This thesis explores the marine trade-wind convection and the clouds forming within by using spatial-high-resolution airborne remote sensing observations taken from the German High Altitude and LOng range research aircraft (HALO). The nadir-pointing HALO Microwave Package (HAMP) is the central tool of this thesis. HAMP comprises a cloud radar and a 26-channel microwave radiometer (MWR, 22–183 GHz), for which the atmosphere and clouds are semitransparent. The shallow cumulus clouds, like they regularly occur in the trade-wind region, are of particular interest for better understanding the climate. Several studies (e.g., Bony and Dufresne, 2005; Schneider et al., 2017) identified such clouds as a main source of model spread in climate projections. The challenge of this kind of ubiquitous clouds in the models is partly due to large spread in global observations which can be related to the small scale of shallow cumuli and the coarse-scale observations from satellites. This thesis combines three studies around HAMP from the characterization of the HAMP MWR, over the development of MWR retrievals for liquid clouds to the application by evaluating two cloud-resolving simulations.
The HAMP MWR is characterized by investigating the random noise of each channel, the covariance within each of the five frequency bands, the brightness temperature (BT) offset, the offset stability, and by suggesting an offset correction. The offset and stability of the HAMP BT acquisitions are studied by comparing the measured BTs to synthetic measurements based on forward-simulated dropsondes. Offsets between −11 and +6 K show a spectral dependency, which repeatedly appears but is shifted between flights. The offsets are most likely caused by uncertainties in the calibration method and changing environmental conditions of the MWR in the belly pod during take-off and ascending. However, an offset correction based on the dropsondes can be developed for each channel as a function of the flight.
To better interpret the HAMP BT observations, novel retrievals are developed based on a realistic database of synthetic measurements and corresponding atmospheric profiles. Retrievals of the liquid water path (LWP), rainwater path (RWP), and integrated water vapor (IWV) are developed to describe the clouds and their environment. The retrieved IWV using the offset-corrected BTs agrees with coincident dropsondes and water vapor lidar measurements by 1.4 kg/m² . The theoretical assessment of LWP shows that the LWP error is below 20 g/m² for LWP below 100 g/m² . The absolute LWP error increases with increasing LWP, but the relative error decreases from 20 % at 100 g/m² to 10 % at 500 g/m². The RWP retrieval, which uses the radar in addition to the MWR, can reliably detect RWP larger than 10 g/m² with a Gilbert skill score > 0.75.
The retrieval results are summarized in a comparison of the clouds and their moisture environment in the two tropical seasons, which are represented by the field experiments in December 2013 (dry season) and in August 2016 (wet season). Clouds were more frequent, and their average LWP and RWP were higher in the dry season than in the wet season. However, deeper convection with the formation of large frozen particles was less frequent in the dry season. It is hypothesized, that the lower degree of cloud organization in the dry season led to smaller systems with more overall cloud cover. The higher degree of randomness in the dry season comes along with less extremes and is reflected by a narrower distribution of IWV. The variability between (especially the wet-season) flights shows, how statistics from airborne campaigns are affected by the choice of the individual flight pattern.
The more homogeneous and cloudy statistics of the dry season are used to assess the representation of shallow cumulus convection and the cloud formation over the ocean in two cloud-resolving simulations generated with the ICON model. The HAMP radar and a backscatter lidar are used for detecting cloud top height (CTH), base height, and precipitation, and the MWR stratifies the cases by LWP. Forward simulators are used to derive the same measurements synthetically from the model data while applying the same instrument-specific cloud-detection thresholds. The analysis reveals a bimodal structure of the CTH. The lower mode relates to boundary layer driven clouds, while the upper mode is driven by moist shallow convection, trapped under the trade inversion at about 2.3 km above sea level. The storm-resolving model (SRM) with 1.25 km horizontal grid spacing resolves the two cloud layers to a limited extend. Most CTHs in the SRM are above the observed lower CTH mode, and top height increases with LWP. The second model with a 300 m grid (large-eddy model, LEM) represents better the observed bimodal distribution of CTH. However, the microphysical schema of neither model can produce in-cloud drizzle-sized particles that were often observed by the radar. This application study shows, how HAMP on HALO provides insightful data to help closing the uncertainty in the models, if interpreted thoroughly
