Fluxes and Budgets of Trace Gases over the Tropical
Pacific
Steve Oncley and Don Lenschow, NCAR
We will attempt to extend the usefulness of the chemical species
measurements from the P-3B by estimating fluxes through the bottom and top
of both the planetary boundary layer (PBL) and overlying buffer layer
(BuL), as well as other variables involved in chemical species budgets.
This will require flying specialized flight tracks, including following an
air mass and sampling at several heights in both layers, to maximize the
number of techniques which can be applied.
Profiles of fluxes within the PBL will be calculated directly using the
eddy-correlation method for scalar quantities for which fast-response
measurements are available (at least temperature and H2O). We will
estimate fluxes in the PBL for species which are measured at lower rates
(down to 1/min) using mixed-layer similarity methods. Budget methods also
will be used to estimate surface and entrainment fluxes as well as
exchange between the PBL and BuL and at the top of the BuL. We also will
examine top-down -- bottom-up (TD-BU) formulations that relate mixed-layer
gradients and variances to fluxes at the surface (emission or deposition
fluxes) and at the top of the PBL (entrainment fluxes). These methods are
summarized below.
- Eddy correlation: Direct flux measurement from the covariance
between vertical velocity measured by TAMMS and fast response sensors
(temperature, H2O, CO2?, O3?).
- Bandpass covariance: Computing a modified Bowen ratio from a
(low-frequency) portion of the concentration cospectrum with vertical
velocity (on continuous sensors with a time constant of 20 seconds or
less), plus all species collected in flasks, if flasks can be filled in
10 seconds or less, and at least 16 flask samples made per flight leg).
- Budget method: Relating the fluxes at the top and bottom of both
layers to the time rate of change of concentration in these layers. It
is necessary to determine the entrainment rates (exchange of air between
layers) from a species which is well measured, such as H2O, as well as
average concentration profiles of other species. To implement this
method requires Lagrangian flight tracks (following the same airmass
throughout the flight) in order to measure the time rates of change.
- Divergence method: Determining the entrainment rates from only
velocity measurements. This approach requires flying circular paths at
several levels in the same air mass, over a large enough area to obtain
a reasonable divergence measurement. We expect this technique to be
useful only in the PBL due to variability in the height of the top of
the BuL.
- Mixed-layer gradient: Integrating Large-Eddy Simulation results to
relate concentration differences in the PBL to the surface and
entrainment fluxes. We expect this approach to work only for species
with lifetimes on the order of a week or less.
- Mixed-layer variance: Similar to the mixed-layer gradient method,
this relates the fluxes to the change in concentration variance with
height. Although this removes the dependence of the measurement to drift
in the chemical analyzer, it introduces errors due to mesoscale effects
- in particular, air modification by clouds.
All of these methods will be tried on as many chemical species as
appropriate for each method to check for consistancy. Hybrid methods may
be developed in the course of data analysis.