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On the summertime planetary boundary layer with
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different thermodynamic stability in China: A
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radiosonde perspective
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Wanchun Zhang1#，Jianping Guo1#*，Yucong Miao1，Huan Liu1，Yu Song2,
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Zhang Fang3, Jing He1, Mengyun Lou1, Yan Yan1, Yuan Li1, Panmao Zhai1* 1
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State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China
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Department of Environmental Science, Peking University, Beijing, 100871, China
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State Key Joint Laboratory of Environmental Simulation and Pollution Control,
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Public Meteorological Service Center, China Meteorological Administration, Beijing 100081, China
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# These co-authors contribute equally to this work.
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Correspondence to:
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Dr./Prof. Jianping Guo, Email:
[email protected]
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Prof. Panmao Zhai, Email:
[email protected]
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Key Points:
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neutral PBL types across China for the first time
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The PBL height is positively (negatively) associated with near-surface temperature (humidity) for both convective and neutral PBLs
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PBL height is estimated from afternoon soundings for convective, stable and
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Clouds tend to cause more stable atmosphere, resulting in lower PBL height 1
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Abstract
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Strongly influenced by thermodynamic stability, the planetary boundary layer (PBL) is
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key to the exchange of heat, momentum, and moisture between the ground surface and
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free troposphere. The PBL with different thermodynamic stability across the whole
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China, however, is not yet well understood. In this study, the occurrence frequency and
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spatial distribution of convective boundary layer (CBL), neutral boundary layer (NBL),
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and stable boundary layer (SBL) were systematically investigated, based on intensive
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summertime soundings launched at 1400 Beijing time (BJT) throughout the China’s
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Radiosonde Network (CRN) for the period 2012 to 2016. Overall, the occurrences of
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CBL, NBL, and SBL account for 70%, 26%, and 4%, respectively, suggesting CBL
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dominates in summer throughout China. In terms of the spatial pattern of PBL height,
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a prominent north-south gradient can be found with higher PBL height in Northwest
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China. Besides, the PBL heights of CBL and NBL were found to be positively
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(negatively) associated with near-surface air temperature (humidity), whereas no
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apparent relationship was found for SBL. Furthermore, clouds tend to reduce the
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occurrence frequency, irrespective of PBL types. Roughly 70% of SBL cases occur
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under overcast conditions, much higher than those for NBL and CBL, indicating that
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clouds govern to some extent the occurrence of SBL. In contrast, except for the
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discernible changes in PBL height under overcast condition relative to that under clear-
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sky conditions, the changes in PBL height under partly cloudy condition are no more
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than 170 m for both NBL and CBL types.
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Keywords: Radiosonde, convective PBL, PBL height, thermodynamic stability, China 2
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1. Introduction
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The planetary boundary layer (PBL), the lowest layer of troposphere close to the
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surface, plays an important role in air pollution, weather and climate (Garratt 1992;
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1994; Stull 1988) . The turbulent PBL is also the main place where the vertical exchange
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of momentum, heat, moisture, and atmospheric pollutants occurs between ground
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surface and free troposphere (e.g., Garratt 1992; Oke 2002; Hu et al. 2010; Zhang et
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al. 2015; Guo et al. 2016a). The turbulent motions in the PBL are responsible for the
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mixing processes in the atmosphere, which affects the vertical redistribution of moisture
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and momentum. This in turn impacts on the formation and evolution of boundary-layer
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clouds such as stratocumulus and cumulus (e.g., Paluch and Lenschow 1991; Eltahir
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1996; Amenu and Kumar 2005; Sherwood et al. 2014; Eitan et al. 2017; Zhang et al.
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2017).
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The PBL is increasingly recognized as one of important parameters that strongly
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influence the air quality (Guo et al. 2009; Sandip and Martial 2015; Li et al. 2017),
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partly account for the varying regional warming rate (Davy and Esau 2016), and
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significantly affect global weather and climate system (Medeiros et al. 2005; Esau et
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al. 2010; Fletcher et al. 2016; Solomon et al. 2017). Previous studies show that the
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heating and cooling of ground surface, cloudiness, and the variability in surface
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characteristics could affect the evolution of the PBL (e.g., Garratt 1994; Miao et al.
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2015a; 2017). The PBL height during the daytime is found to be significantly negatively
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correlated with surface relative humidity, but positively correlated with surface
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temperature (Zhang et al. 2013). Likewise, on the seasonal and annual timescales, the 3
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PBL height is positively associated with surface temperature and 10-m wind speed, but
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negatively associated with surface pressure (Seidel et al. 2012; Guo et al. 2016b).
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The PBL height is traditionally determined from the vertical profiles of temperature,
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humidity, and wind from atmospheric soundings. In addition, several other methods
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based on new data sources (e.g., lidar, doppler radar, and sodar) have been developed to
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elucidate the PBL structure/processes (e.g., Holzworth 1964; Coulter 1979; Beyrich
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1997; Steyn et al. 1999; Seibert 2000; Asimakopoulos et al. 2004; Hennemuth and
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Lammert 2006; Lammert and Bösenberg 2006; Dandou et al. 2009; Mao et al. 2009;
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McGrath-Spangler and Denning 2012; Chan and Wood 2013; Sawyer and Li 2013;
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Zhang et al. 2014; Guo et al. 2016b; Zhang et al. 2016).
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Based on the atmospheric soundings collected in several major field campaigns, the
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diurnal variation of three dominant PBL types, namely convective boundary layer
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(CBL), stable boundary layer (SBL), and neutral boundary layer (NBL) in the
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continental U.S., were documented (Liu and Liang 2010): the CBL mainly occurs in
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the daytime with a peak at 1500 local time, in contrast to the SBL dominating at
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nighttime. Seidel et al. (2010) calculated the PBL height by applying seven commonly
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methods to the profiles of temperature, potential temperature, virtual potential
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temperature, relative humidity, specific humidity, and refractivity, and found there
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existed large uncertainties with respect to various methods to estimate PBL height .
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Then the climatology of the PBL height over Europe and the continental U.S. were
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compiled using the bulk Richardson number method (Seidel et al. 2012). The same
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method has also been utilized by Guo et al. (2016b) to develop a radiosonde-based PBL 4
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height climatology in China, which exhibits large diurnal, seasonal, and spatial
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variations. This could be at least in part in association with the strong variations in
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aerosol pollution in China (Guo et al. 2011; Guo et al. 2016c) and atmospheric
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thermodynamic stability (Tokinaga et al. 2006; Tang et al. 2016). Although the PBL
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height over China has been well characterized from space-borne lidars (e.g., Liu et al.
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2015; Zhang et al. 2016) and radiosondes (Guo et al., 2016b), most of other PBL
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characteristics remain poorly understood, including the thermodynamic stability and its
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associated meteorological factors.
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Since 2011, intensive radiosonde observational campaigns at 1400 Beijing time (BJT)
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in summer (May-June-July-August) have been performed at most sites of the newly-
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updated L-band China Radiosonde Network (CRN) as operated by the China
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Meteorological Administration (CMA) (Guo et al. 2016b; Zhang et al. 2016). The
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afternoon soundings provide us a unique opportunity to look into the thermodynamic
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stability of PBL. Thus, one of the main objectives of this study is to unravel the
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occurrence frequency and spatial distribution of CBL, SBL, and NBL across China
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using long-term fine-resolution atmospheric soundings. Another objective is to
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understand the relationships between meteorological variables and PBL height that
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differs by three PBL types.
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The rest of the present paper is organized as follows. Section 2 describes the data and
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method used in this study, as well as the uncertainty analysis for the PBL height
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retrievals. Section 3 presents the climatology of different PBL types in China, and its
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association with other atmospheric variables. In section 4, detailed discussions with 5
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regard to the PBL height climatology and the potential influential factors on the
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development of PBL will be provided. Finally, the key findings are summarized in
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section 5.
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2. Data and Methods
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2.1 Radiosonde and meteorological data
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The L-band radiosondes of CRN are conventionally launched twice a day at 0800
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and 2000 BJT, and additional soundings are made at 1400 BJT in summer (May-June-
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July-August) at most radiosonde sites mainly for improving the capability of predicting
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high-impact weather in China. The L-band radiosondes are produced by the Nanjing
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Daqiao Co. LTD., and the Shanghai Changwang Meotech Co. LTD. (Xu et al. 2007),
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which provide fine resolution (1s) profiles of temperature, humidity, wind speed and
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wind direction (Guo et al. 2016b; Zhang et al. 2016). As the previous studies (Tao et al.
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2006; Xing et al. 2009; Bian et al. 2011; Ma et al. 2011) pointed out, the accuracy of
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the temperature profiles within lower portion of troposphere is similar to that of GPS
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RS92 radiosonde (produced by Vaisala), which is less than 0.1 K. Such good data
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accuracy provides us enough confidence to investigate PBL climatology in China using
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these L-band soundings of CRN.
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In this study, the characteristics of afternoon PBL during summer were investigated
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using the 1400 BJT soundings from 2012 to 2016 collected from CRN, due to most of
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the exchanges of energy, moisture and air pollutants occurring in the PBL in the
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afternoon (Garratt 1994; Guo et al. 2016b). Considering the time consistency, the near6
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surface meteorological observations from the collocated radiosonde sites are used to
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understand the factors governing the PBL development, including near-surface
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temperature (Tsfc), near-surface relative humidity (RHsfc), near-surface pressure (Psfc)
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and 10-m wind speed (WS10). The former three meteorological factors were calculated
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by taking average over the five lowest consecutive soundings starting from the ground
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surface, whereas WS10 calculated by taking average over the wind profiles from the
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second lowest to third lowest soundings.
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Given the substantial difficulties and uncertainties in determining the PBL types
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caused by precipitation, the 1400 BJT sounding data have been excluded for the days
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when precipitation occurred from 1200 BJT to 1400 BJT. Additionally, the sites that
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have less than 30 valid soundings at 1400 BJT from 2012 to 2016 are not considered.
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As a result, there are 5399 valid summertime soundings collected from the 92
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radiosonde sites of CRN at 1400 BJT. The total cloud cover (CLD) data measured at
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1400 BJT at the same radiosonde sites have been used, in addition to the hourly rain
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gauge measurements recorded at the hours from 1200 BJT to 1400 BJT.
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To understand the impacts of land surface processes on the development of PBL in
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the afternoon, the dataset from Global Land Data Assimilation System (GLDAS, Rodell
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et al., 2004) were analyzed. The GLDAS contains a series of geophysical parameters
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reflecting the state of land surface (e.g., soil moisture and surface temperature) and flux
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(e.g., evaporation and sensible heat flux), based on four land surface models (CLM,
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Mosaic, Noah and VIC). The surface sensible heat fluxes (SHFsfc) and soil moisture
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fields in China derived from the Noah model (Chen and Dudhia 2001) were used as 7
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well.
2.2 Determination of PBL type
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During the daytime, due to the sufficient solar radiation reaching the land surface,
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the CBL typically dominates over land, which can reach a few kilometers in the
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afternoon (Chen and Houze 1997). By contrast, when the surface cools by the nocturnal
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radiation processes, the SBL takes over the bottom portion of the troposphere ( Zhang
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et al. 2011; Miao et al. 2015b). The residual layer is neutrally stratified, resulting in
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turbulence that is nearly of equal intensity in all directions ( Sivaraman et al. 2013;
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Blay-Carreras et al. 2014), referred to as the NBL.
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Despite the dominance of CBL in the afternoon, the SBL and NBL may form under
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certain meteorological conditions (Medeiros et al. 2005; Poulos et al. 2002; Stull 1988).
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The complicated connections between the PBL structure and meteorology make it
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imperative that the PBL types be determined upfront.
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Here the determination of PBL types is largely based on the methods developed by
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Liu and Liang (2010), where the soundings they used are at a vertical resolution of ~5
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hPa, which is in sharp contrast to the vertical resolution of L-band radiosonde (~1 hPa)
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to be used in the present study. Therefore, subsample operation in the vertical has been
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performed by resampling 5 consecutive original 1-hPa measurements to one 5-hPa
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measurement. As such, a series of coarser-resolution soundings (5-hPa) are derived,
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and Figure S1 is a case in point. Then, the PBL types were determined by calculating
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the near-surface potential temperature difference (PTD) between the fifth lowest 8
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measurement and second lowest measurement. The threshold value of PTD is set to be
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0.1 K, and the thermodynamic stability of PBL has to be further determined using the
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bulk Richardson number based on the original lowest 100 m radiosonde measurements.
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Specifically, if the PTD lies between -0.1 K and 0.1 K, the PBL is identified as NBL; if
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the PTD is higher than 0.1 K and the bulk Richardson number is positive, the PBL types
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is identified as SBL; other PBL cases can be considered as CBL (Vogelezang and
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Holtslag 1996; Eresmaa et al. 2006; Tang et al. 2015).
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2.3 PBL height estimation and uncertainty analysis
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The estimation method of PBL height differs by varying PBL types. Similar to Liu
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and Liang (2010), the PBL height for both CBL and NBL is calculated as the height at
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which an air parcel rising adiabatically from the surface becomes neutrally buoyant
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(Stull 1988). Starting from the surface, the height where the gradient of potential
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temperature firstly becomes greater than a certain gradient threshold (GT) of potential
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temperature is considered as the estimated PBL height.
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For the SBL, the determination of the PBL height is much more uncertain.
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Turbulence in the SBL can result from either buoyancy forcing or wind shear (Bonner
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1968; Garreaud and Muñoz 2005). If both the stability-derived and wind-shear-derived
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PBL heights are derived, the lowest of which is estimated to be the PBL heights for
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SBL (Liu and Liang 2010; Sivaraman 2013). Figure S2 illustrates the typical profiles
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of CBL, NBL, and SBL, and their corresponding PBL heights are 1650, 721, and 249
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m above ground level (AGL), respectively. 9
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To quantify the uncertainties in the estimation of PBL heights, sensitivity analysis
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by selecting various values of GT has been performed (Argentini et al. 2005). Figures
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1a-b compared the PBL heights calculated using 3.5 K/km, 4 K/km 4.5 K/km as GT.
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As expected, the PBL heights of GT = 4.5 K/km (3.5 K/km) are generally higher (lower)
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than those of GT= 4 K/km. The heights of NBL and CBL based on different GT are
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significantly correlated with each other (R = 0.98). In contrast, the heights of SBL based
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on various GT have relatively weaker correlations with each other.
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We further checked the changes in occurrence frequency of SBL, NBL and CBL
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caused by the different critical GT values applied. The changes in their corresponding
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PBL heights were examined as well, which is summarized in Table S1. Overall, the
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deviation of frequency for SBL, NBL and CBL is less than 13%, and the changes in
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PBL heights is not more than 177 m, which could make sense for SBL cases. However,
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the uncertainties can be negligible for NBL and CBL, since most of their PBL heights
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are much higher.
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3. Results
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This section presents the basic climatology of three PBL types (i.e., SBL, NBL, and
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CBL). Comparisons of radiosonde-based PBL height climatology under different CLD
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conditions are made to elucidate the impacts of cloud on the development of PBL in the
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afternoon during the summer. Then we present comprehensive analysis results with
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regard to the correlations between PBL heights and four typical meteorological
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variables, including Tsfc, Psfc, RHsfc, and WS10. 10
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3.1 Climatology of PBL height for various PBL types
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Figure 2 shows the frequency distribution and cumulative frequency distribution of
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PBL heights for SBL, NBL, and CBL. At 1400 BJT, 96 % of SBL are lower than 500
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m, while 83 % (76 %) of CBL (NBL) are higher than 1000 m. On the whole, all PBL
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heights estimated from soundings are lower than 5 km at 1400 BJT across China.
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In terms of the numbers of valid soundings at each site at 1400 BJT during summer,
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the numbers of soundings at 69 sites (about 75%) range from 30 to 60 (Figure 3). In
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particular, the valid soundings at Beijing (39.80°N, 116.47°E) and Shanghai (31.40°N,
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121.48°E) are larger than 150. Figures 3b-d presents the spatial distributions of
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occurrence frequency with respect to CBL, NBL and SBL at 1400 BJT across China,
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which on average are 70 %, 26 %, and 4 %, respectively. The occurrence frequency of
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NBL is generally less than 40 % at most sites of CRN. By comparison, the CBL
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dominates over most sites of CRN.
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Irrespective of PBL regimes, Figure 4a displays the spatial distribution of mean PBL
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height at 1400 BJT in China. The averaged PBL height of all sites is 1694 m AGL,
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indicative of high tendency of well-developed PBL in the afternoon. A prominent north-
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south PBL height gradient is found, with higher PBL height in northwestern China,
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which is consistent with spatial pattern revealed by previous PBL studies derived from
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the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) onboard CALIPSO
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(Huang et al. 2009; Liu et al. 2015; Zhang et al. 2016). The climatological PBL height
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in northwest China is generally greater than 1800 m AGL, whereas the values are 11
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usually less than 1400 m AGL over southeast and coastal areas. The high soil moisture
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may be one of the causes accounting for the relatively shallower daytime PBL over the
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east coast (McGrath-Spangler and Denning 2012; Wang and Wang 2014; 2016). In
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contrast, the low soil moisture in the north and west regions tends to favor the PBL
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development by partitioning more solar radiation to sensible heat flux (Wang and Wang
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2014).
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Figures 4b-d illustrate the spatial distributions of PBL height of SBL, NBL, and CBL.
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Specifically, Figure 4b shows that the PBL height for SBL is merely 152 ± 100 m AGL
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(mean value ± standard deviation). However, the values increase sharply to 1656 ± 562
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m AGL for NBL, and 1803 ± 550 m AGL for CBL (Figures 4c-d). In other words, the
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mean PBL heights tend to be on average reduced by ~150 m in the presence of NBL as
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compared with those for CBL. Although the SBL develop more frequently after sunset,
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which can also be established during the daytime under certain synoptic conditions.
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To reveal the spatial discrepancy in PBL, we further selected six typical regions of
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interest (ROIs, defined and shown in Figure S3), including (1) the North China Plain
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(NCP), (2) the Yangtze River Delta (YRD), (3) the Pearl River Delta (PRD), (4) the
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Taklamakan Desert (TKD), (5) the Tibetan Plateau (TBP), and (6) the Sichuan Basin
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(SCB). The occurrence frequency and averaged PBL height of CBL, NBL and SBL at
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these six ROIs are illustrated in Figure 5 and Table 1. Except for PRD, in which the
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1400 BJT soundings were only launched in May and June, the 1400 BJT soundings in
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other ROIs are all launched throughout the whole summer season (June-July-August),.
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Overall, the CBL occurs most frequently in summer (60%), about 2 times higher than
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the frequency of NBL (less than 30%), let alone the frequency of SBL (less than 10%).
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Among these six ROIs, the TBP has the highest occurrence frequency of CBL (Figure
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5), characterized by an averaged PBL height of 2222 m AGL (Table 1). Although both
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PRD and TKD have similar occurrence frequencies for all the three PBL types, the
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mean PBL heights are quite different, which are 1067 and 2248 m AGL for PRD and
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TKD, respectively (Table 1). This large difference in PBL height over PRD and TKD
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could be relevant to the large discrepancy of RHsfc and SHFsfc. The mean RHsfc over
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PRD and TKD is 50 % and 22 %, respectively, whereas the mean SHFsfc 113 W/m2 and
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290 W/m2 at these two ROIs. In the afternoon, less moisture in the PBL of arid TKD is
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associated often with lower RH and higher SHF, which generally facilitates the
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development of PBL (Stull 1988; Liu et al. 2004; Dirmeyer et al. 2014). This could be
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likely the main reason why the high PBL height occurred in TKD rather than in PRD.
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3.2 The potential impact of cloud on the development of various PBL regimes
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It is well known that clouds alter the solar radiation reaching the land surface, which
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further modulates the PBL in the afternoon. Figure 6 shows the mean PBL height and
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frequency for SBL, NBL, and CBL under different CLD conditions. It is intriguing to
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note that the PBL height does not change much when CLD ranges from 0 % to 80 %,
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no matter what the PBL regime is. Only when the CLD becomes higher than 80%,
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which is referred to as the overcast condition, the PBL height for SBL, NBL and CBL
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decreases by a certain magnitude. As compared with that under clear-sky condition 13
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(CLD < 20%), the averaged PBL height at 1400 BJT under overcast condition decreases
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by ~400 m and ~300 m for NBL and CBL, respectively. In contrast, the depth of SBL
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keeps almost constant. Relative to PBL under clear-sky condition, the changes in PBL
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height under partly cloudy condition are less than 170 m for both NBL and CBL regimes
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(Table S2).
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Figure 6b shows the occurrence frequency of SBL, NBL, and CBL under different
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CLD conditions in summer. In general, the occurrence frequency of CBL is higher than
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40 % under overcast conditions, almost twice the frequency under clear-sky conditions.
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Similarly, the occurrence frequencies of NBL are ~20 % and ~50 % for the clear-sky
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and overcast conditions, respectively. For SBL, the occurrence frequency is merely ~10 %
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under clear-sky conditions, as compared to ~70 % under overcast conditions. Such huge
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differences existing in the occurrence frequency of CBL, NBL, and SBL suggested that
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the CLD could play a significant role in modulating the development of PBL. The
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presence of clouds tends to reduce the solar radiation reaching surface in the afternoon,
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and thus weaken the vertical turbulent mixing, which in turn suppress the PBL
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development (Li et al. 2017). Therefore, a reduced PBL height typically ensues (Wetzel
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et al. 1996; Freedman et al. 2001; Zhou and Geerts 2013).
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To figure out the spatial distribution of potential cloud influences on CBL, 29
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radiosonde sites evenly scattered across the China were selected, in which the number
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of valid CBL sounding is more than 30 for both overcast and clear-sky conditions. As
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shown in Figure 7, one noticeable feature is that the PBL height under clear-sky
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condition is systematically lower than that under overcast condition throughout all these 14
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29 sites, even though the PBL height under both conditions ranges from 800 m to 2600
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m. On average, the difference of mean PBL height is 336 m, as indicated in Figure 7c.
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3.3 Relationships between PBL height and meteorological variables
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In this section, we will analyze the relationships between PBL height and several
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meteorological parameters, including Tsfc, RHsfc, Psfc, and WS10. To better understand
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the development of SBL in the afternoon, the meteorological factors for the days with
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SBL were compared with those with CBL, including CLD, Tsfc, RHsfc, WS10 and SHFsfc.
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Comparing with the CBL regime, the SBL tends to be formed under the meteorological
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conditions characterized by relatively high CLD and RHsfc, and low Tsfc, WS10 and
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SHFsfc (Figure 8).
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Figure 9 shows the correlation coefficient (R) of PBL height and various
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meteorological factors in the six ROIs for different PBL regimes, and the gray shaded
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areas indicate the R values are statistically significant at 95% confidence level.
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Typically, the PBL height is positively correlated with Tsfc in six ROIs for both NBL
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and CBL, including the NCP (R = 0.37, 0.25 for NBL and CBL, respectively), YRD (R
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= 0.55, 0.62), PRD (R = 0.73, 0.66), SCB (R = 0.68 under CBL), and TBP (R = 0.19,
320
0.24), and the lowest correlation (R = 0.02, 0.13) is found in TKD. For SBL, only the
321
correlation of YRD region show here (R = -0.30). More details on the correlations can
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be found in Table S3.
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In contrast, negative relationships are observed between the PBL height and RHsfc in
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summer for NBL and CBL (Figure 9), and the R values are mostly lower than -0.6 15
325
except for those in TKD (R = -0.39 for NBL, -0.51 for CBL) regions. By comparison,
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the positive correlations (R = 0.36) in TKD between PBL height and RHsfc are found in
327
summer for SBL. For a specific location, the high RHsfc may be associated with high
328
soil moisture and high latent heat flux, which will inhibit the occurrence of convection
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and turbulence within SBL, and thus lead to a relatively deeper SBL. For CBL and NBL,
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higher Tsfc and lower RHsfc have been found to be linked to more sensible heat fluxes,
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favoring the development of PBL during the daytime (Zhang et al. 2013).
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Besides, the relationships are ubiquitously negative between the PBL height and Psfc
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in summer for CBL, NBL and SBL. Similar correlation analyses using WS10 show that
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these parameters are not significantly correlated with PBL height at 1400 BJT in
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summer.
336
4. Discussions
337
Regarding the large discrepancy in PBL height in PRD and TKD (Figure 5), it is
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known that PRD is characterized by richer vegetation cover compared with TKD, the
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relatively strong surface evaporation in PRD can substantially increase the near-surface
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humidity, and lead to less surface sensible heat flux, which suppress the PBL height
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over PRD (Zhang et al. 2013; Dirmeyer et al. 2014).
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In addition to the local-scale atmospheric processes, the large-scale synoptic
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conditions have been well recognized to be able to govern the development of the PBL
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(Whiteman and Doran 1993; Garratt 1994; Hoover et al. 2015; Miao et al. 2017). For
345
example, the subsidence associated with the high-pressure systems (e.g., anti-cyclone) 16
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may suppress the development of PBL, while the large-scale upward motions induced
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by the low-pressure systems (e.g., fronts, cyclone) would favor the growth of PBL (Stull
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1988). Specifically, the cold front passage generally comes with CBL, since the strong
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winds within PBL would favor the development the PBL dynamically. In contrast,
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sometimes the presence of warm front could facilitate the formation of SBL, since warm
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air mass behind the front would climb over the local colder air, leading to an increase
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in atmospheric stability and a suppressed growth in PBL to some extent (Keyser and
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Anthes 1982; Stull 1988; Sun et al. 2002). Conversely, the stable PBL is also favorable
354
for the low-level stratus cloud formation and maintenance (Lin et al. 2016).
355
Furthermore, clouds (especially under overcast condition) are generally associated
356
with lower PBL height (Figure 6). As shown in Figure 10, compared with the negligible
357
association between CLD and WS10, high CLD was found to be associated with low
358
Tsfc and higher RHsfc (Betts et al. 2017; Garcia-Carreras et al. 2017), which also
359
accompanied with the lower PBL height in the afternoon (Zhang et al. 2013). Although
360
high wind speed is recognized to be positively associated with annual averaged PBL
361
height in the morning or afternoon (Guo et al. 2016b), this positive relation cannot be
362
apparently seen from the afternoon PBL during summer. There may exist complicated
363
coupling effects between the near-surface meteorological factors, CLD, and PBL in the
364
afternoon. For instance, high CLD is often associated with high RHsfc and low PBL
365
height, due to the modulation of surface solar radiation. Meanwhile, the low PBL height
366
may limit the vertical exchange of water vapor between the moister surface layer and
367
upper dryer free troposphere, leading to the relatively high RHsfc (Hohenegger et al. 17
368
2009；Zhou and Geerts 2013).
369
In addition, the soil moisture may be another important factor dictating the
370
development of PBL, since it affects the land surface energy budget (e.g., surface
371
sensible heat flux and latent heat flux). For a specific location, the lower soil moisture
372
generally comes with high surface sensible heat flux, which facilitates the PBL
373
development in the afternoon (McCumber and Pielke 1981; Sanchez-Mejia and Papuga
374
2014; Rihani et al. 2015). As illustrated in Figure S4, the northwest of China is mainly
375
covered by bare land, characterized by low soil moisture, high SHFsfc, high Bowen ratio,
376
and low latent heat flux, all of which favors the development of PBL. On the contrary,
377
the high soil moisture, high latent heat flux, low SHFsfc and low Bowen ratio in the
378
southern China are unfavorable to the development of PBL. Besides, the high latent
379
heat flux tends to favor cloud formation, and the increased clouds will in turn results in
380
reduced surface temperature, increased soil moisture and decreased PBL height (Betts
381
et al. 1996; Eltahir 1998; Pal and Eltahir 2001; Zhou and Geerts 2013). Additionally,
382
the different land covers in the northwest and southeast of China may also play a role
383
in modulating the land surface processes and PBL development. The complex
384
relationships/connections between the land surface properties and PBL in China need
385
more studies in the future.
386
5. Conclusions
387
Based on the 1400 BJT soundings in summer collected from the CMA radiosonde
388
network across China from 2012 to 2016, this study investigated the occurrence
18
389
frequency, spatial distribution of different PBL types (i.e., SBL, NBL, and CBL) in
390
China. Besides, the relationships between the PBL heights and other meteorological
391
variables were examined, including CLD, Tsfc, Psfc, RHsfc and WS10.
392
The occurrence frequency of CBL is about 70 %, followed by 26 % for NBL, and 4 %
393
for SBL. At 1400 BJT, the spatial distributions of PBL heights for NBL and CBL are
394
quite similar, demonstrating a prominent north-south gradient of PBL height. The depth
395
of SBL (152 ± 100m) is generally lower than those of NBL (1656 ± 562 m) and CBL
396
(1803± 550 m). At 1400 BJT, clouds tend to reduce the occurrence frequency of CBL
397
and NBL. In terms of cloud impact on the frequency of SBL, it is found to be merely
398
~10 % under clear-sky conditions, as compared to ~70 % under overcast conditions,
399
indicating that clouds are one of the key factors dictating the frequency of SBL. With
400
the exception of the discernible changes in PBL height under overcast condition relative
401
to the PBL height under clear-sky condition, the PBL height under partly cloudy
402
condition does not change much (less than 170 m) under CBL and NBL regimes. This
403
indicates that high cloud cover could be an important factor modulating PBL
404
development. Besides, the PBL heights of CBL and NBL were found to be positively
405
correlated with the near-surface meteorological factors such as Tsfc, but negatively
406
associated with RHsfc. For a specific location, the low RHsfc usually accompanies with
407
low CLD and low soil moisture, which could favor the development of PBL in the
408
afternoon with high surface sensible heat flux.
409
Although the CBL dominates over China in the summer afternoon, the NBL and SBL
410
could also be established and cannot be overlooked. This study merely investigated the 19
411
afternoon PBL in summer, more efforts should be made to expand the climatology of
412
different PBL types to other seasons in China when more sounding data are available
413
in the future.
414
Acknowledgements
415
This work was supported by the National Natural Science Foundation of China under
416
Grants 41771399, 41705002, 91544217 and 41471301, Ministry of Science and
417
Technology under Grant 2014BAC16B01, and Chinese Academy of Meteorological
418
Sciences under Grants 2017Z005 and 2017Y002. The authors would like to
419
acknowledge Chinese Meteorological Administration for providing the radiosonde
420
dataset. Last, but not least, special thanks go to four anonymous reviewers for their
421
valuable comments and suggestions that helped improve the quality of our manuscript.
20
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700
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30
707
Figure captions:
708
Figure 1. Scatter plots showing (a) PBL heights computed using 4 K/km (in the x-axis)
709
and 4.5 K/km (in the y-axis) as critical potential temperature gradient threshold (GT),
710
and (b) the PBL heights computed using 4 K/km (in the x-axis) and 3.5 K/km (in the y-
711
axis) as critical GT. The results are based on 5399 soundings across China in summer
712
(May-June-July-August) from 2012 to 2016.
713
Figure 2. Frequency distribution (a) and cumulative frequency distribution (b) of PBL
714
heights for SBL (in black), NBL (in red), and CBL (in blue) at 1400 BTJ in summer for
715
the period of 2012-2016. The number of soundings (N) and mean value at each
716
observed time are also given.
717
Figure 3. Spatial distributions of (a) # of soundings from CMA radiosonde sites and
718
the frequency (in percent) for PBL states classified by atmospheric stability: (b) SBL,
719
(c) NBL and (d) CBL at 1400 BJT in summer for the period of 2012-2016, respectively.
720
Figure 4. Spatial distribution of mean PBL height for (a) all types, (b) SBL, (c) NBL,
721
and (d) CBL at 1400 BJT in summer for the period of 2012-2016.
722
Figure 5. The occurrence frequency of SBL, NBL and CBL in six ROIs, including (1)
723
the North China Plain (NCP), (2) the Yangtze River Delta (YRD), (3) the Pearl River
724
Delta (PRD), (4) the Taklamakan Desert (TKD), (5) the Tibetan Plateau (TBP), and (6)
725
the Sichuan Basin (SCB), at 1400 BJT during in summer (May-June-July-August) for
726
the period of 2012 to 2016, respectively. The bars show the frequency of different PBL
727
types (SBL in black, NBL in gray, and CBL in white). The pie charts show the frequency
728
percentage distribution for each month.
729
Figure 6. The bar charts showing the mean PBL height (a) and the occurrence
730
frequency (b) for three PBL types, including SBL (in gray), NBL (in red), and CBL (in
731
blue) under different cloud cover (CLD) conditions (with a bin size of 20%) in summer
732
for the period of 2012-2016. Note that the error bar in (a) denotes the standard deviation
733
of PBL height. 31
734
Figure 7. The spatial distributions of mean PBL heights at 1400 BJT in summer for the
735
period 2012-2016 for CBL days under clear-sky (0 ≤ CLD ≤ 20% (a) and overcast
736
conditions (80%≤CLD≤100% (b). Panel (c) shows the spatial distribution with respect
737
to the differences of PBL heights between clear-sky and overcast conditions.
738
Figure 8. The box plots comparing such meteorological factors as (a) cloud cover
739
(CLD), (b) surface temperature (Tsfc), (c) relative humidity (RHsfc), (d) 10-m wind
740
speed (WS10), and (e) surface sensible heat flux (SHFsfc) between the days with SBL
741
and those with CBL, as averaged over 67 radiosonde sites with valid SBL observations.
742
Figure 9. The correlation coefficient of PBL heights and meteorological parameters in
743
six ROIs under SBL (in black), NBL (in red), and CBL (in blue) conditions. The
744
meteorological variables include surface temperature (Tsfc), surface pressure (Psfc),
745
surface relatively humidity (RHsfc), 10-m wind speed (WS10), V component of 10-m
746
wind speed (WS(V)10), U component of 10-m wind speed (WS(U)10). The gray
747
shadings indicate the correlations are statistically significant at 95% confidence level.
748
Figure 10. The box plots comparing several meteorological factors between clear-sky
749
days and overcast days, including (a) surface temperature (Tsfc), (b) surface relative
750
humidity (RHsfc), (c) 10-m wind speed (WS10), and (d) surface sensible heat flux
751
(SHFsfc). Note that only the radiosonde sites that have more than 30 observations for
752
both clear-sky and overcast days were considered.
32
753
Table list
754
Table 1. The statistics of mean value and standard deviation (sd) of PBL heights (in
755
unites of m AGL) for SBL, NBL and CBL in six ROIs, including (1) the North China
756
Plain (NCP), (2) the Yangtze River Delta (YRD), (3) the Pearl River Delta (PRD), (4)
757
the Taklamakan Desert (TKD), (5) the Tibetan Plateau (TBP), and (6) the Sichuan Basin
758
(SCB) at 1400 BJT in summer (May-June-July-August) for the period of 2012 to 2016. NCP mean (sd)
YRD mean (sd)
PRD mean (sd)
TKD mean (sd)
TBP mean (sd)
SCB mean (sd)
SBL
109(110)
199(184)
174(163)
114(98)
211(197)
-
NBL
1529(712)
1241(455)
999(330)
1982(1020)
2188(752)
1300 (338)
CBL
1703(739)
1297(474)
1067(365)
2248(1004)
2222 (751)
1317(409)
759
33
(a) Critical GT(4 vs. 4.5)
PBL height(m, GT=4.5)
2000
SBL(R=0.73)
1500 1000 500 0
0
5000
NBL(R=0.98)
4000
5000
3000
3000
2000
2000
1000
1000
500 1000 1500 2000 PBL height(m, GT=4)
0
0
CBL(R=0.99)
4000
1000 2000 3000 4000 5000 PBL height(m, GT=4)
0
0
1000 2000 3000 4000 5000 PBL height(m, GT=4)
PBL height(m, GT=3.5)
(b) Critical GT(4 vs. 3.5) 1500
SBL(R=0.92)
1000 500 0
0
760
5000 4000
NBL(R=0.98)
5000 4000
3000
3000
2000
2000
1000
1000
500 1000 1500 PBL height(m, GT=4)
0
0
1000 2000 3000 4000 5000 PBL height(m, GT=4)
0
CBL(R=0.98)
0
1000 2000 3000 4000 5000 PBL height(m, GT=4)
(c) Subsample of radiosonde profile
762
1000
5000
5000
800
4000
4000
600
3000
3000
200
1000
1000
Figure 1. SBL(R=1) Scatter plots showing (a)NBL(R=1) PBL heights computed using 4 K/km (in the x-axis) CBL (R=1) PBL height (m)
761
and 4.5 K/km (in the y-axis) as critical potential temperature gradient threshold (GT), 400 2000 2000
763
and (b) the PBL heights computed0 using 4 K/km (in the x-axis) and 3.5 K/km (in the y0 0
764
axis) as critical GT. The results are based on 5399 soundings across China in summer
765
(May-June-July-August) from 2012 to 2016.
0
200 400 600 800 1000 PBL height(m)
0
1000 2000 3000 4000 5000 PBL height(m)
766
34
0
1000 2000 3000 4000 5000 PBL height(m)
30
(a)
Relative Frequency(%)
Frequency (%)
25
100 SBL (N=217, Mean=152 m) NBL (N=1404, Mean=1656 m) CBL (N=3778, Mean=1803 m)
20 15 10
80
60
40
20
5 0
0 0
767
(b)
1000
2000 3000 4000 PBL height (m)
5000
0
1000
2000 3000 4000 PBL height (m)
5000
768
Figure 2. Frequency distribution (a) and cumulative frequency distribution (b) of PBL
769
heights for SBL (in black), NBL (in red), and CBL (in blue) at 1400 BTJ in summer for
770
the period of 2012-2016. The number of soundings (N) and mean value at each
771
observed time are also given.
35
(a)
(b) 50
Latitude (°N)
Latitude (°N)
50 40 30 150 - 180 120 - 150 90 - 120 60 - 90 30 - 60
20
75
85
40 30 20
95
105
115
125
15% - 20% 10% - 15% 5% - 10% 0% - 5%
75
135
85
(c)
Latitude (°N)
Latitude (°N)
115
125
135
125
135
50
40 30 80% - 100% 60% - 80% 40% - 60% 20% - 40% 0% - 20%
75
772
105
(d)
50
20
95
Longitude (°E)
Longitude (°E)
85
40 30 20
95
105
115
125
135
Longitude (°E)
80% - 100% 60% - 80% 40% - 60% 20% - 40% 0% - 20%
75
85
95
105
115
Longitude (°E)
773
Figure 3. Spatial distributions of (a) # of soundings from CMA radiosonde sites and
774
the frequency (in percent) for PBL states classified by atmospheric stability: (b) SBL,
775
(c) NBL and (d) CBL at 1400 BJT in summer for the period of 2012-2016, respectively.
36
(a)
(b) 50
Latitude (°N)
Latitude (°N)
50 40 30 20
40 30 400 300 200 100 0 -
20
75
85
95
105
115
125
135
75
- 500 - 400 - 300 - 200 100
85
Longitude (°E)
115
125
135 Unit:m
(d) 50
Latitude (°N)
50
Latitude (°N)
105
Longitude (°E)
(c)
40 30
40 30 20
20 75
776
95
85
95
105
115
125
135
75
Longitude (°E)
85
95
105
115
125
135
3000 - 3200 2800 - 3000 2600 - 2800 2400 - 2600 2200 - 2400 2000 - 2200 1800 - 2000 1600 - 1800 1400 - 1600 1200 - 1400 1000 - 1200 800 - 1000 600 - 800 400 - 600 200 - 400 0 - 200
Longitude (°E)
777
Figure 4. Spatial distribution of mean PBL height for (a) all types, (b) SBL, (c) NBL,
778
and (d) CBL at 1400 BJT in summer for the period of 2012-2016.
37
779
100
59
Frequency (%)
80
28
28
N=468 6
44
19
22
N=171
N=433
N=352
7
18
52
47 53
87
N=70
N=691
30
14
49
37 (%)
60
May June July August SBL NBL CBL
40
20
0
780
NCP
YRD
PRD
TKD
TBP
SCB
781
Figure 5. The occurrence frequency of SBL, NBL and CBL in six ROIs, including (1) the North
782
China Plain (NCP), (2) the Yangtze River Delta (YRD), (3) the Pearl River Delta (PRD), (4) the
783
Taklamakan Desert (TKD), (5) the Tibetan Plateau (TBP), and (6) the Sichuan Basin (SCB), at 1400
784
BJT during in summer (May-June-July-August) for the period of 2012 to 2016, respectively. The
785
bars show the frequency of different PBL types (SBL in black, NBL in gray, and CBL in white). The
786
pie charts show the frequency percentage distribution for each month.
38
PBL Height (m)
4000
(a)
SBL NBL CBL
3000 2000 1000 0 0
Frequency(%)
80
20
40
CLD (%)
60
(b)
80
100
80
100
SBL NBL CBL
60 40 20 0 0
20
40
60 CLD (%)
787 788
Figure 6. The bar charts showing the mean PBL height (a) and the occurrence
789
frequency (b) for three PBL types, including SBL (in gray), NBL (in red), and CBL (in
790
blue) under different cloud cover (CLD) conditions (with a bin size of 20%) in summer
791
for the period of 2012-2016. Note that the error bar in (a) denotes the standard deviation
792
of PBL height.
793
794
39
Latitude (°N)
(a) 50
50
40
40
30
30
20
20 75
Unit:m
(b)
85
95
105
115
125
135
75
Longitude (°E)
3000 - 3200 2800 - 3000 2600 - 2800 2400 - 2600 2200 - 2400 2000 - 2200 1800 - 2000 1600 - 1800 1400 - 1600 1200 - 1400 1000 - 1200 800 - 1000 600 - 800 400 - 600 200 - 400 0 - 200
85
95
105
115
125
135
Longitude (°E)
(c)
Latitude (°N)
50
40
1600 - 1800 700 - 800 600 - 700 500 - 600 400 - 500 300 - 400 200 - 300 100 - 200 0 - 100
30
20 75
795
85
95
105
115
125
135
Longitude (°E)
796
Figure 7. The spatial distributions of mean PBL heights at 1400 BJT in summer for the
797
period 2012-2016 for CBL days under clear-sky (0 ≤ CLD ≤ 20% (a) and overcast
798
conditions (80%≤CLD≤100% (b). Panel (c) shows the spatial distribution with respect
799
to the differences of PBL heights between clear-sky and overcast conditions.
800
40
(a) All sites 100
100 40
20
(a) SBL
0
CBL
(b)
500
WS10(m/s)
2
SHFsfc (W/m )
15 10 5
40
0
SBL
600
(d)
60
20
10
20
20
RHsfc (%)
o
40
0
80
30
60
Tsfc ( C)
CLD (%)
80
CBL
(c) SBL
CBL
(e)
400 300 200 100 0
0
SBL
CBL
-100
SBL
CBL
801 802
Figure 8. The box plots comparing such meteorological factors as (a) cloud cover
803
(CLD), (b) surface temperature (Tsfc), (c) relative humidity (RHsfc), (d) 10-m wind
804
speed (WS10), and (e) surface sensible heat flux (SHFsfc) between the days with SBL
805
and those with CBL, as averaged over 67 radiosonde sites with valid SBL observations.
41
R
0.8 (c)PRD 0.4 0.0 -0.4 -0.8
0.8 (d)TKD 0.4 0.0 -0.4 -0.8
0.8 (e)TBP 0.4 0.0 -0.4 -0.8 Tsfc
0.8 (f)SCB 0.4 0.0 -0.4 -0.8 Tsfc WS(V)10WS(U)10
R
0.8 (b)YRD 0.4 0.0 -0.4 -0.8
R
SBL 0.8 (a)NCP 0.4 0.0 -0.4 -0.8
806
Psfc
RHsfc WS10
Psfc
NBL
CBL
RHsfc WS WS(V) WS(U)10 10 10
807
Figure 9. The correlation coefficient of PBL heights and meteorological parameters in
808
six ROIs under SBL (in black), NBL (in red), and CBL (in blue) conditions. The
809
meteorological variables include surface temperature (Tsfc), surface pressure (Psfc),
810
surface relatively humidity (RHsfc), 10-m wind speed (WS10), V component of 10-m
811
wind speed (WS(V)10), U component of 10-m wind speed (WS(U)10). The gray
812
shadings indicate the correlations are statistically significant at 95% confidence level.
42
40
100
(a)
80 RHsfc (%)
o
Tsfc ( C)
30 20
40 20
Clear-sky
Clear-sky
Overcast
Clear-sky
Overcast
(d)
400
2
SHFsfc (W/m )
WS10(m/s)
500
(c)
10 5 0
0
Overcast
15
813
60
10 0 20
(b)
300 200 100 0
Clear-sky
Overcast
814
Figure 10. The box plots comparing several meteorological factors between clear-sky
815
days and overcast days, including (a) surface temperature (Tsfc), (b) surface relative
816
humidity (RHsfc), (c) 10-m wind speed (WS10), and (d) surface sensible heat flux
817
(SHFsfc). Note that only the radiosonde sites that have more than 30 observations for
818
both clear-sky and overcast days were considered.
819
43
