Turbulence regimes and the validity of similarity theory in the stable boundary layer over complex terrain of the Loess Plateau, China

Abstract

To gain an insight into the characteristics of turbulence in a stable boundary layer over the complex terrain of the Loess Plateau, data from the Semi-Arid Climate and Environment Observatory of Lanzhou University are analyzed. We propose a method to identify and efficiently isolate nonstationary motions from turbulence series, and then we examine the characteristics of nonstationary motions (nonstationary motions refer to gusty events on a greater scale than local shear-generated turbulence). The occurrence frequency of nonstationary motions is found to depend on the mean flow, being more frequent in weak wind conditions and vanishing when the wind speed, U, is greater than 3.0 m s À1. When U exceeds the threshold value of 1.0 m s À1 for the gradient Richardson number Ri ≤ 0.3 and 1.5 m s À1 for Ri > 0.3, local shear-generated turbulence on timescales of less than 4 min depends systematically on U with an average rate of 0.05 U. However, for the weak wind condition, neither the mean wind speed nor the stability is an important factor for local turbulence. Then turbulence is categorized into three regimes based on the behaviors of nonstationary motions and local turbulence. Regime 1 considers stationary turbulence with a wind speed greater than 3.0 m s À1, and the Monin-Obukhov similarity theory (MOST) can be used to calculate the turbulence momentum flux. Regime 2 examines intermittent turbulence where the MOST is competent to evaluate the local turbulence momentum flux but not nonstationary motions. Regime 3 involves wind speed that is less than the threshold value, where nonstationary motions are dominant, local turbulence is independent of the mean flow, and where the MOST may well be invalid.

Characteristics and mechanisms of the sudden warming events in the nocturnal atmospheric boundary layer: A case study using WRF

“…1). It comprises a large set of instruments that measure boundary layer meteorological variables, surface radiation, and surface fluxes (Huang et al, 2008;Wang et al, 2010;Liang et al, 2014). The boundary layer meteorological measurements include wind speed (014A-L, Met One), air temperature, and relative humidity (HMP45C-L, Vaisala) at 1, 2, 4, 8, 12, 16, and 32 m above ground level (AGL), and wind direction (034B-L, Met One) at 8 m AGL.…”

Impact of Mesoscale Motions on Monin-Obukhov Similarity and Surface Energy Balance over a Homogeneous Surface

“…The Monin-Obukhov similarity theory (MOST) [1,2] is the most fundamental theory in the study of atmospheric turbulence and has been adopted for landsurface processes in the current numerical models. However, the application of MOST in the stable boundary layer or on the heterogeneous surface is often limited [3][4][5]. With the advance of observational research, the energy balance closure problem [6][7][8][9], noticed in the 1980s, further complicated the application of MOST.…”

“…Mesoscale motions are nonstationary and independent of the gradient transport in the vertical direction, having larger time scale than turbulence. When the turbulent fluxes observed in the boundary layer are contaminated by mesoscale motions, the turbulent fluxes often deviate greatly from the estimated values of mean flows [16], and the statistical characteristics of the turbulence also show various forms [4,12]. Vickers and Mahrt [15] developed the multiresolution decomposition (MRD) method to exclude the mesoscale motions involved in the turbulent fluxes based on the fact that mesoscale motions have larger timescale than turbulence.…”

“…Vickers and Mahrt [15] developed the multiresolution decomposition (MRD) method to exclude the mesoscale motions involved in the turbulent fluxes based on the fact that mesoscale motions have larger timescale than turbulence. With the help of the MRD method, the influence of mesoscale motions on MOST has been widely analyzed (Table 1) [4,12,17,18], and the results show that excluding mesoscale motions can reduce the scatter in the similarity relations in stable conditions. Meanwhile, the turbulent fluxes can also be affected by mesoscale motions.…”

Understanding the Partitioning of the Available Energy over the Semi-Arid Areas of the Loess Plateau, China

“…As the largest arid and semi-arid zone in China, the Loess Plateau is sensitive to climate change [21][22][23]. Land-atmosphere interactions in this area do not only have a significant impact on the regional climate but also affect the monsoon circulation in China [24].…”