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Cloud-Base Height Derived from a Ground-Based Infrared Sensor and a Comparison with a Collocated Cloud Radar

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Cloud-Base Height Derived from a Ground-Based Infrared Sensor and a Comparison with a Collocated Cloud Radar VOLUME 35 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY APRIL 2018 Cloud-Base Height Derived from a Ground-Based Infrared Sensor and a Comparison with a Collocated Cloud Radar ZHE WANG Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, CMA Key Laboratory for Aerosol–Cloud–Precipitation, and School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing, and Training Center, China Meteorological Administration, Beijing, China ZHENHUI WANG Collaborative Innovation Center on Forecast and Evaluation of Meteorological Disasters, CMA Key Laboratory for Aerosol–Cloud–Precipitation, and School of Atmospheric Physics, Nanjing University of Information Science and Technology, Nanjing, China XIAOZHONG CAO,JIAJIA MAO,FA TAO, AND SHUZHEN HU Atmosphere Observation Test Bed, and Meteorological Observation Center, China Meteorological Administration, Beijing, China (Manuscript received 12 June 2017, in final form 31 October 2017) ABSTRACT An improved algorithm to calculate cloud-base height (CBH) from infrared temperature sensor (IRT) observations that accompany a microwave radiometer was described, the results of which were compared with the CBHs derived from ground-based millimeter-wavelength cloud radar reflectivity data. The results were superior to the original CBH product of IRT and closer to the cloud radar data, which could be used as a reference for comparative analysis and synergistic cloud measurements. Based on the data obtained by these two kinds of instruments for the same period (January–December 2016) from the Beijing Nanjiao Weather Observatory, the results showed that the consistency of cloud detection was good and that the consistency rate between the two datasets was 81.6%. The correlation coefficient between the two CBH datasets reached 0.62, based on 73 545 samples, and the average difference was 0.1 km. Higher correlations were obtained for thicker clouds with a larger echo intensity. A low-level thin cloud cannot be regarded as a blackbody because of its high transmittance, which results in higher CBHs derived from IRT data. Because of a smaller cloud radiation effect for high-level thin cloud above 8 km, the contribution of the atmospheric downward radiation below the cloud base to the IRT cannot be ignored, as it results in lower CBHs derived from IRT data. Owing to the seasonal variation of atmospheric downward radiation reaching the IRT, the difference between the two CBHs also has a seasonal variation. The IRT CBHs are generally higher (lower) than the cloud radar CBHs in winter (summer). 1. Introduction height for climate statistical analysis in an attempt to identify signs of climate change (Ramanathan et al. 1989; Clouds are of great interest to atmospheric science re- Poli et al. 2000). Cloud height is also a key factor, such as search, as they have a significant impact on atmospheric in meteorological forecasting, civil aviation, weather dynamics and thermal processes, water vapor cycling, and modification, and so on (Zhou and Zhao 2008; Yan et al. the radiation balance at the surface (Cess et al. 1989), 2012). Therefore, accurate and timely access to long-term which is a key driver of climate change (Naud et al. 2003; and continuous cloud height data is very important. Hawkinson et al. 2005; Stephens 2005). Many studies have Observations of clouds are currently made by ground- been conducted using long-term observations of cloud based, sounding, or satellite-based remote sensing techniques. Each method has its own strengths and Corresponding author: Zhe Wang, [email protected] weaknesses related to the method of observation, DOI: 10.1175/JTECH-D-17-0107.1 Ó 2018 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). 689 Unauthenticated | Downloaded 09/27/21 06:47 AM UTC 690 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 35 instrument performance, and the methods used for cal- radiation information. Although the observation band is culation and retrieval. It is therefore important to study in the atmospheric window, the accuracy of cloud mea- synergistic cloud measurements to improve cloud ob- surements is influenced by a number of factors. It is clear servations and to verify the results of different methods that the impact of water vapor is significant. Many to improve their reliability (Lü et al. 2003; Lu et al. studies on water vapor measurements of clear sky show 2012). As observations have gradually become more that water vapor content in the atmosphere is highly automated, the development of ground-based remote correlated with clear-sky infrared radiation, including sensing cloud equipment and technology has gradually the atmospheric window (Shaw et al. 2005; Maghrabi replaced traditional observations, and it has provided and Clay 2010; Mims et al. 2011). The long-wave in- more possibilities for synergistic cloud measurements. frared (LWIR) window has ozone absorption centered The current ground-based cloud remote sensing equip- at 9.6 mm, but the overall envelope (atmospheric trans- ment includes ceilometers, millimeter-wavelength cloud mittance vs wavelength) of the LWIR transmission radars, infrared/visible cloud imagers, and microwave window is governed by the amount of water vapor in the radiometers. Many studies have evaluated and com- atmosphere at any given time and place (Shaw and pared the capabilities of these instruments for cloud Nugent 2013). Thin clouds, which cannot be regarded as height measurement (Lhermitte 1987; Zhong et al. 2009; an equivalent blackbody because of their high trans- Gao et al. 2010; Huang et al. 2013; Costa-Surós et al. parency, also make the accuracy of cloud infrared radi- 2014; Oh et al. 2016). ation measurements questionable (Ahn et al. 2015). Microwave radiometers are mainly used for the Aerosols usually have little influence on the thermal measurement of atmospheric temperature and humidity infrared cloud signature, but water vapor condensed on profiles, water vapor content, and cloud water content sulfate and other hygroscopic aerosols can significantly (Ware et al. 2003). For microwave remote sensing, increase the aerosol optical thickness of the atmosphere clouds have a negative effect on the retrieval of atmo- (Tang 1996). Therefore, it is necessary to evaluate the spheric temperature and humidity profiles (Han and accuracy of IRT CBH measurements. Westwater 1995; Hewison 2007). Therefore, a micro- Millimeter-wavelength cloud radar is able to detect wave radiometer is typically equipped with an infrared small particles, such as cloud, fog, and dust storms. Since temperature sensor (IRT) for obtaining cloud-base the birth of the first millimeter-wavelength radar, sci- height (CBH) information, which, combined with the entists have been constantly testing their cloud obser- liquid water content and cloud optical thickness, is re- vation capabilities (Petrocchi and Paulsen 1966; Hobbs garded as useful for improving the vertical profile re- et al. 1985; Kropfli et al. 1990; Nakamura and Inomata trieval accuracy. Compared with conventional balloon 1992; Sekelsky and Mcintosh 1996). Obviously, cloud sounding, a microwave radiometer has the ability to radar can penetrate clouds to describe their three- obtain atmospheric vertical profiles with high temporal dimensional structure, providing information on the resolution, which can meet the needs of small- and vertical structure and microphysical parameters of medium-scale weather analysis, numerical forecasting, clouds, such as cloud height, cloud thickness, cloud and services related to catastrophic weather. Therefore, particle size, droplet distribution, and so on, which is a its application is expected to become increasingly im- powerful tool for atmospheric cloud and precipitation portant and widespread. Although the IRT is auxiliary research (Kollias et al. 2007). Haper (1966) studied the to a microwave radiometer, it can be used operationally cloud height retrieval methods with an 8.6-mm radar. for CBH measurement, and this can be regarded as a Clothiaux et al. (1995) used a 94-GHz radar to study the supplement to synergistic cloud measurements. characteristics of clouds and retrieved the CBH/cloud- According to the wavelength and width of the band, top height (CTH) using the radar reflectivity data com- the downward infrared radiation measured by a ground- bined with lidar data. Kropfli and Kelly (1996) provided based infrared cloud radiometer can be divided into liquid water profiles in warm marine stratus clouds, wide (Dürr and Philipona 2004) and narrow (Brocard combining radar and microwave radiometer data. et al. 2011; Klebe et al. 2014) widths. Most infrared ra- Moran et al. (1998) used an unattended cloud-profiling diometers work between 8 and 14 mm (wide band). The radar at the Department of Energy’s ARM Cloud atmospheric transmission rate is very high in this band, and Radiation Testbed (CART) sites to examine the so there is no strong absorption of water vapor or carbon radiative impacts of clouds on climate. Hollars dioxide. The IRT installed on top of a ground-based et al. (2004) compared observations of cloud height microwave radiometer works in the 9.6–11.5-mm (nar- from 35-GHz millimeter-wavelength cloud radar and row) band, which further improves the atmospheric Himawari-5 [Geostationary Meteorological Satellite-5 transmittance, and can more accurately receive cloud (GMS-5)] and analyzed the difference
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