Output from the ALEXI Model

Instantaneous Fluxes

Figure 2 demonstrates that the first-order outputs from the ALEXI model are the instantaneous fluxes at the times of the two surface temperature observations, t1 and t2. All fluxes are small at time t1 (sunrise+1hr), so more interesting are the flux predictions at time t2 (noon-1.5hr).

Figure 4 shows an example of the instantaneous sensible heat flux predictions at time t2 for a day in June of 1995 over the central U.S.:

The blacked pixels in the SE corner of the map indicate regions that were flagged as cloudy at or between times t1 and t2. The ALEXI model cannot make predictions for such regions: clouds at times t1 and t2 corrupt the radiometric temperature measurements, while clouds in the intervening time will cause the assumption of a morning linear rise in sensible heating to be violated. These regions are filled in later with an interpolation algorithm applied between bracketing clear days (see next page ).

Daily Integrated Fluxes

While the partitioning of instantaneous fluxes gives us some information about the overall surface soil moisture status, estimates of the daytime-integrated surface fluxes are often of more practical use.

Here, extrapolation of instantaneous fluxes to daytime totals is accomplished by assuming that the "evaporative fraction", given by the ratio between latent heat and the available energy flux

Fev = LE/(RN-G)

is stable during the daytime hours (Anderson et al., 1997). An estimate of the daytime total <RN>-<G> can be computed from a time series of satellite-derived net radiation estimates, and the remaining daily budget components can be estimated with

<LE> = Fev ( <RN>-<G> )
<H> = <RN>-<G>-<LE>.

In fact, examination of surface flux data shows that holding Fev constant tends to underestimate daytime LE by 5-10% (see references in Anderson et al. 1997), so in extrapolating from the instantaneous flux estimates at time t2 to daytime surface fluxes, we have used an evaporative fraction given by

Fev = 1.1 LE2/(RN2-G2).

Other Outputs

Besides the system, soil, and canopy flux components, other outputs from ALEXI include the soil and canopy temperatures and the air temperature at the interface between the surface and boundary layer models (~50m). In addition, comparisons between "actual" and potential evaporative fluxes are interpreted as signals of soil moisture status (see next page).

Previous page || Next Page

ALEXI TUTORIAL

Home ||ALEXI || U.S || IHOP || SMEX02
AmeriFlux || DisALEXI || Publications || Contact Us