Nocturnal boundary layer turbulence regimes analysis during the BLLAST campaign

Jesús Yus-Díez,Maria Rosa Soler,Mireia Udina,Jielun Sun,Erik Nilsson,Marie Lothon,Joan Bech

doi:10.5194/acp-19-9495-2019

Abstract

Abstract. A night-time turbulence regime classification, the so-called “HOckey-Stick Transition ” (HOST) theory, proposed by Sun et al. (2012) from the Cooperative Atmosphere–Surface Exchange Study-1999 (CASES-99) is explored using data from the Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign which took place during summer 2011 in the central French Pyrenean foothills. Results show that the HOST turbulence relationships for the BLLAST field campaign data are strongly dependent on both the meteorological and orographic features. The HOST pattern only appears for nights when a stably stratified boundary layer can be developed, corresponding to fair-weather and clear-sky nights, when the flow is generated by the nearby orography, from the south and the south-east. Those flows strongly influenced by the orography may generate intermittent or enhanced turbulence. When considering the whole nocturnal dataset for these flow directions, several enhanced turbulence points are found to be associated with sudden wind speed and directional shear transitions. In contrast, flows from other directions do not reproduce the HOST relationships and the turbulence relationship is almost linear, independent of vertical temperature gradients, corresponding to flows driven by synoptic scales. In addition we identify examples of gravity waves and top-down turbulent events that lead to transitions between the turbulence regimes.

Highlights

With the aim to investigate and categorize turbulence patterns generated by wind shear in the atmospheric stable boundary layer (SBL), Sun et al (2012) analysed a month-long dataset collected in Kansas from the Cooperative Atmosphere– Surface Exchange Study-1999 (CASES-99) in October 1999 (Poulos et al, 2002)
As the HOckeyStick transition” (HOST) was originally derived from measurements taken in a relatively flat area, where the CASES-99 field campaign was held (Sun et al, 2012), the aim of this study is to explore the validity of the HOST theory in an area with complex terrain, for flows that are influenced by the nearby orography
To further understand the HOST pattern, we explore the effect of stratification on the turbulence relationship for the whole Boundary-Layer Late Afternoon and Sunset Turbulence (BLLAST) dataset, as previously carried out in Sect. 4 for the nocturnal IOPs (nIOPs)

Summary

Introduction

With the aim to investigate and categorize turbulence patterns generated by wind shear in the atmospheric stable boundary layer (SBL), Sun et al (2012) analysed a month-long dataset collected in Kansas from the Cooperative Atmosphere– Surface Exchange Study-1999 (CASES-99) in October 1999 (Poulos et al, 2002) They found that, depending on the relationship between the turbulence intensity of the flow and the wind velocity, turbulent mixing from the surface to the last level of measurement (55 m) can be categorized in three regimes (Fig. 1). The close relationship between turbulence intensity and mean wind speed suggests that turbulence under moderate winds responds to the bulk shear V (z)/z near the ground (Sun et al, 2012, 2016) The eddies dominating this stronger turbulence regime have a larger scale of z, leading to a well-mixed layer below this height and producing a near-neutral stratification.

Objectives

Results

Conclusion