NOAA/National Severe Storms Laboratory


Hypothesis (Ziegler-4) concerning the role of land use and soil moisture gradients in developing thermal solenoids

Conrad Ziegler on August 27, 1997 at 22:14:39:

Horizontal virtual temperature gradients and thermal solenoids in the boundary layer are generated by horizontal gradients of surface layer heat flux, which in turn arise from the joint variability of land use and soil moisture.

Evaluate component of thermal solenoid normal to the mesoscale boundary with aircraft stepped traverses and mobile soundings. Estimate surface fluxes using soil moisture and land use data in an appropriate mesoscale model. Correlate thermal solenoids with surface layer heat flux profiles.
Either thermal solenoids are absent despite variable surface heating, or thermal solenoids exist in the absence of surface heating contrasts, in a sufficient number of cases.

The following appear in order; discussion points may directly refer to one or more comments preceeding it.

Steve Koch 16:55:04 10/22/97

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 11/03/97

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 11/11/97

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 11/17/97

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.

Roger A. 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 formed.

Click here to comment on this hypothesis. Please reference: ZIEGLER-4.