (NARVAL2) are used to highlight the ability of HALO, when configured in this fashion, to characterize not only the distribution of water condensate in the atmosphere, but also its impact on radiant energy transfer and the covarying large-scale meteorological conditions—including the large-scale velocity field and its vertical component. The NARVAL campaigns with HALO demonstrate the potential of airborne cloud observatories to address long-standing riddles in studies of the coupling between clouds and circulation and are helping to motivate a new generation of field studies.
A 35-GHz radar has been operating at the Meteorological Observatory Lindenberg (Germany) since 2004, measuring cloud parameters continuously. The radar is equipped with a powerful magnetron ...transmitter and a high-gain antenna resulting in a high sensitivity of -55 dBZ at 5-km height for a 10-s averaging time. The main purpose of the radar is to provide long-term datasets of cloud parameters for model evaluation, satellite validation, and climatological studies. Therefore, the system operates with largely unchanged parameter settings and a vertically pointing antenna. The accuracy of the internal calibration (budget calibration) has been appraised to be 1.3 dB. Cloud parameters are derived by two different approaches: macrophysical parameters have been deduced for the complete period of operation through combination with ceilometer measurements; a more enhanced target classification and the calculation of liquid and ice water contents are realized by algorithms developed in the framework of the European CloudNet project.
This study gives a summary of lessons learned during the absolute calibration
of the airborne, high-power Ka-band cloud radar HAMP MIRA on board
the German research aircraft HALO. The first part ...covers the internal
calibration of the instrument where individual instrument components are
characterized in the laboratory. In the second part, the internal calibration
is validated with external reference sources like the ocean surface
backscatter and different air- and spaceborne cloud radar instruments. A key component of this work was the characterization of the spectral
response and the transfer function of the receiver. In a wide dynamic range
of 70 dB, the receiver response turned out to be very linear
(residual 0.05 dB). Using different attenuator settings, it covers
a wide input range from −105 to −5 dBm. This
characterization gave valuable new insights into the receiver sensitivity
and additional attenuations which led to a major improvement of the absolute
calibration. The comparison of the measured and the previously estimated
total receiver noise power (−95.3 vs. −98.2 dBm)
revealed an underestimation of 2.9 dB. This underestimation could
be traced back to a larger receiver noise bandwidth of 7.5 MHz
(instead of 5 MHz) and a slightly higher noise figure
(1.1 dB). Measurements confirmed the previously assumed antenna
gain (50.0 dBi) with no obvious asymmetries or increased side lobes.
The calibration used for previous campaigns, however, did not account for a
1.5 dB two-way attenuation by additional waveguides in the airplane
installation. Laboratory measurements also revealed a 2 dB higher
two-way attenuation by the belly pod caused by small deviations during
manufacturing. In total, effective reflectivities measured during previous
campaigns had to be corrected by +7.6 dB. To validate this internal calibration, the well-defined ocean surface
backscatter was used as a calibration reference. With the new absolute
calibration, the ocean surface backscatter measured by HAMP MIRA agrees very
well (<1 dB) with modeled values and values measured by the GPM
satellite. As a further cross-check, flight experiments over Europe and the
tropical North Atlantic were conducted. To that end, a joint flight of
HALO and the French Falcon 20 aircraft, which was equipped with the RASTA cloud
radar at 94 GHz and an underflight of the spaceborne CloudSat at
94 GHz were performed. The intercomparison revealed lower
reflectivities (−1.4 dB) for RASTA but slightly higher
reflectivities (+1.0 dB) for CloudSat. With effective
reflectivities between RASTA and CloudSat and the good agreement with GPM,
the accuracy of the absolute calibration is estimated to be around
1 dB.
Abstract
This paper presents an experimental analysis of the antenna system effects on polarimetric measurements conducted with cloud radars operating in the linear depolarization ratio (LDR) mode. ...Amplitude and phase of the copolar and cross-polar antenna patterns are presented and utilized. The patterns of two antennas of different quality were measured at the Hungriger Wolf airport near Hohenlockstedt, Germany, during the period from 28 January to 1 February 2014. For the measurements a test transmitter mounted on a tower and the scanning 35-GHz (Ka band) cloud radar MIRA-35, manufactured by METEK GmbH and operated in the receiving mode, were used. The integrated cross-polarization ratios (ICPR) are calculated for both antennas and compared with those measured in light rain. Correction algorithms for observed LDR and the co-cross-channel correlation coefficient
ρ
are presented. These algorithms are aimed at removing/mitigating polarization cross-coupling effects that depend on the quality of radar hardware. Thus, corrected LDR and
ρ
are primarily influenced by scatterer properties. The corrections are based on the decomposition of the coherency matrix of the received signals into fully polarized and nonpolarized components. The correction brings LDR values and the co-cross-channel correlation coefficients from two radars with different antenna systems to a close agreement, thus effectively removing hardware-dependent biases. Uncertainties of the correction are estimated as 3 dB for LDR in the range from −30 to −10 dB. In clouds, the correction of the co-cross-channel correlation coefficient
ρ
results in near-zero values for both vertically pointed radars.
Cloud Doppler radars are increasingly used to study cloud and precipitation microphysical processes. Typical bulk cloud properties such as liquid or ice content are usually derived using the first ...three standard moments of the radar Doppler spectrum. Recent studies demonstrated the value of higher moments for the reduction of retrieval uncertainties and for providing additional insights into microphysical processes. Large effort has been undertaken, e.g., within the Atmospheric Radiation Measurement (ARM) program to ensure high quality of radar Doppler spectra. However, a systematic approach concerning the accuracy of higher moment estimates and sensitivity to basic radar system settings, such as spectral resolution, integration time and beam width, are still missing. In this study, we present an approach on how to optimize radar settings for radar Doppler spectra moments in the specific context of drizzle detection. The process of drizzle development has shown to be particularly sensitive to higher radar moments such as skewness. We collected radar raw data (I/Q time series) from consecutive zenith-pointing observations for two liquid cloud cases observed at the cloud observatory JOYCE in Germany. The I/Q data allowed us to process Doppler spectra and derive their moments using different spectral resolutions and integration times during identical time intervals. This enabled us to study the sensitivity of the spatiotemporal structure of the derived moments to the different radar settings. The observed signatures were further investigated using a radar Doppler forward model which allowed us to compare observed and simulated sensitivities and also to study the impact of additional hardware-dependent parameters such as antenna beam width. For the observed cloud with drizzle onset we found that longer integration times mainly modify spectral width (Sw) and skewness (Sk), leaving other moments mostly unaffected. An integration time of 2 s seems to be an optimal compromise: both observations and simulations revealed that a 10 s integration time – as it is widely used for European cloud radars – leads to a significant turbulence-induced increase of Sw and reduction of Sk compared to 2 s integration time. This can lead to significantly different microphysical interpretations with respect to drizzle water content and effective radius. A change from 2 s to even shorter integration times (0. 4 s) has much smaller effects on Sw and Sk. We also find that spectral resolution has a small impact on the moment estimations, and thus on the microphysical interpretation of the drizzle signal. Even the coarsest spectral resolution studied, 0. 08 ms−1, seems to be appropriate for calculation moments of drizzling clouds. Moreover, simulations provided additional insight into the microphysical interpretation of the skewness signatures observed: in low (high)-turbulence conditions, only drizzle larger than 20 µm (40 µm) can generate Sk values above the Sk noise level (in our case 0.4). Higher Sk values are also obtained in simulations when smaller beam widths are adopted.
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