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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

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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. several physically related parameters is shown in section 4; Beginning from the early 1990s, ASOS has gradually and the concluding remarks and a discussion of the re- replaced the manned weather stations in North America sults are presented in section 5. as well as in some other parts of the world. ASOS contains a set of automated instruments for observing sky 2. Data condition (i.e., cloud amount and cloud-base height), as well as other variables such as temperature, dewpoint, The data used in this study are from the ASOS precipitation, wind, visibility, and pressure (NWS 1998). 5-min dataset (NWS 1998). ASOS employs standard The ceilometer introduced by ASOS has become one laser ceilometers (Vaisala models CT12K or CL31; of the most common sources of surface cloud observa- Vaisala Inc., Woburn, Massachusetts) to observe the tions in the United States. Ceilometers from the widely sky conditions. A ceilometer sends laser pulses verti- distributed ASOS stations provide continuous data with cally upward toward the sky and determines whether high quality for the study of diurnal, seasonal, and in- the return signals are from cloud bases (Fig. 1). terannual variations of cloud frequency and cloud-base Knowing the speed of light, cloud-base height can be height over a large region. In addition, the simultaneous obtained by the time delay between the launch of the observations of temperature and precipitation from laser pulse and the detection of the backscatter signal. ASOS can be used to examine the variability of cloud The ASOS sky condition algorithm (NWS 1998)pro- frequency and cloud-base height. cesses the sensor signals into 30-s sample ‘‘hits.’’ The This paper uses cloud frequency and cloud-base algorithm calculates the cloud amount and cloud-base height data collected from ASOS ceilometers from height values using the most recent 30 min of the 30-s 2000 to 2015 to analyze the statistics of cloud frequency data samples, and data in the last 10 min are double and cloud-base height over the contiguous United weighted.

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ASOS provides sky condition information, including whether the sky is clear or cloudy. When the sky is cloudy, the information will also include the cloud amount and cloud-base height for at most three layers and up to 12 000 ft (approximately 3600 m) above the surface. With the exception of clear-sky (CLR) conditions, the cloud amounts reported by ASOS have four values: few (FEW), scattered (SCT), broken (BKN), and overcast (OVC). These values are determined by the ratio of the number of cloud hits to the total number of hits in the range of 5%–25%, 25%–50%, 50%–87%, and 87%–100% (NWS 1998). FIG. 2. Map of six climate zones of the contiguous United States The precision of cloud amount observed by ASOS is used in this study (see Table 1). The climate zones are generated ö low; therefore, we did not use the cloud amount directly. based on the K ppen–Geiger climate classification. The dots are the 186 stations selected to analyze the trends from 2000 to 2015. In this research, we separated clouds into four categories The stations are evenly distributed. by the four cloud amounts (FEW, SCT, BKN, and OVC). Subsequently, we divided the number of the cases when ASOS reported these four categories by the total number Precipitation and fog may attenuate the signal of of valid observations to calculate the cloud frequency in ceilometers. According to the ceilometer technical each category; the total cloud frequency was calculated manuals (Vaisala 1989), CT12K ceilometers perform by summing the frequencies of the four categories. algorithms to compensate for the attenuation of the In this research, we analyzed only the lowest layer of cloud-base signal caused by obstructions, such as pre- the cloud-base height because ceilometers have been re- cipitation and fog. When the cloud base is completely ported to exhibit inherent limitations (Costa-Surósetal. obscured by these obstructions, the algorithm will 2013) that prevent them from completely describing the calculate a height of vertical visibility (VV) instead of a upper layers of clouds when multiple layers of clouds occur. cloud-base height. During 2008–12, approximately The data used in this research underwent rigorous 18.9%, 7.2%, 89.2%, and 1.5% of the VV measurements quality control. First, we determined the valid days; we occurred with precipitation, mist, fog, and haze, re- rejected the data recorded on the days when the daily spectively. We excluded the VV measurements under valid data numbers were less than 50% of the total haze and included the rest of the VV measurements that number of daily observations. For the analysis of in- occurred with precipitation, mist, and fog in the category terannual variations and trends, we selected 186 stations of OVC. The annual mean occurrence frequency of VV from ASOS that had abundant data for the period from measurements was 0.9% for all 719 stations during 2000 to 2015 and determined whether the data met the 2008–12. It was not comparable with annual mean cloud following criterion: sufficient data for each season (more frequency (39%). Therefore, in general, including VV than 45 valid days in a season) for more than 10 yr within had little influence on the distributions and variations of the 16-yr period. Finally, because the number of ASOS cloud frequency and cloud-base height. stations has increased in recent decades, we selected The macrocharacteristics of clouds are mainly de- 719 stations with sufficient data for all seasons from pendent on the atmospheric temperature and humidity 2008 to 2012 to calculate the climatology of the clouds condition. To better analyze the geographic differences, and determined whether the data met the following based on the Köppen–Geiger climate classification criterion: sufficient data for each season (more than 45 (Kottek et al. 2006), we merged similar climate regions valid days in a season) for more than 3 yr of the 5-yr that have the same main climate types and precipitation period. The selected stations were evenly distributed types (i.e., have the same first and second letter of throughout the contiguous United States. Köppen–Geiger climate classification) and separated ASOS uses a variety of sensors to identify different the contiguous United States into six climate zones types of weather (e.g., rain, snow, and frozen rain), (Fig. 2; Table 1). The hot-summer Mediterranean cli- identify the obstructions to vision (e.g., mist, fog, and mate (Csa) and warm-summer Mediterranean climate haze), and infer the intensity of precipitation. We also (Csb) are merged into one climate zone because they checked the precipitation, obstruction, temperature, have obvious features of Mediterranean climate. The and dewpoint datasets from the ASOS 5-min data to humid continental climate–dry cool summer (Dsb) and support the analyses of cloud frequency and cloud-base continental subarctic–cold dry summer (Dsc), which height from laser ceilometers. together contain only two stations, are also counted as

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TABLE 1. Climate zones used in this study, the corresponding climate regions of the Köppen–Geiger climate classiﬁcation for each climate zone, and the number of stations selected for analyzing interannual variability and for analyzing seasonal and diurnal variability. The percentages in parentheses refer to the ratios of the numbers of selected stations to the total numbers of stations in each climate zone and over the entire contiguous United States.

Corresponding Köppen–Geiger No. of sites for analyzing No. of sites for analyzing Zone No. Climate zone climate classiﬁcation interannual variability seasonal and diurnal variability I Mediterranean climate Csa, Csb, Dsb, and Dsc 19 (18.3%) 81 (77.9%) II Arid steppe climate BSk and BSh 33 (26.6%) 102 (82.3%) III Arid desert climate BWk and BWh 3 (14.3%) 18 (85.7%) IV Snow climate Dfa, Dfb, and Dfc 56 (19.9%) 207 (76.4%) V Warm and humid climate Cfa and Cfb 73 (24.3%) 301 (82.0%) VI Equatorial climate Am and Aw 2 (13.3%) 10 (66.7%) Total — — 186 (20.6%) 719 (79.7%) the Mediterranean climate zone in this research because height for the surface–3600-m layer over the contiguous they both have dry summers. The tropical savanna cli- United States as well as their relationships with mate (Aw) and tropical monsoon climate (Am) were precipitation. merged into one climate zone because they both have a. Seasonal distribution of cloud frequency dry winters, and the area of the equatorial climate in the contiguous United States is not large enough to separate The geographical distributions of the average surface– it into two climate zones. 3600-m total cloud frequency for 719 stations recorded during different seasons over the contiguous United States for ﬁve years (2008–12) are shown in Fig. 3. The 3. Results results indicate that seasonal variability and geo- In this section, we show the seasonal, diurnal, and in- graphical differences occurred. The total cloud fre- terannual variability of cloud frequency and cloud-base quency is higher (over 40%) in the northwestern coastal

FIG. 3. Mean total cloud frequency (%) for the surface–3600-m layer of 719 stations over the contiguous United States in (a) spring (March–May), (b) summer (June–August), (c) autumn (September–November), and (d) winter (December–February) during the period from 2008 to 2012.

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FIG. 4. Mean seasonal and annual (ANN) cloud frequency (%) for the surface–3600-m layer in the different climate zones during the period from 2008 to 2012: (a) total cloud frequency, (b) cloud frequency of FEW, (c) cloud frequency of SCT, (d) cloud frequency of BKN, (e) cloud frequency of OVC, and (f) cloud frequency during precipitation. Climate zones I–VI are as in Fig. 2. Error bars designate the standard error of the value in a climate zone and season. Cloud frequency during precipitation in (f) is calculated by dividing the number of cases when ASOS reported precipitation and clouds by the total number of precipitation cases. The areas with slanted lines indicate the frequency (%) of the VV observations during precipitation. region, the Great Lakes region, and southern Florida FEW and SCT in the equatorial climate zone located in throughout the year (Fig. 3). In arid areas, the total cloud south Florida are much higher than in the other climate frequency is relatively low throughout the year. The an- zones. The cloud frequencies of FEW and SCT are nual average surface–3600-m total cloud frequencies for generally higher in summer than in winter. The cloud the arid steppe climate zone and the arid desert climate frequency of BKN is also higher in the equatorial cli- zone are 27% and 15%, respectively, whereas those for mate zone, but it is higher in winter than in summer in the Mediterranean climate zone, the snow climate zone, the equatorial climate zone, the arid steppe climate the warm and humid climate zone, and the equatorial zone, and the arid desert climate zone. The cloud fre- climate zone are 39%, 45%, 39%, and 50%, respectively. quency of OVC is higher in the snow climate zone in As shown in Fig. 4a, the total cloud frequency is generally the northeast and in the Mediterranean climate zone higher in winter and lower in summer. In winter, the av- on the West Coast of the contiguous United States erage total cloud frequency for all 719 stations is 43%, but is lower in the equatorial climate zone. The cloud whereas the average is 33% in summer. frequency of OVC is much higher in winter than In Figs. 4b–e, the cloud frequencies for the four in summer. categories (FEW, SCT, BKN, and OVC) within the Figure 4f shows the surface–3600-m cloud occur- surface–3600-m layer have different seasonal and rence frequency during precipitation occurrence. It is geographical distributions. The cloud frequencies of calculated by dividing the number of cases when ASOS

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FIG.5.AsinFig.3, but for mean cloud-base height (m). detected precipitation as well as clouds by the total detected by ceilometers are shown in Fig. 5;itdisplays number of precipitation cases. The areas with slanted data collected from 719 stations over the contiguous lines indicate the frequency of VV observations during United States from 2008 to 2012 during different sea- precipitation. Although the VV frequency is not com- sons. The regional means of the cloud-base height for patible with cloud frequency, it accounts for an impres- all categories of clouds, cloud-base height for each sive proportion of precipitation. VV during precipitation of the four categories, and cloud-base height during occurred more in winter and in cold areas. The VV fre- precipitation for the surface–3600-m layer in the quency during precipitation in the equatorial climate different climate zones are shown in Figs. 6a–f, zone is very low and therefore not apparent in Fig. 4f.The respectively. cloud frequency during precipitation is under 100% In general, the cloud-base height is lower in the east mainly because a small portion of precipitation events are and higher in the west except on the West Coast of the generated by higher-level clouds that ceilometers do not contiguous United States (Fig. 5). In the arid steppe observe. Precipitation is also transported by wind and can climate zone and the arid desert climate zone in the occur when clouds are not right above the ceilometers, west, the annual average surface–3600-m cloud-base although this precipitation makes only a slight contribu- heights are 1884 and 2390 m, respectively. In the snow tion to the total. The total cloud frequencies during pre- climate zone, the warm and humid climate zone, and cipitation are over 90% in many seasons and many parts the equatorial climate zone, which occur primarily in of the contiguous United States, which conﬁrms that the eastern part of the contiguous United States, the clouds below 3600 m play an important role in pre- annual average surface–3600-m cloud-base heights are cipitation. Not all clouds that produce precipitation are 1485, 1373, and 1175 m, respectively. Along the West detected by ASOS ceilometers. In the arid desert zone Coast of the contiguous United States, the cloud-base and the equatorial climate zone, especially in summer, height is mainly less than 1200 m during every season 20%–40% of the precipitation occurs without the de- (Fig. 5), which is due to the high frequencies of stratus, tection of clouds by the CT12K ceilometers from ASOS. stratocumulus, and fog along the west coastal area (Leipper 1994; Warren et al. 1986). Lower values for b. Seasonal distribution of the cloud-base height the cloud-base heights are also observed in the coastal Geographical distributions of the average surface– areas along the Gulf of Mexico and the Great Lakes 3600-m cloud-base height for all categories of clouds area. As shown in Fig. 6a,mostoftheregionsofthe

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FIG. 6. Mean seasonal and annual cloud-base height (m) for the surface–3600-m layer in different climate zones in period from 2008 to 2012: (a) cloud-base height of all clouds, (b) cloud-base height of FEW, (c) cloud-base height of SCT, (d) cloud-base height of BKN, (e) cloud-base height of OVC, (f) cloud-base height during precipitation. Climate zones I–VI are as in Fig. 2. Error bars designate the standard error of the value in a climate zone and season. contiguous United States have higher cloud-base summer, when the average cloud-base heights during heights in summer than in winter except in the equa- precipitation reach about 2000 m (Fig. 6f). torial climate zone. c. Diurnal variations of cloud frequency and The average cloud-base heights of the four categories cloud-base height of FEW, SCT, BKN, and OVC for the surface–3600-m layer are both higher in arid areas such as the arid steppe For most stations within a climate zone, the patterns of climate zone and the arid desert climate zone and lower the diurnal cycle of cloud frequency and cloud-base height in more humid areas such as the tropical climate zone are similar. Therefore, we analyzed the average diurnal and the warm and humid climate zone (Figs. 6b–e). cycle of cloud frequency and cloud-base height from all Their geographical distributions are similar and not stations in every climate zone. All the diurnal cycles are shown in the ﬁgure. The annual average surface–3600-m obtained based on local standard time (LST). cloud-base heights of all the 719 stations during 2008–12 Figure 7 shows the diurnal cycles of cloud frequency for the four categories (FEW, SCT, BKN, and OVC) are within the surface–3600-m layer of the four categories 1941, 1802, 1672, and 1212 m, respectively. The mean (FEW, SCT, BKN, and OVC) for different climate cloud-base height becomes lower when cloud amount zones. The diurnal frequency of FEW, SCT, and BKN increases. is allied to the diurnal cycle of cumulus clouds (e.g., The annual average cloud-base height during pre- Eastman and Warren 2014, their Fig. 10). The occur- cipitation at all the 719 stations during 2008–12 is 851 m. rence frequencies of these types of clouds begin to in- However, it is much higher in the arid steppe climate crease in the early morning as the sun rises and the zone and the arid desert climate zone, especially in solar radiation heats the surface and drives convection

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FIG. 7. Diurnal variations of cloud frequency (%) of (a) FEW, (b) SCT, (c) BKN, and (d) OVC within the surface–3600-m layer for six climate zones (I–VI, as in Fig. 2) during 2008–12. in the boundary layer. The value peaks just after noon in the morning. This time corresponds to the cumuliform and decreases in the afternoon. Except in summer in clouds’ onset time. The newly and vastly generated the equatorial climate shown in Fig. 7d(6), the diurnal cumulus clouds in the lower atmosphere drive down the cycles of cloud frequencies of OVC generally peak in average cloud-base height at that time. Then, clouds the morning, which corresponds to the diurnal cycle of are lifted during the daytime and reach peaks in the late stratiform clouds (e.g., Eastman and Warren 2014,their afternoon or early evening. As shown in Fig. 8, in general, Fig. 10). The peak in the morning is associated with the the minima of the diurnal cycles of cloud-base height diurnal peak frequency of drizzle and nonshowery pre- occur earlier and the peak values occur later during the cipitation in the morning (e.g., Dai 2001, their Fig. 6). day in summer than in winter because sunrise is earlier The diurnal cycles of cloud-base heights within the and sunset is later in summer. The exceptions in the surface–3600-m layer have similar patterns in different equatorial climate zone may be due to few samples of climate zones for different categories (Fig. 8). Daily stations as well as the small difference between minima generally appear around 0900 LST (0700–1100 LST) summer and winter in a tropical area. The exceptions in the

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FIG. 8. Diurnal variations of cloud-base height (m) of (a) FEW, (b) SCT, (c) BKN, and (d) OVC within the surface–3600-m layer and (e) precipitation frequencies for six climate zones (I–VI, as in Fig. 2) during 2008–12. The asterisks in the ﬁgure represent the cloud-base height value when the value of temperature minus dewpoint temperature reaches its diurnal peak value. arid desert climate zone may be due to the fact that least squares regressions (Table 2; Figs. 9 and 10). As some of the clouds higher than 3600 m are not taken shown in Table 2, from 2000 to 2015, the total cloud into account in the diurnal cycles especially in summer. frequency increased while the cloud-base height de- The contrasts between surface temperature and creased. These changes in cloud frequency and cloud- dewpoint are used to estimate the lifting condensation base height are especially obvious in winter, which level: the larger the contrasts, the higher the lifting showed an average increase of 6.6% in surface–3600-m condensation level, and the higher the convective cloud- cloud frequency and an average decrease of 149 m base height (Craven et al. 2002). The cloud-base height in surface–3600-m cloud-base height for all 186 sta- peaks later than the time when the contrast between the tions during the 16 studied years. As shown in Fig. 9, temperature and dewpoint reaches its daily maximum in the increasing cloud frequency trend in winter gen- the midafternoon (shown by the asterisks in Fig. 8). erally occurs in the eastern part of the United States. In winter, over 50% of stations have signiﬁcant (i.e., d. Interannual variations of cloud frequency and passed the two-tailed Student’s t test at the 90% level) cloud-base height increasing trends of cloud frequency in the snow climate zone and the warm and humid climate zone, while We calculated the linear trends of surface–3600-m cloud-base height shows a decreasing trend over the total cloud frequency and cloud-base height using eastern part of the contiguous United States (Fig. 10).

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21 21 21 TABLE 2. Seasonal and annual trends of cloud-base height (m yr ), cloud frequency (% yr ), and precipitation days (days yr ) in different climate zones from 2000 to 2015. Trends in boldface are statistically signiﬁcant at the 0.1 level.

2 2 2 Trends of cloud-base height (m yr 1) Trends of total cloud frequency (% yr 1) Trends of precipitation days (days yr 1) Climate zone MAM JJA SON DJF Annual MAM JJA SON DJF Annual MAM JJA SON DJF Annual I 26 22 22 211 25 20.03 0.22 20.01 0.21 0.10 20.01 0.14 0.29 20.08 0.09 II 0 22021 21 0.08 0.14 20.13 0.02 0.08 0.17 0.43 0.09 0.05 0.19 III 5 3 215 211 25 0.28 1.35 0.49 20.00 0.13 0.05 0.32 0.15 20.30 0.05 IV 21 23 22 29 24 0.11 0.47 0.28 0.64 0.36 20.12 0.32 0.04 0.26 0.13 V 28122 213 25 0.26 0.23 0.32 0.47 0.35 0.18 0.07 0.09 0.19 0.13 VI 244123 21 0.40 0.53 0.36 0.57 0.47 0.46 0.16 0.64 0.53 0.45 Total 24 21 22 29 24 0.15 0.31 0.20 0.41 0.28 0.07 0.22 0.10 0.15 0.14

The correlations between the trends during daytime calculated for a season (Table 2; Fig. 12). The results and nighttime were tested. Figure 11 shows the scatter- indicate that from 2000 to 2015, precipitation days in- plots of nighttime and daytime trends of surface–3600-m creased for a large part of the contiguous United States. total cloud frequency as well as cloud-base height. An increase of 2.4 precipitation days in winter occurred The trends of daytime and nighttime total cloud fre- during the 16 years from 2000 to 2015. Figure 13 illus- quency seem to be consistent, with a correlation co- trates the scatterplots between the significant trends of efficient of 0.75. The same conclusion is also found surface–3600-m total cloud frequency and the trends of for the trends of cloud-base height, with a correla- precipitation days. The figure shows that most of the tion coefficient of 0.66 between daytime and night- stations with significant positive (negative) trends of time trends. cloud frequency also show positive (negative) trends of The precipitation days that have precipitation for precipitation days. The correlation between the trends more than half an hour were summed within a sea- of precipitation days and total cloud frequency in son, and then the trends of precipitation days were summer is weak. In winter, the correlation between the

21 FIG. 9. Linear trends of total cloud frequency (% yr ) for the surface–3600-m layer of 186 stations over the contiguous United States of (a) spring, (b) summer, (c) autumn, and (d) winter for the period from 2000 to 2015. Trends are calculated using least squares regression. The dots indicate that the trend of a station has passed the two- tailed Student’s t test at the 90% level, and the triangles indicate that the trend fails to pass.

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21 FIG. 10. As in Fig. 9, but for linear trends of cloud-base height (m yr ). trends of cloud frequency and precipitation days is We calculated the correlations between the detrended 0.50 (p , 0.05), and the plots are mostly located in the monthly average surface–3600-m total cloud frequency

ﬁrst quadrant, which indicates the close connection and detrended monthly average RHsfc,DTR,andTsfc between the increasing cloud frequency and increasing as well as the correlations between the detrended snow and rainfall days in winter. monthly average surface–3600-m cloud-base height and

detrended monthly average RHsfc,DTR,andTsfc at every station over the contiguous United States during 2000–15 4. Temporal relationships with physically related in different seasons. The RH was calculated from parameters at surface sfc temperature and dewpoint observations from the ASOS Cloud datasets in the United States are confronted 5-min dataset and the DTR was determined by daily with inhomogeneity problems because of the changes to maximum temperature minus the minimum temperature observation practices and the introduction of ASOS from the ASOS 5-min dataset. (Free and Sun 2013). Therefore, many studies tested the In general, the correlations between cloud-base height long-term variations of cloud datasets with physically and RHsfc, DTR, and Tsfc (Figs. 14b,d,f and 15b,d,f) are related surface air parameters to identify a homoge- opposite to the correlations between cloud frequency neous climate record for cloud (Free and Sun 2014; Sun and these parameters (Figs. 14a,c,e and 15a,c,e), which is and Groisman 2004; Sun et al. 2000, 2007). Interannual due to the negative correlation of cloud frequency and variations of cloud macrophysical properties have been cloud-base height within the surface–3600-m layer. examined and proved to be closely related to parameters When the RHsfc is higher, there is a high possibility for such as surface air temperature Tsfc, relative humidity more water vapor condensed as clouds and resulting in RHsfc, and diurnal temperature range (DTR) (Dai et al. more cloud amount. The positive correlations between 2006; Sun et al. 2000, 2007). However, most of their total cloud frequency and RHsfc are consistent among all analyses are generally calculated based on area-averaged 186 stations in both summer and winter except for a very cloud macrophysical properties and area-averaged sur- few stations that are not signiﬁcant or have negative face air parameters in a certain country or a region. With correlations. high density of ASOS stations, we want to test the re- Nearly all 186 stations over the contiguous United lationships at every station to see if there are apparent States present negative correlations between cloud fre- spatial variations over the contiguous United States. quency and DTR in either summer or winter seasons.

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FIG. 11. (a) Scatterplots of linear trends of nighttime surface–3600-m total cloud frequency 2 (% yr 1) as a function of linear trends of daytime surface–3600-m total cloud frequency 2 2 (% yr 1). (b) Scatterplots of linear trends of nighttime surface–3600-m cloud-base height (m yr 1) 2 as a function of linear trends of daytime surface–3600-m cloud-base height (m yr 1). Each dot represents a station for one season. The red line is the best-ﬁt line calculated using least squares regression.

The surface–3600-m layer contains mainly low clouds, States. This is likely due to the decoupling of clouds and which are especially efficient in reducing the daily surface conditions (i.e., the clouds resulting from the land maximum temperature and DTR for their high capability and sea breeze). in reflecting solar radiation. As shown in Figs. 14b,d and The correlations between cloud frequency and Tsfc are 15b,d, the correlations between cloud-base height different between summer and winter. In summer, and RHsfc as well as DTR are not significant in many nearly all the stations of the entire contiguous United stations along the West Coast of the contiguous United States present negative correlations between total cloud

21 FIG. 12. Linear trends of precipitation days (days yr ) for 186 stations over the contiguous United States of (a) spring, (b) summer, (c) autumn, and (d) winter for the period from 2000 to 2015. Trends are calculated using least squares regression. The dots indicate that the trend of a station has passed the two-tailed Student’s t test at the 90% level, and the triangles indicate that the trend fails to pass.

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21 FIG. 13. Scatterplots of linear trends of precipitation days (days yr ) as a function of linear 2 trends of surface–3600-m total cloud frequency (% yr 1) when the linear trends of surface– 2 3600-m total cloud frequency (% yr 1) is signiﬁcant in different seasons: (a) spring, (b) summer, (c) autumn, and (d) winter. Each dot represents a station for a season. The different colors of the dots indicate the climate zones of the stations corresponding to Fig. 2. The blue line is the best-ﬁt line calculated using least squares regression.

frequency and Tsfc as well as positive correlations between Our results showed that the peak of cloud-base cloud-base height and Tsfc (Figs. 14e,f). However, in height lags the peak of the difference between tem- winter, many stations in the east present positive corre- perature and dewpoint in the diurnal cycle, which may lations between total cloud frequency and Tsfc (Fig. 15e). be explained by convection constantly lifting the clouds The geographical difference can also be found for the regardless of whether convection is increasing and correlations between cloud-base height and Tsfc (Fig. 15f). decreasing during the daytime. Another explanation may be that the convective precipitation that has higher frequencies in the late afternoon [Figs. 8e(1)–(6)] 5. Conclusions and discussion dissipates the lower portions of the strong vertically Based on ASOS observations of clouds, we analyzed developed clouds and keeps the upper portions of clouds the seasonal, diurnal, and interannual variations in cloud at a higher level. frequency and cloud-base height over the contiguous The surface–3600-m layer contains mainly low clouds, United States. The results show that ceilometers from which are connected to surface parameters through ASOS can monitor clouds with high spatial and tem- turbulence. The surface–3600-m cloud frequency and poral resolution within their measurement range. cloud-base height are closely related to RHsfc as well as We separated clouds into four categories by their DTR, and the correlations are consistent in most of the cloud amounts (FEW, SCT, BKN, and OVC). The cloud stations over the contiguous United States in different frequencies of the four categories show different geo- seasons. The relationship between cloud frequency and graphical distributions and seasonal and diurnal varia- Tsfc is negative in summer because of the cooling effect tions. The categories of FEW, SCT, and BKN may of low clouds. In winter, cloud frequency is positive in contain more cumuliform clouds, whereas OVC may relation to Tsfc in the eastern part of the contiguous contain more stratiform clouds according to the result. United States. A possible explanation for the in- The mean cloud-base height decreases as cloud amount conformity in winter may be the different synoptic sit- increase from FEW to OVC. However, the geographical uations during winter. Cold outbreaks containing dry distribution and seasonal and diurnal variations of and cold arctic air routinely plunge south into many cloud-base height have similar features for the four parts of the central and eastern contiguous United States categories. (Kunkel et al. 2013), which results in lower temperatures

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FIG. 14. Correlations between detrended monthly mean total cloud frequency and (a) detrended monthly mean

relative humidity, (c) DTR, and (e) Tsfc; correlations between detrended monthly mean cloud-base height and (b) detrended monthly mean relative humidity, (d) DTR, and (f) Tsfc for 186 stations during summer (June–August) from 2000 to 2015 over the contiguous United States. The dots indicate that correlation is significant, and the triangles indicate that the correlation is not significant at the 0.05 level. and more clear days, causing a positive relationship Previous research on long-term trends of cloud amount between cloud frequency and Tsfc. Sun et al. (2000) and cloud-base height was generally based on daytime found that cloud cover and Tsfc were positively corre- observations. Our results show that the nighttime trends lated at midlatitudes in winter because changes in snow of cloud frequency and cloud-base height are consistent cover affect Tsfc by altering the surface albedo. This is with the corresponding daytime trends. Sun et al. (2015) consistent with our result. reported that cloud amount decreased at a trend From 2000 to 2015, the average total cloud frequency of 20.40% from 1982 to 2007, and Sun et al. (2007) in the surface–3600-m layer over the contiguous United showed that cloud-ceiling height in the surface–3600-m States increased while the cloud-base height decreased level significantly increased after the early 1970s until and was accompanied with an increase of precipitation 2003. Cloud-ceiling height is defined as the base height days. The increase of cloud frequency and decrease of of the lowest layer when BKN or OVC of opaque clouds cloud-base height are very noticeable in the eastern part are reported. Based on this definition, we calculated 2 of the contiguous United States in the winter season. the trend of cloud-ceiling height (22myr 1)overthe

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FIG. 15. As in Fig. 14, but during winter (December–February). contiguous United States during 2000–2015 from ASOS to 2012. Our results show the increase of cloud fre- ceilometer observation. This is consistent with the quency is closely related to the increase of precipitation negative trend of cloud-base height we calculated that is days in spring and winter. In summer when convection also statistically signiﬁcant at the 0.1 level. Combining the is strong, more than 20% of precipitation may occur result of this study and earlier studies suggests that de- higher than 3600 m, which may help to explain the weak cadal variabilities in cloud properties have been ongoing correlation between the trends of total cloud frequency over the contiguous United States: decreasing cloud with the trends of precipitation days in summer. amount from the 1980s to the late 1990s and then in- Therefore, the observation scope (3600 m) of the creasing up to 2015 as well as increasing cloud-base height CT12K ceilometer used by ASOS may not be adequate from 1970s to the late 1990s and then decreasing up to study clouds and their interactions with weather and to 2015. climate. At present, ASOS only reports clouds under 12 000 ft Next-generation ceilometer sensors (CL31) from (approximately 3600 m). Clouds in this vertical range Vaisala replaced the old-generation ceilometer sensors play an important role in precipitation. On average, (CT12K) at ASOS stations, and they provide more reli- 94% of precipitation is accompanied by clouds within able information in the lower atmosphere up to 25 000 ft the surface–3600-m layer for all 719 stations from 2008 (about 7600 m). The new-generation ceilometers have

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