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Downloaded 10/06/21 04:17 PM UTC 3726 JOURNAL of CLIMATE VOLUME 30 15 MAY 2017 A N E T A L . 3725 Observed Variability of Cloud Frequency and Cloud-Base Height within 3600m above the Surface over the Contiguous United States NING AN College of Global Change and Earth System Science, Beijing Normal University, Beijing, China, and Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland KAICUN WANG AND CHUNLÜE ZHOU College of Global Change and Earth System Science, Beijing Normal University, Beijing, China RACHEL T. PINKER Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland (Manuscript received 28 July 2016, in final form 2 January 2017) ABSTRACT The geographic and temporal variability of the surface–3600-m cloud frequency and cloud-base height over the contiguous United States for a 5-yr period (2008–12) and the interannual variations for a 16-yr period (2000–15) are described using information from the Automated Surface Observing System (ASOS) observations. Clouds were separated into four categories by the cloud amount reported by ASOS: few (FEW), scattered (SCT), broken (BKN), and overcast (OVC). The geographic distributions and seasonal and diurnal cycles of the four categories of surface–3600-m cloud frequency have different patterns. Cloud frequency of FEW, SCT, and BKN peaks just after noon, whereas the frequency of OVC peaks in the early morning. However, the geographic distributions and seasonal and diurnal cycles of the four categories of the surface–3600-m cloud-base height are similar. The diurnal cycles of the cloud-base height within the surface–3600-m level present a minimum in the morning and peak in the late afternoon or early evening. Cloud frequency and cloud-base height within this range are closely related to surface air temperature and humidity conditions. From 2000 to 2015, the cloud frequency in the contiguous United States showed a 2 2 positive trend of 0.28% yr 1 while the cloud-base height showed a negative trend of 24myr 1 for the 2 surface–3600-m level, accompanied with a positive trend of precipitation days (0.14 days yr 1). Moreover, the increase of cloud frequency and the decrease of cloud-base height were most obvious in winter in the eastern half of the contiguous United States. 1. Introduction by absorbing thermal infrared radiation from below and reemitting it back to space and the surface. During The global climate is largely determined by Earth’s daytime, whether a cloud cools or warms the surface energy budget (Trenberth et al. 2009). The absorbed depends on the cloud type associated with the cloud solar shortwave radiation heats the planet while the height (Lee et al. 1997). At night, the impact of clouds on emitted longwave radiation cools it. Clouds play a major solar shortwave radiation is zero; thus, all types of clouds role in regulating the energy flows into and out of the have a warming effect. As a result, the general magni- earth–atmosphere system. They cool the earth by re- tude and sign of the cloud radiative effect depends on flecting solar radiation into space. They warm the earth the cloud amount, the cloud height (base and top), and the time of occurrence during a day (Ramanathan et al. Denotes content that is immediately available upon publica- 1989; IPCC 2013). tion as open access. Our current knowledge of cloud processes is limited; thus, cloud represents one of the largest uncertainties in Corresponding author e-mail: Dr. Kaicun Wang, kcwang@bnu. simulating past climate change and predicting future edu.cn climate change (Cess et al. 1990; Fasullo and Trenberth DOI: 10.1175/JCLI-D-16-0559.1 Ó 2017 American Meteorological Society. For information regarding reuse of this content and general copyright information, consult the AMS Copyright Policy (www.ametsoc.org/PUBSReuseLicenses). Unauthenticated | Downloaded 10/06/21 04:17 PM UTC 3726 JOURNAL OF CLIMATE VOLUME 30 2012; Sherwood et al. 2014). Observations provide an Henderson-Sellers 1992). Moreover, the lack of il- opportunity to assess how clouds contribute and respond lumination at night may introduce inconsistencies to climate change. The most common sources of cloud when analyzing the diurnal cycle. observations are from satellites and surface weather Radiosonde data can also be used to infer cloud in- stations. formation, such as cloud vertical structure and cloud Passive satellite visible and thermal infrared sensors frequency, based on profiles of temperature, relative principally derive information regarding cloud amount, humidity, and pressure (Costa-Surós et al. 2014; cloud optical thickness, cloud-top pressure, and other Chernykh et al. 2001; Minnis et al. 2005; Wang and cloud properties from radiance measurements (Rossow Rossow 1995; Zhang et al. 2012). However, these data and Schiffer 1991). These sensors have the advantage are restricted by radiosonde observation frequencies of observing cloud-top height over the entire globe (usually twice daily) and therefore are not able to depict (Forsythe et al. 2000). However, these satellite sensors the diurnal changes of cloud properties. cannot pierce through clouds; thus, assumptions on Previous studies have analyzed the climatology of cloud thickness are required (Welch et al. 2008)to cloud properties using ground-based and remote determine the cloud-base height (Hutchison 2002; sensing methods. Stubenrauch et al. (2013) accessed Kokhanovsky and Rozanov 2005), and these assump- 12 state-of-the-art satellite cloud datasets and ana- tions introduce large uncertainties (Welch et al. 2008). lyzed the latitudinal variations, seasonal and diurnal Moreover, many of these satellite sensors have differ- cycles, and interannual variability of cloud amount, ent spectral channels for day and night retrievals, which cloud-top location, and other cloud properties. How- introduce intrinsic errors to the observations of cloud ever, the climatology of cloud-base height has not diurnal variations. New airborne active sensors, such as been properly analyzed via satellite observations. the Cloud Profiling Radar on board the CloudSat sat- Warren et al. (1986, 1988) produced a database from ellite and Cloud–Aerosol Lidar with Orthogonal Po- surfacelandstationandshipmeasurementsandfur- larization on board the CALIPSO satellite, can provide ther analyzed the long-term and seasonal and diurnal multiple cloud-layer-top and cloud-layer-base heights variations of cloud cover, cloud types, and cloud-base (Kim et al. 2011). However, the samples from these height based on this database (Eastman and Warren sensors are spatially sparse. 2014; Eastman et al. 2011; Warren et al. 2007). How- Traditionally, synoptic weather reports via the ever, these surface databases have only four or eight global telecommunication system (GTS) from surface daily observations and may be influenced by poor il- stations contain cloud informationsuchastotalcloud lumination at night. cover, the predominate cloud types for three layers For regions of the United States, studies have focused (low, middle, and high), the cloud amount, and the on the total cloud cover, cloud amounts, and cloud- cloud-base height of the lowest layer, according to ceiling height changes based on different ground-based World Meteorological Organization (WMO) codes and remote sensing methods (Dai et al. 2006; Free and (WMO 1974). Nevertheless, the ways cloud observa- Sun 2014; Sun 2003; Sun et al. 2015; Sun and Groisman tions were made vary from country to country (Sun 2004; Sun et al. 2001, 2007). Long-term interannual et al. 2001). For example, in the United States, his- variations of these parameters are generally based on torically human observers were allowed to report up to daytime cloud observations only. four or more cloud layers and the cloud-base height of Studies have shown that cloud microphysical charac- each layer; for augmentation by human observers at teristics (e.g., cloud particle size, ice/water content, and the Automated Surface Observing System (ASOS) cloud infrared absorption coefficient) and macro- sites, up to six cloud layers and the cloud-base height of physical characteristics (e.g., low cloud amount, high each were provided. Those rich-information observations cloud amount, and high cloud-top height) can sub- were archived, but modifications and abridgements were stantially change (Bender et al. 2012; Brient and Bony made on-site to generate transmitted WMO synoptic 2012; Platt 1989; Zelinka and Hartmann 2010), which in data over GTS. turn introduce important climatic feedback. Cloud ver- Routine observations of clouds have long-term his- tical structures (i.e., the cloud-top and cloud-base toric records. The cloud types they report have strong height) are also sensitive to climate change (Chepfer indications for cloud dynamic processes. However, these et al. 2014; Ockert-Bell and Hartmann 1992; Williams datasets that generally based on human visual observa- et al. 2015). Cloud-base height is an important feature tions are somewhat subjective and have potential that describes the impact of clouds in a changing climate. inhomogeneity related to changes in observational For example, its variations directly impact cloud radia- practice (Dai et al. 2006; Free and Sun 2013; tive properties and thus affect the global radiation Unauthenticated | Downloaded 10/06/21 04:17 PM UTC 15 MAY 2017 A N E T A L . 3727 FIG. 1. Example of (left) an ASOS meteorological station and (right) the ceilometer working principle (station image is from http://www.wrcc.dri.edu/images/Station_pics/Utah/milford_ut_asos.jpg). balance (Slingo and Slingo 1988; Viúdez-Mora et al. States. The data and methods are briefly discussed in 2015). Moreover, cloud-base height is important for section 2; the seasonal and diurnal variations and in- weather forecasting and aviation control (Ellrod and terannual variability results are presented in section 3; Gultepe 2007; Mittermaier 2012; Vislocky and Fritsch an analysis of temporal relationships between cloud 1997). Hence, there is a need to better monitor and properties (cloud frequency and cloud-base height) and understand cloud-base height variability.
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