The following appear in order; discussion points may directly
refer to one or more comments preceeding it.
Steve Koch 16:55:04
We will not possess the ability to measure surface layer heat
profiles and soil moisture with sufficient coverage to address
this hypothesis. The CASES and SGP-97 projects contain the kind
of data that are needed (e.g., the LSTAR aircraft remote sensing
of areal coverage of soil moisture).
Curtis Marshall 19:48:06
The Oklahoma Mesonet has recently received a $2 million NSF grant
to augment the network. Specifically, this additional instrumentation
will allow for the estimation of surface-layer heat fluxes. Additionally,
soil moisture and temperatures are already being measured at more
than half the sites. The augmentation for measuring heat fluxes
should be completed by TIMEX phase II.
Steve Koch 17:11:41
We must be careful to distinguish shallow from root zone soil
moisture. Unless the OK Mesonet will be profiling deep soil moisture
at a large number of sites, then there will be no way to determine
if transpiring vegetation (such as the winter wheat) is effectively
tapping into the deeper soil moisture and releasing this into
the atmosphere. See my paper in 1997 MWR concerning the numerical
modeling of the effects of differential cloud cover and evapotranspiration
gradients on the simulation of a frontal squall line - notice
the importance that the vegetation distribution (measured as NDVI
by satellite) plays in the total mosisture availability field.
Furthermore, it is important to understand that the Mesonet is
not fully equipped with soil moisture systems, and this reduces
the effective spatial sampling to probably more than 50 km spacing.
If we are to study thermally forced circulations at the meso-beta
scale, this will not permit analysis of the boundary forcing at
scales smaller than about 300 km. Will this satisfy TIMEX objectives?
By the way, I am keenly interested in this topic, so I am just
posing questions, not being critical.
Curtis Marshall 14:30:24
I agree with your concerns. It is nearly impossible to tell what
portion of the surface latent heat flux is resulting from canopy
transpiration, and what part is a result of bare soil evaporation.
Let me clarify a few things I had said about the Mesonet upgrades.
I forgot to mention in my original posting that part of the new
effort includes equipping every Mesonet Site with soil moisture
and soil temperature sensors. Soil Moisture will be measured at
5, 25, 60, and 75 cm. At the current time, we measure these quantities
at half the sites, but the rest of the network should be completed,
if not by TIMEX phase I, then certainly by phase II. Also, every
site will have the instruments that allow estimation of the surface
heat and moisture fluxes via Monin-Obukhov Similarity theory,
along with net radiometers and ground heat flux plates for more
careful analysis of the surface energy budget. Additionally, Bowen
Ratio systems will be placed at selected sites as another means
for monitoring the fluxes. Average Mesonet station spacing is
roughly 30 km. I know of no other fixed facility in the world
at that density for measuring surface fluxes and soil parameters.
We are also taking great care to be as compatible as possible
with the instrument platforms of the ARM/CART network.
Pielke Sr. 13:14:57 12/08/97
I would like to add that there is no need to know details of
the deep soil (ie.root zone) moisture when the plants are not
stressed from lack of water in this zone. Using satellite (AVHRR)
estimates of LAI and knowing vegetation type,effective spatial
resolutions of around 3-4 kilometers are possible. Using observed
T and other met parameters with a coupled SVAT will provide reasonably
accurate values of transpiration. We have a paper in press in
JAS (Eastman et al.) that discusses how point measurements can
also be used to refine transpiration/evaporation/sensible heat
flux estimates. I urge TIMEX to consider this surface forcing
and not just rely on atmospheric fields after boundaries have
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Please reference: ZIEGLER-4.