Climatology of turbulence in the boundary layer at the ARM Eastern North Atlantic (ENA) site
Submitter:
Ghate, Virendra Prakash — Argonne National Laboratory
Area of research:
Vertical Velocity
Journal Reference:
Science
Long-term observations made at the ARM Eastern North Atlantic (ENA) site were used to characterize the turbulence in the boundary layer and its modulating factors in the region. Turbulence exhibits a distinct annual and diurnal cycle. It is primarily modulated by winds, surface fluxes, and clouds on monthly timescales, while it is only modulated by clouds on hourly timescales. Analysis also shows the observations being influenced by the island during wind directions between 90o and 310o, and a substantial impact of evaporating drizzle on below-cloud vertical air motion during drizzling stratocumulus cloud conditions.
Impact
Analysis shows significant impact of drizzle on vertical mass transport during drizzling stratocumulus cloud conditions, which is absent in Earth system models (ESMs) that treat sub-grid-scale precipitation as a diagnostic variable. About two-thirds of the observations made at the ENA site are affected by the island, and since the island is not represented in most ESMs, caution must be taken while using the ENA data for evaluating ESM output.
Summary
Six years of data collected at the ARM’s Eastern North Atlantic observatory are used to characterize turbulence in the boundary layer. Analysis shows the measurements to be affected by the island during wind direction (wdir) between 90o and 310o. Data collected during the marine conditions (wdir<90o or wdir>310o) show that the turbulence is stronger during the winter months than during the summer months due to higher cloudiness, wind speeds, and surface fluxes. During marine conditions the variance of vertical velocity and cloud fraction exhibited a distinct diurnal cycle with higher values during the nighttime than during the daytime. Detailed analysis of 32 cases of drizzling marine stratocumulus clouds showed that for a similar amount of radiative cooling at the cloud top, 1) drizzle tends to predominantly fall in downdrafts with an increase in the rain rates, and 2) the strength of the downdrafts increases with increasing rain rates. The results have implications for climatological and model evaluation studies conducted at the ENA site, along with efforts of accurately representing drizzle-turbulence interactions in a range of atmospheric models.
Figure 1. (Left) Profiles of vertical air motion as binned by surface wind direction with the island effect visible during directions (90o-310o), and (right) topography of the Graciosa Island and sector of wind direction during which observations are unaffected by the island. From journal.
Figure 2. Box-whisker plot of updraft (red), downdraft (blue), and all vertical air motion (black) as binned by rain rate for stratocumulus cloud conditions with cloud top radiative cooling between -110 W/m2 and -90 W/m2. From journal.