MMM SEMINAR NCAR
THE
ENTRAINMENT PROCESS OF CARBON DIOXIDE IN THE ATMOSPHERIC BOUNDARY LAYER
Jordi
Vila-Guerau de Arellano
Wageningen
University
Abstract
Aircraft and surface measurements of turbulent thermodynamic
variables and carbon dioxide (CO_2) were taken above a grass land in a
convective atmospheric boundary layer. The observations were analyzed to assess
the importance of the entrainment process for the distribution and evolution of
carbon dioxide in the boundary layer. From the observations, we were able to
estimate the vertical profiles of the fluxes, the correlation coefficients and
the skewness of the virtual potential temperature, the specific humidity and
the carbon dioxide These profiles indicate that important entrainment events
occurred during the observed period. The data were also used to estimate the
budgets for heat, moisture and carbon dioxide.
By studying this observational data, we find that the
entrainment of air parcels containing lower concentrations of water vapor and
carbon dioxide significantly dries and dilutes the concentration of these two
constituents in the boundary layer. This process is particularly important in
the morning hours which are characterized by a rapidly growing boundary layer.
The observations show that the CO_2 concentration in the boundary layer is
reduced much more effectively by the ventilation with entrained air than by
CO_2 uptake by the vegetation. We quantify this effect by calculating the ratio
of the entrainment flux of CO_2 to the surface flux of CO_2
(beta_c=-(wc)_e/(wc)_o). A value of beta_c equal to 2.9 is estimated at around
13 UTC from the vertical profile of the carbon dioxide flux. We corroborate
this observational evidence by reproducing the observed situation using a mixed
layer model. The mixed layer model also yields the variation in time of beta_c.
During the morning, the ventilation process is more important than the
CO_2-uptake by the vegetation (beta_c>1) whereas in the afternoon the
assimilation by grass at the surface becomes the dominant processes
(beta_c<1). This research points out the relevance of the entrainment
process on the budget of carbon dioxide in the lower troposphere and the
relevance of boundary layer dynamics in controlling the diurnal variation of
carbon dioxide.
Thursday,
June 24, 3:30 p.m.
Refreshments
at 3:15 p.m.
NCAR-Foothills
Laboratory
3450
Mitchell Lane
Bldg 2, Rm 1001