The Atmospheric radiation measurement (ARM) program network of microwave radiometers: instrumentation, data, and retri vals

The Climate Research Facility of the US Department of Energy’s Atmospheric Radiation Measurement (ARM) Program operates a network of ground-based microwave radiometers. Data and retrievals from these instruments have been available to the scientific community for almost 20 yr. In the past five years the network has expanded to include a total of 22 microwave radiometers deployed in various locations around the world. The new instruments cover a frequency range between 22 and 197 GHz and are consistently and automatically calibrated. The latest addition to the network is a new generation of three-channel radiometers, currently in the early stage of deployment at all ARM sites. The network has been specifically designed to achieve increased accuracy in the retrieval of precipitable water vapor (PWV) and cloud liquid water path (LWP) with the long-term goal of providing the scientific community with reliable, calibrated radiometric data and retrievals of important geophysical quantities with well-characterized uncertainties. The radiometers provide high-quality, continuous datasets that can be utilized in a wealth of applications and scientific studies. This paper presents an overview of the microwave instrumentation, calibration procedures, data, and retrievals that are available for download from the ARM data archive. 1 Overview of the microwave radiometers network For nearly two decades the ARM Climate Research Facility has been operating a network of 2-channel (23.8 and 31.4 GHz) ground-based MicroWave Radiometers (MWR). The radiometers have provided decadal time series of precipitable water vapor (PWV) and cloud liquid water path (LWP) at the Atmospheric Radiation Measurement (ARM) Program fixed sites: Southern Great Plains (SGP, Oklahoma, USA), North Slope of Alaska (NSA, Alaska, USA), and the Tropical Western Pacific (TWP, Darwin, Manus Island, Nauru). In addition, ARM MWRs provided year-long time series during the Surface Heat Budget of the Arctic (SHEBA) campaign (Liljegren, 2000a), and during various ARM mobile facility deployments. These data are critically important because of the key role that water vapor and liquid water path play in the earth’s radiative budget (Turner et al., 2007a), in cloud-aerosol interaction (McComiskey et al., 2009) and in the climate system in general. Since their deployment, data from the MWRs have served as the reference for several ARM-sponsored water vapor studies (Revercomb et al., 2003; Mattioli et al., 2007), for comparisons of various water vapor measurement techniques involving sun photometers (Michalsky et al., 1995; Schmid et al., 2001) and the Global Positioning System (Keihm et al., 2002; Braun et al., 2003; Liou et al., 2001), as well as liquid water measurement techniques (Greenwald et al., 1999). Data from the MWRs have been widely used by the scientific community to improve gas spectroscopy in the microwave region (Liljegren et al., 2005; Payne et al., 2008; Marchand et al., 2003), to develop new retrievals of precipitable water vapor and liquid water path (Turner at al., 2007b; Turner, 2007), and for climate studies (Del Genio and Wolf, 2000; Doran et al., 2002). The MWRs also serve as the water vapor calibration reference for ARM-launched radiosondes (Turner et al., 2003; Cady-Pereira, 2008) and the operational Raman lidars at the SGP and TWP sites (Turner and Goldsmith, 1999). Consistencies in the instruments’ calibration (Liljegren, 2000b) and in the data quality control (Peppler et al., 2008) have been Published by Copernicus Publications on behalf of the European Geosciences Union. 2360 M. P. Cadeddu et al.: The ARM program network of microwave radiometers key elements to ensure the repeatability and continuity of the measurements and therefore the scientific relevance of the retrievals. Although data obtained with the 2-channel MWRs span a wide dynamic range of PWV and LWP with good sensitivity, spatial, and temporal resolution, their widespread use has revealed the need for improved measurements of PWV in very dry locations such as the Arctic (Westwater, 2001) and for improved measurements of LWP when thin liquid water clouds are present (Turner et al., 2007a). Accordingly, in recent years the ARM Program has added two MicroWave Radiometer High Frequency (MWRHF) units operating at 90 and 150 GHz, two G-band Vapor Radiometers (GVR and GVRP) operating in a frequency range between 170 and 197.3 GHz, and more recently the 3-channel radiometers (MWR3C) with frequencies at 23.834, 30, and 89 GHz. The choice of frequencies and design of the new radiometers has been driven by the scientific need to improve the accuracy in the retrieval of low amounts of integrated water vapor and cloud liquid water (Crewell and Löhnert, 2003; Racette et al., 2005; Cimini et al., 2007). The ARM infrastructure has also added two microwave temperature and humidity profilers (MWRP) with multiple frequencies between 22 and 60 GHz. Frequencies between 22 and 52 GHz are mostly sensitive to atmospheric water in vapor and liquid phase, frequencies between 51 and 60 GHz are sensitive to atmospheric temperature due to the absorption of atmospheric oxygen. A view of the microwave spectrum covered by the ARM radiometers is shown in Fig. 1. It is evident that the channels near 183.3 GHz are 10–100 times more sensitive to water vapor than the 23.8 GHz channel of the MWR. Similarly, frequencies near 90 and 150 GHz are 3–5 times more sensitive to liquid water than the 31.4 GHz channel of the MWR (Löhnert and Crewell, 2003). All ARM microwave radiometers are co-located with several active and passive sensors such as lidars, radars, and infrared interferometers. An array of surface meteorological sensors provides continuous measurements of surface temperature, pressure, and humidity. In addition, radiosondes are regularly launched multiple times a day at each site. A summary of the radiometric network with the locations and available retrievals is shown in Table 1. Some of the radiometers can be relocated for short-term campaigns upon request. For example the GVRP was recently deployed in a research vessel in the South East Pacific during the VAMOS OceanCloud-Atmosphere-Land Study (VOCALS) (Zuidema et al., 2011) and was then deployed to Chile for the Radiative Heating in Underexplored Band Campaign II (Turner and Mlawer, 2010). Three MWRs were recently employed in a cloud-tomography experiment at the Southern Great Plains site in Oklahoma (Huang et al., 2010). Data from all radiometers are daily reviewed by a Data Quality office (Peppler et al., 2008) that provides a first layer of data review. Additional data analysis ensures continuFig. 1. The electromagnetic spectrum covered by the ARM microwave radiometers for an atmosphere with PWV of 2 mm and liquid water path of 100 g m−2. Vertical lines indicate the frequency location of the instruments MWR and MWRP (light blue), MWRHF (yellow), GVR and GVRP (purple). ity in the calibration and provides reprocessing of the data as needed. Monthly reports on the instrument’s status are also available from the instrument’s web pages. Redundancy of instrumentation in several locations ensures meaningful comparison between independent measurements, therefore offering additional insurance on the quality of long-term instrument performance. It also provides continuity in the retrievals in case one instrument malfunctions. In the following sections, we provide an overview of the network, the calibration algorithms, data and retrievals currently available and under development. 2 Radiometers and their calibration To ensure high accuracy of the retrievals, the calibration of each of the radiometers is closely monitored. Records of automated and manual calibrations are kept in a calibration database and most calibration records are also recorded in the data files. In addition, raw data files containing detector voltages and diagnostic data are stored in the data archive and are available upon request and for recalibration purposes. To eliminate the introduction of operator’s judgment from the calibration process, all radiometers that employ tip curves (with the exception of the MWRP) use a self-calibration approach where acquired tip curves are continuously monitored and incorporated in the calibration algorithm with an automated process (Liljegren, 2000b). The algorithms are specifically written for the ARM radiometers and are documented in the instruments’ handbooks. With the automated algorithm all radiometers are calibrated in a consistent (if not identical) fashion despite differences in manufacturers and designs. Atmos. Meas. Tech., 6, 2359–2372, 2013 www.atmos-meas-tech.net/6/2359/2013/ M. P. Cadeddu et al.: The ARM program network of microwave radiometers 2361 Table 1. Location, frequency range, date range of the ARM microwave radiometers and currently available retrievals. MWR MWRP GVR GVRP MWRHF MWR3C Sites All NSA, AMF NSA NSA SGP, AMF All Center Frequencies 23.8, 31.4 22–60 183.3± 1,3,7,14 170–183.3 90, 150 23.8, 30, 90 (GHz) Number of channels 2 12 4 15 2 3 Retrievals PWV, LWP PWV, LWP, T/H Profiles PWV PWV PWV, LWP PWV, LWP Algorithms Statistical MWRRET Statistical NN NN MWRRET NN MWRRET Date Range 1993–present 2004–present 2006–present 2008–present 2008–present 2011–present 2.1 The MWR and MWRP The MWR, manufactured by Radiometrics Corp., is a twochannel microwave radiometer (WVR-1100 series) that operates at 23.8 and 31.4 GHz. The radiometer receiver is composed of a Gaussian optical antenna, a noise diode injection device, and two Gunn diode oscillators used for frequency selection. Each ARM fixed site and each mobile facility is equipped with one MWR (a total of 7 units). The instrument’s field-of-view varies between 5 and 6 degrees depending on the frequency. The MWRs rely entirely on tip curves for calibration. Tip curves are processed with a self-calibration algorithm that continuously monitors the gain and the receiver temperature as described in Liljegren (2000b). The algorithm applies real time corrections to account for the temperature dependence of the calibration and maintains a record of the most recent tip curves, but does not update the calibration every time a new tip curve is collected. Instead a dataset of recent successful tip curves are statistically analyzed and a median value is used to calibrate the brightness temperatures. This procedure avoids jumps in the brightness temperatures when the calibration is updated. The benefits of not updating the instantaneous tip curves are particularly evident in the highfrequency radiometers (Sect. 4). Figure 2 (top) shows one month of estimated instantaneous noise diode temperatures at the SGP. Figure 2 (bottom) shows the residual temperature dependence of the noise diode temperature (this dependence is estimated and accounted for in the calibration algorithm). During times when tip curves are not collected (mostly during prolonged times of cloudy conditions), the receiver gain is monitored through frequent viewing of the internal black body target. Noise diodes used in the radiometers are usually very stable and will work for months without drift. Therefore during times of reduced tip curves it is sufficient to calibrate the drift in the gain. If drifts in the noise diode occur data are recalibrated or flagged in Data Quality Reports (DQRs). The Root-Mean-Square (RMS) error of the calibrated brightness temperatures have been estimated to be ∼ 0.3 K (Liljegren, 2000b). The Microwave Radiometer Profiler (MWRP) manufactured by Radiometrics Corp. has 12 calibrated channels of which five (22–30 GHz) are sensitive to water vapor and Fig. 2. Self-calibration of the ARM MWRs. Top: brown points are instantaneous noise injection temperature values derived from tip calibration. The black solid line is the median calculated value and the dashed lines are 2 standard deviations of the measurements. Bottom: dependence of the noise diode temperature on ambient temperature. The data were collected at the SGP site during the month of January 2012. cloud liquid water and the remaining seven (50–60 GHz) cover a frequency range that is mostly sensitive to atmospheric temperature. The frequency tuning is achieved with a frequency synthesizer that cycles through the selected frequencies serially. Frequencies between 22 and 30 GHz are calibrated with tip curves that are monitored monthly and updated if necessary. Frequencies between 50 and 60 GHz are calibrated with liquid nitrogen every 3–4 months. Liquid nitrogen (LN2) calibration is a challenging aspect of the radiometers operations and the accuracy achieved by it is affected by several factors such as environment conditions and length of calibration (Solheim, 1993). LN2 calibration is less accurate where the channels are more transparent with uncertainty in the brightness temperature of approximately 1– 2 K. In addition, the LN2 calibration must be carried out with care by experienced personnel to avoid errors or injury. Two www.atmos-meas-tech.net/6/2359/2013/ Atmos. Meas. Tech., 6, 2359–2372, 2013 2362 M. P. Cadeddu et al.: The ARM program network of microwave radiometers MWRPs are part of the network: one is located at the NSA site, and the other is part of the first ARM mobile facility (AMF1).