Ground-based remote sensing of precipitation using a multi-polarized FM-CW Doppler radar

Electrical Engineering, Mathematics and Computer Scienc

High-resolution polarimetric X-band weather radar observations at the Cabauw Experimental Site for Atmospheric Research

In 2007, the horizontally scanning polarimetric X-band radar IDRA (IRCTR Drizzle Radar) was installed on top of the 213 m high mast at the Dutch meteorological observatory Cabauw Experimental Site for Atmospheric Research (CESAR) at Netherlands. This radar complements a large variety of measurement instruments at CESAR by providing information on the horizontally spatial distribution and the temporal evolution of precipitation around the site. IDRA is a frequency-modulated continuous-wave radar developed at TU Delft's International Research Centre for Telecommunications and Radar (IRCTR). IDRA is designed to provide a high spatial resolution (down to 3 m in range) at a temporal resolution of 1 min. Its central frequency of 9.475 GHz, sensitive receivers with a large dynamic range, and the possibility to adjust the power of the transmitted signal permit IDRA to measure the whole spectrum of meteorological echoes from low-level clouds and drizzle to heavy convective rain. Similarly to most data collected at CESAR, also the data collected by IDRA are freely available for scientific purposes. IDRA data are stored at the Dutch 3TU.Datacentrum in order to make it easily accessible for everyone. In this article, we outline the IDRA dataset, including details on the data acquisition, processing, and possible applications.Geoscience & Remote SensingCivil Engineering and Geoscience

A framework for cloud - Aerosol interaction study

Aerosols can indirectly influence climate either by cloud albedo or lifetime effect. In order to have better understanding of these processes it is crucial to measure detailed vertical profiles of the radiative transfer and the microphysical evolution of clouds. Best results can be achieved by using advanced sensor synergy techniques. Essential remote sensing instruments used in this study include cloud radars and different types of lidar to obtain vertical structure of the atmosphere, as well as microwave radiometers and radiation sensors for improving the accuracy of the retrieved profiles. Several advanced combined retrieval algorithms will be used to quantify the physical characterization of water clouds and aerosols. Further work will be required on the understanding of the relation between cloud and aerosol.Geoscience and Remote ControlCivil Engineering and Geoscience

Novel method of drizzle formation observation at large horizontal scales using multi-wavelength satellite imagery simulation

The observations of on-board satellite imaging radiometers are representative of a far-reaching two-dimensional cloud top properties, however with a cutback in the capacity of profiling the cloud vertically. A combination of simulated radiances calculated at the top of the cloud in the near-infrared (IR) and thermal infrared part of the spectra, is used as a proxy to estimate in-cloud droplet growth stage and ongoing precipitation intensity at the water cloud base. We present a drizzle observational technique that is built on simulated satellite imaging radiometry via the EarthCARE SIMulator (ECSIM). A period of 40 hours of the modeled cloud field evolution (using Dutch Atmospheric Large Eddy Simulator - DALES) for the case study during the Atlantic Stratocumulus Transition Experiment campaign (ASTEX) is used to create a series of cloud scenes of a transitioning Sc into a Sc topped Cumulus (Cu) fields. Drizzle appears throughout the cloud evolution, evaporating on its way to the surface, depleting the cloud droplets at the cloud base. Longwave radiation model from ECSIM is applied to the ingested three-dimensional cloud scenes of a Stratocumulus evolution. The cloud top brightness temperatures are calculated using a three-dimensional, Monte Carlo, long-wave Radiative Transfer Model (RTM). The simulated Brightness Temperatures Difference (BTD) between the channels 3.9 and 11mm is then used to highlight the cloud top droplet size spatial variability, during the production of drizzle near the cloud base. The observed correlation of the BTD with the droplet size variability is used to interpret the conditions at which the precipitation at the cloud base is triggered or went through a change in the intensity. Tracking the process of evolution of the cloud droplet into a precipitating drop is likely due to the sensitivity of the 3.9mm channel to the particle size and cloud phase, near the cloud top. A comparison of the observed BTD with the vertically averaged cloud droplet size from the imported cloud scene is done. It is used to examine if a single pixel value of BTD from the cloud top (as retrieved via RTM) can be representative of the inhomogeneous microphysical vertical structure of a Sc deck and the potential precipitation intensity at the cloud base. A range in BTD for the two infra-red channels, between 0 and 2K is correlated to the presence of effective radius of the cloud droplets larger than 15mm, treated as drizzle drops in this paper.Geoscience & Remote SensingCivil Engineering and Geoscience

Ground-based remote sensing scheme for monitoring aerosol–cloud interactions (discussion)

A method for continuous observation of aerosol–cloud interactions with ground-based remote sensing instruments is presented. The main goal of this method is to enable the monitoring of cloud microphysical changes due to the changing aerosol concentration. We use high resolution measurements from lidar, radar and radiometer which allow to collect and compare data continuously. This method is based on a standardised data format from Cloudnet and can be implemented at any observatory where the Cloudnet data set is available. Two example study cases were chosen from the Atmospheric Radiation Measurement (ARM) Program deployment at Graciosa Island, Azores, Portugal in 2009 to present the method. We show the Pearson Product–Moment Correlation Coefficient, r, and the Coefficient of Determination, r2 for data divided into bins of LWP, each of 10 g m?2. We explain why the commonly used way of quantity aerosol cloud interactions by use of an ACI index (ACIr,? = dln re,?/dln?) is not the best way of quantifying aerosol–cloud interactions.Geoscience & Remote SensingCivil Engineering and Geoscience

Attenuation correction for a high-resolution polarimetric X-band weather radar

In 2007, IRCTR (Delft University of Technology) installed a new polarimetric X-band LFMCW radar (IDRA) at the meteorological observation site of Cabauw, The Netherlands. It provides plan position indicators (PPI) at a fixed elevation with a high range resolution of either 3 m or 30 m at a maximum observation range of 1.5 km and 15 km, respectively. IDRA aims to monitor precipitation events for the long-term analysis of the hydrological cycle. Due to the specifications of IDRA, the spatial and temporal variability of a large range of rainfall intensities (from drizzle to heavy convective rain) can be studied. Even though the usual observation range of IDRA is limited to 15 km, attenuation due to precipitation can be large enough to seriously affect the measurements. In this contribution we evaluate the application of a combined method to correct for the specific and the differential attenuation, and in the same vein estimate the parameters of the raindrop-size distribution. The estimated attenuations are compared to a phase constraint attenuation correction method.TelecommunicationsElectrical Engineering, Mathematics and Computer Scienc

Monitoring aerosol-cloud interactions at the CESAR Observatory in the Netherlands

The representation of aerosol-cloud interaction (ACI) processes in climate models, although long studied, still remains the source of high uncertainty. Very often there is a mismatch between the scale of observations used for ACI quantification and the ACI process itself. This can be mitigated by using the observations from groundbased remote sensing instruments. In this paper we presented a direct application of the aerosol-cloud interaction monitoring technique (ACI monitoring). ACI monitoring is based on the standardised Cloudnet data stream, which provides measurements from ground-based remote sensing instruments working in synergy. For the data set collected at the CESAR Observatory in the Netherlands we calculate ACI metrics. We specifically use attenuated backscatter coefficient (ATB) for the characterisation of the aerosol properties and cloud droplet effective radius (re) and number concentration (Nd) for the characterisation of the cloud properties. We calculate two metrics: ACIr Dln(re)/ln(ATB) and ACIN Dln(Nd)/ln(ATB). The calculated values of ACIr range from 0.001 to 0.085, which correspond to the values reported in previous studies. We also evaluated the impact of the vertical Doppler velocity and liquid water path (LWP) on ACI metrics. The values of ACIr were highest for LWP values between 60 and 105 gm-2. For higher LWP other processes, such as collision and coalescence, seem to be dominant and obscure the ACI processes. We also saw that the values of ACIr are higher when only data points located in the updraught regime are considered. The method presented in this study allow for monitoring ACI daily and further aggregating daily data into bigger data sets.Atmospheric Remote SensingGeoscience and Remote Sensin

Towards the improvement of cloud microphysical retrievals using simultaneous Doppler and polarimetric radar measurements

Radar-based retrievals are often employed to characterize the microphysical properties of cloud hydrometeors, i.e. their phases, habits, densities as well as their respective size and orientation distributions. These techniques are based on a synergetic use of different cloud observation sensor(s) and microphysical model(s) where the information extracted from both sensors and models is combined and converted into microphysical cloud properties. However, the amount of available information is often limited, which forces current microphysical retrieval techniques to base their algorithms on several microphysical assumptions which affect the retrieval accuracy. By simultaneously combining Doppler and polarimetric measurements obtained from fully Doppler polarimetric radars, it is possible to create spectral polarimetric parameters. Although these parameters are easily contaminated with unwanted echoes, this work shows that, from a correct radar signal processing based on filtering and averaging techniques, spectral polarimetric parameters can be correlated to microphysical cloud properties. In particular, preliminary results suggest that particle orientations and habits can be determined from the sole use of such spectral polarimetric parameters. Therefore, such additional spectral polarimetric information offers an opportunity to improve current microphysical retrievals by reducing the number of microphysical assumptions in them.TelecommunicationsElectrical Engineering, Mathematics and Computer Scienc

Narrow-Band Clutter Mitigation in Spectral Polarimetric Weather Radar

In this paper, a new clutter suppression method, named the moving double spectral linear depolarization ratio (MDsLDR) filter, is put forward to mitigate narrow-band clutter in weather radars. The narrow-band clutter observed in the Doppler domain includes: 1) stationary clutter such as ground clutter and 2) nonstationary clutter such as artifacts caused by the radar system itself or external sources. These artifacts are difficult to remove, because they are not confined to specific azimuth and range bins. Based on the difference of the spectral-polarization feature and the spectral continuity of precipitation and clutter, the MDsLDR filter can remove ground clutter, artifacts, and noise. The performance of the newly proposed filter is assessed by data collected by the Doppler-polarimetric IRCTR Drizzle Radar. Three precipitation cases are considered in this paper: moderate/light precipitation, convective precipitation with hook-echo signature, and light precipitation with severe artifact contamination. Furthermore, the implementation of the MDsDLR filter requires relatively low computation complexity, so that the MDsLDR filter can be operated in real time.Atmospheric Remote SensingGeoscience and Remote Sensin