ARM Measurements Reveal Details of Nocturnal Stable Boundary Layer

Summary

The atmospheric boundary layer, which is the lowest layer of the atmosphere, directly feels the Earth’s surface and is strongly affected by processes such as large-scale dynamics, solar heating and nocturnal radiative cooling, evapotranspiration, and frictional drag. Accurate prediction of the boundary layer’s height and characteristics is important for a wide range of atmospheric processes including surface temperature; cloud formation; aerosol mixing, transport, and transformation; and chemical mixing, transport, and transformation. The structure of the boundary layer varies, becoming more stable and less convective at night as the surface starts to cool. The nocturnal stable boundary layer (SBL) generally can be classified into the weakly stable boundary layer (wSBL) and very stable boundary layer (vSBL) regimes. Within the wSBL, turbulence is relatively continuous, whereas in the vSBL, turbulence is intermittent and not well characterized. Better understanding of the differing characteristics of each SBL type is needed so they can be accurately simulated in numerical weather and climate models.

Scientists analyzed thermodynamic and kinematic data collected by a suite of instruments at the Department of Energy’s Atmospheric Radiation Measurement (ARM) Southern Great Plains site in north central Oklahoma to better understand both SBL regimes and their differentiating characteristics. In particular, the team examined the relationship between wind speed and SBL characteristics. Composite normalized profiles of potential temperature, wind speed, vertical velocity variance, and the third-order moment of vertical velocity were produced for weak, moderate, and strong turbulence regimes. The team found that a threshold wind speed must be exceeded at lower heights (down to the surface) in order for strong turbulence to develop. Within the wSBL, turbulence is generated at the surface and transported upward. In the vSBL, values of vertical velocity variance are small throughout the entire boundary layer, likely due to the strong surface inversion that typically forms after sunset. The temperature profile tends to be approximately isothermal in the lowest portions of the wSBL and does not substantially change over the night. Within both SBL types, stability in the residual layer tends to increase as the night progresses. This stability increase is likely due to differential warm air advection, which frequently occurs in the southern Great Plains when southerly low-level jets and a typical north–south temperature gradient are present. Differential radiative flux divergence also contributes to this increase in stability. This increased understanding of different SBL characteristics can be used to evaluate and improve weather and climate models.

References

Bonin, T. A., W. G. Blumberg, P. A. Klein, and P. B. Chilson. 2015. “Thermodynamic and Turbulence Characteristics of the Southern Great Plains Nocturnal Boundary Layer Under Differing Turbulent Regimes,” Boundary-Layer Meteorology 157, 401–420. DOI: 10.1007/s10546-015-0072-2.