PEM Tropics A Mission (Aug - Sept 1996)

Data

The PEM Tropics A data is available for all PEM Tropics A Flights, excluding Flights 1-3 & 9 (Test Flights) and Flights 11-16 (Instrument Problems) in an 18.7 hz archive file. The file, in ASCII zipped format, included the following parameters:

The data is available thru the GTE Project Office or by e-mailing a request to Lee Thornhill

PEM TROPICS A ANALYSIS and DISCUSSION

by Geoff Considine, former TAMMS PI

A single day of flights in the vicinity of Christmas Island provided the basis for a study of the transport properties of this Tropical marine boundary layer. The flight plan during Flight 7 consisted of a series of 30-minute circles at three heights in the mixed layer (77 m, 160 m, and 310 m), along with vertical soundings. The turbulent transport across the mixed layer (about 600 m deep) was heavily influenced by mesoscale processes. Further, the mesoscale component of the vertical transport of momentum and water vapor changed significantly with height above the surface, so that the vertical flux divergence is strongly affected by mesoscale variability. The results suggest that mesoscale processes have a large impact on the rate of entrainment into the mixed layer. The findings represent a single day's data, but they suggest that mesoscale variability strongly modulates the exchange of momentum and moisture between the surface and the free troposphere. Since bulk transfer equations for computing turbulent tend not to perform well in environments where mesoscale processes play a large role, our results support the need for application of improved bulk aerodynamic equations such as proposed by Mahrt and Sun (Monthly Weather Review, 123, 3032-3041).

A notable feature of the turbulent transport is that the surface humidity flux remained relatively constant throughout the 8 hour period of the flights, whereas the flux at 300 m, half way through the mixed layer, increased by 50%. This increase coincided with an increase in the dynamical scales associated with the fluxes. If the increase in the fluxes were simply due to a larger source or sink of moisture, with no change in the dynamics, the distribution of scales in the co-spectra would remain constant. The change in the characteristic dynamical scales in the co-spectra is highly correlated with the flux, suggesting that changing dynamical processes are what cause the increases in the flux.

We also examined the effects of using an alternative scale decomposition for filtering and analysis of scale in the turbulence data. Some work has been done comparing the decomposition of scales using Fourier transforms with moving averages (Howell and Mahrt, 1997; Sun et al., 1996). The moving average (MA) technique (described in detail with software to compute it below) uses a localized non-orthogonal basis to determine the fraction of variance and covariance associated with different dynamical scales. Fourier modes are not localized in physical space (they have no edges), so there is reason to examine the extent to which a localized basis set (moving averages in this case) yield a different distribution of scales. Since the decomposition of turbulence also is used to remove the largest scales of variability that are unreliably resolved because of our finite sampling scale, we have examined the effect on fluxes when we use the MA vs. Fourier approaches. We find differences of up to 6% of the latent heat between the Fourier and MA derived fluxes.

For a discussion of these results, please contact G. Considine for a copy of a paper that is currently in the review process at The Journal of Geophysical Research.

A cursory look at satellite imagery from GOES 9 satellite suggests the presence of clouds on a wide range of scales (bright in VIS and cold in IR). The images below are 15 degrees by 15 degrees. Don Lenschow and collaborators have analyzed this day's flights and found evidence that cumulus convection appears to be measurably affecting the transport features of the boundary layer. 

IR Image and the VIS Image

Imagery from GOES 9. Yellow and red mark flight track during Xmas Island. These images are fairly degraded, but the full images are available from the GTE web page:

Visible Image

IR Image

Mission7, Christmas Island Flight Log

This was a quasi-Lagrangian flight, with the pilots following predominant winds over the course of the mission. The main maneuvers performed were 30-minute circles, referred to as flux circles, or FC's. Note that FC's 2, 5, and 10, had 5 minute legs alternating between altitudes. 

Local Time = GMT - 10 hours

Responsible NASA Official(TAMMS): Bruce Anderson
Curator(TAMMS): Lee Thornhill