Atmospheric Research 75 (2005) 183–200 www.elsevier.com/locate/atmos A network suitable microwave radiometer for operational monitoring of the cloudy atmosphere Thomas Rosea,1, Susanne Crewellb,*, Ulrich Lo¨hnertb,2, Clemens Simmerc,3 aRadiometer Physics GmbH, Meckenheim, Birkenmaarstrasse 10, 53340 Meckenheim, Germany bMeteorological Institute, University of Munich, Theresienstr.37, 80333 Mu¨nchen, Germany cMeteorological Institute, University Bonn, Auf dem Hu¨gel 20, 53121 Bonn, Germany Received 1 February 2004; received in revised form 28 September 2004; accepted 3 December 2004 Abstract The implementation of an operational network of microwave radiometers is presently hampered by the cost and complexity of the available instruments. For this reason, the definition and design of a low-cost microwave radiometer suitable for automatic, high-quality observations of liquid water path (LWP) were one objective of the BALTEX cloud liquid water network: CLIWA-NET. In the course of the project, it turned out that a full profiling radiometer with 14 channels can be produced at only about 30% higher cost than a classical dual-channel IWV/LWP radiometer. The profiling capability allows simultaneous observations of LWP and the lower tropospheric (0–5 km) humidity and temperature profiles with a temporal resolution of less than 10 s and a vertical resolution from 100 m to 1 km in the planetary boundary layer depending on height and atmospheric conditions. The latter is possible due to an elevation scan capability and by the implementation of a new filter bank design. The radiometer has several additional sensors * Corresponding author. Tel.: +49 89 2180 4210; fax: +49 89 2805508. E-mail addresses: [email protected] (T. Rose)8 [email protected] (S. Crewell)8 [email protected] (U. Lo¨hnert)8 [email protected] (C. Simmer). 1 Tel.: +49 2225 99981 22; fax: +49 2225 99981 99. 2 Tel.: +49 89 2180 4574; fax: +49 89 2805508. 3 Tel.: +49 228 735181; fax: +49 228 735188. 0169-8095/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.atmosres.2004.12.005 184 T. Rose et al. / Atmospheric Research 75 (2005) 183–200 (temperature, humidity, pressure, rain detector and GPS) which guarantee, together with a flexible software package, the operational performance of the system with maintenance intervals of about every 3 months. The performance of the first prototype has been verified during a 3-week campaign at Cabauw, The Netherlands. D 2005 Elsevier B.V. All rights reserved. Keywords: Remote sensing; Microwave radiometry; Radiometer technology; Boundary layer profiling 1. Introduction The prototype of a European Cloud Observation Network (ECON) was implemented as part of CLIWA-NET (Crewell et al., 2002) by co-ordinating the use of existing, mostly operational, ground-based passive microwave radiometers. In total, 14 different radio- meters – most of them self-built by the partners – were used within the CLIWA-NET measurement campaigns. The radiometers were constructed with very different specifi- cations in terms of frequency, bandwidth, integration time, angular resolution and accuracy, e.g., in total about 50 frequencies were observed. Because of the different specifications, the generation of comparable liquid water path (LWP) products from the observed brightness temperatures (TB) proved to be a major effort. This became obvious when the results were used for the evaluation of atmospheric models (van Meijgaard and Crewell, 2005). The diversity of different microwave radiometers also reflects the fact that the implementation of an operational network of microwave radiometers is presently hampered by the cost of commercially available instruments. It was therefore a major objective of CLIWA-NET to develop a network suitable low-cost microwave radiometer for measuring LWP—the key variable within CLIWA-NET. The liquid water path can be estimated from atmospheric emission measurements in the microwave region since at these frequencies, the cloud contribution to the signal strongly increases with frequency (Fig. 1). Therefore, standard dual-channel systems measuring at two frequencies, with one close to the 22.235-GHz water vapour line and the other in a window region at higher frequencies, can simultaneously observe LWP and the integrated water vapour (IWV) (Westwater, 1978). The channel frequencies are selected in a way that the signal at the water vapour channel frequency has a nearly constant response to water vapour with height while the second channel is in an atmospheric window between the water vapour line and the strong oxygen complex at 60 GHz. Thus, the water vapour channel can be regarded as a correction term in the LWP retrieval and vice versa. Generally, the optimum frequency for IWV retrieval along the water vapour wing depends slightly on the site statistics and the beam elevation angle (Elgered, 1993). The frequencies are mostly chosen in (still) protected frequency bands reserved for passive remote sensing. LWP accuracy depends on the frequency combination, brightness temperature measure- ment accuracy and observation angle, and is roughly 25 g mÀ2; it can be improved to less than 15 g mÀ2 when an additional 90-GHz channel is used (Lo¨hnert and Crewell, 2003; Crewell and Lo¨hnert, 2003). By combining microwave radiometer-derived LWP and cloud radar measurements, profiles of liquid water content (LWC) can be retrieved (Frisch et al., 1998; Lo¨hnert et al., 2001, 2004). T. Rose et al. / Atmospheric Research 75 (2005) 183–200 185 Fig. 1. Brightness temperatures at microwave frequencies simulated for clear sky (solid lines) and standard atmospheric conditions. The effect of a cloud with LWP=250 g mÀ2 is shown by the dashed lines. Values are given for zenith observations (908) and for an elevation of 29.18. The different possible radiometer frequencies of HATPRO are marked by vertical lines. The blocks indicate the frequency range for humidity and temperature profiling. Profiles of water vapour and temperature can be derived from multi-spectral measurements at the wings of absorption lines or bands of water vapour and oxygen by so-called profilers, see for example (Solheim et al., 1998; Crewell et al., 2001; Ware et al., 2003). The vertical resolution of these profiles is about 1 km at 1 km height and degrades with increasing height (Gu¨ldner and Spa¨nkuch, 2001). The vertical resolution of the temperature profile in the boundary layer can be improved by performing elevation scans at relatively opaque frequencies (Westwater et al., 1999). Since TB varies only by a few degrees at these frequencies during one scan, the radiometer needs to have very low noise. In order to optimally exploit their information content, the brightness temperatures can be combined with auxiliary information (cloud radars, lidar ceilometer, ground measure- ments) within the Integrated Profiling Technique (IPT) (Lo¨hnert et al., 2004) to give simultaneous profiles of temperature, humidity and LWC. Within the design studies for the network suitable microwave radiometer cost estimates showed that a microwave profiler is only about 30% more expensive than a dual-channel system. In the following, we will focus on HATPRO (Humidity And Temperature Profiler), which has been designed as a profiling extension of the dual-channel system developed within the framework of CLIWA-NET. The paper is organized as follows: on the basis of the experiences gained in CLIWA- NET, we first describe the requirements for a network-suited radiometer in Section 2. The microwave radiometer design that fulfills these requirements and its auxiliary components are introduced together with the control software and the retrieval process in Section 3. The performance and first atmospheric observations of a prototype instrument are 186 T. Rose et al. / Atmospheric Research 75 (2005) 183–200 presented in Section 4. Before we conclude the paper in Section 6, we introduce the possibilities of HATPRO for boundary layer profiling. 2. Requirements and solutions Since personnel costs contribute strongly to the total operation costs, the most important requirement for the use of any instrument within an operational network is a low-maintenance level. To guarantee operation control and data access even when the radiometer is situated at a remote field site, a network connection should be implemented. On the other hand, data backup within the radiometer should be possible in case of external problems. The maintenance interval is basically determined by the absolute radiometer calibration. Since absolute calibrations frequently involve the handling of liquid nitrogen, a person at the site is required. This kind of calibration is explicitly necessary for optically thick frequency channels, e.g., along the oxygen line, which cannot be calibrated using the so-called tipping curve method (Han and Westwater, 2000). To achieve maintenance intervals of several months, the radiometer needs to be extremely well thermally stabilized. Exact timing of the measurements is necessary for almost all applications, in particular when different observations from different instruments will be combined. Therefore, synchronization of the radiometer to a standard time is necessary. If no time server can be used, GPS clocks are the preferred solution. The radiometer must be able to operate within the range of expected environmental conditions. In Europe, e.g., outdoor temperatures typically range from À30 8C to +45 8C. For stable operation in this range, the inner part of the radiometer needs to be well insolated and thermally stabilized with high accuracy (F0.1 K) (see also Section 2.2). The European upper temperature threshold might be too low for tropical applications, but by increasing the lower threshold and by taking higher power consumption into account, the upper threshold can be easily shifted by 10 K. In order to resist strong winds, a robust construction of the radiometer box and a solid stand is required. The weakest structural part of the radiometer is the radome through which microwave radiation enters the receiver.