Abstract
Abstract. Mixing layer height (h) is an important parameter for understanding the transport process in the troposphere, air pollution, weather and climate change. Many methods have been proposed to determine h by identifying the turning point of the radiosonde profile. However, substantial differences have been observed in the existing methods (e.g. the potential temperature (θ), relative humidity (RH), specific humidity (q) and atmospheric refractivity (N) methods). These differences are associated with the inconsistency of the temperature and humidity profiles in a boundary layer that is not well mixed, the changing measurability of the specific humidity and refractivity with height, the measurement error of humidity instruments within clouds, and the general existence of clouds. This study proposes a method to integrate the information of temperature, humidity and cloud to generate a consistent estimate of h. We apply this method to high vertical resolution (~ 30 m) radiosonde data that were collected at 79 stations over North America during the period from 1998 to 2008. The data are obtained from the Stratospheric Processes and their Role in Climate Data Center (SPARC). The results show good agreement with those from N method as the information of temperature and humidity contained in N; however, cloud effects that are included in our method increased the reliability of our estimated h. From 1988 to 2008, the climatological h over North America was 1675 ± 303 m with a strong east–west gradient: higher values (generally greater than 1800 m) occurred over the Midwest US, and lower values (usually less than 1400 m) occurred over Alaska and the US West Coast.
Highlights
The atmospheric boundary layer is the section of the atmosphere that is sensitive to the varying conditions on the Earth’s surface over a short timescale (Stull, 1988; Seibert et al, 2000; Sportisse, 2010)
relative humidity (RH), q and N are shown. (a) Station #14898 at an0d0:N00(bUlaTcCk soonlid13linAeu) gauresta2ls0o0s6h,o(wbn).st(aa)tioSntat#io1n48#91488a9t80a0t:0000 UUTTCC oonn inet al., 2011, 2010; McGrath-Spangler We suggest that the presence of clouds and Denning, 2013). is an important factor
The h0 corresponds to the highest gradients, which means that the h0 is consistent with that determined by the individual standards of θ, RH, q and N in most cases
Summary
The atmospheric boundary layer is the section of the atmosphere that is sensitive to the varying conditions on the Earth’s surface over a short timescale (hours) (Stull, 1988; Seibert et al, 2000; Sportisse, 2010). The land surface cools at a faster rate than the atmosphere above it because the surface emits more long-wave radiation, which causes the temperature to increase with height above the surface. This temperature inversion depresses the turbulence between the surface and atmosphere; the resulting stable layer is referred to as the “nocturnal stable boundary layer”. The sensible heat flux transfers the surface-absorbed heat, including solar short-wave (Wang et al, 2012) and long-wave radiation (Wang and Dickinson, 2013), to the atmosphere; the latent heat flux moistens the atmosphere. An important feature of the mixing layer is the transition zone, which is characterised by a large variability in pollutants and ambient temperature and humidity values that are between those of the well-mixed boundary layer and the stable free troposphere (Seibert et al, 2000; Kim et al, 2007)
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