Turbulence decay during the evening transition:Comparison of time scales from observations and models

The convective boundary layer has been well studied using in situ measurements, remote sensing techniques, and models applying a range of resolutions. Periods with transitions, however, are often neglected in part due to the lack of appropriate measurements. Recently the deployment of remote sensing systems, such as Doppler lidar, can provide continuous measurements that can be applied to periods of transitions. In this study, we utilize a Doppler lidar deployed by the US Department of Energy's (DOE) Atmospheric Radiation Measurement (ARM) User Facility as well as a suite of models including large eddy simulation (LES) and the DOE Energy Exascale Earth System Model (E3SM) to compare measurements of the decay of vertical velocity variance during the evening transition.

Three days have been selected from the DOE Holistic Interactions of Shallow Clouds, Aerosols, and Land Ecosystems (HI-SCALE) field study. These days were selected because they had nearly clear skies and weak forcing. Data from the Doppler lidar was analyzed to provide the vertical velocity variance. Simulations were made using the Weather Research and Forecasting (WRF) model in LES mode as well as E3SM.

Overall, there was large day-to-day variability in the observed rate of decay of vertical velocity variance, with time constants of the decay ranging between approximately 30 minutes and 2 hours over the three-day period. The LES had much shorter decay periods ranging from 24 to 48 minutes, but still showed a significant amount of variability. The time constant from E3SM was intermediate and varied little from day-to-day. Additional analysis suggests that differences in the rate of decay could be related to a range of meteorological conditions including the amount of wind shear in the residual layer and the magnitude of the static stability in the evening.