Some interesting facts...as below:

Experimental evaluation of latent heat flux during night-time radiative hoarfrost . Boundary-Layer Meteorology 1980, Volume 19, Issue 1, pp 119–124

Abstract:A method to evaluate the latent heat flux during favourable conditions to frost formation has been tested with measurements made on two consecutive nights. The method is based on a surface thermal budget. Results show sign changes in the computed latent heat flux according to the observed cycle of frost formation and thaw. Flux estimates of 0.045 cal cm-2 min-1, with a mean relative error of 30%, have been obtained during the period of maximum frost accretion.

The impact of vegetation and soil parameters in simulationsof surface energy and water balance in the semi-arid sahel:A case study using SEBEX and HAPEX-Sahel data.Journal of Hydrology 320 (2006) 23-32

Abstract: A series of numerical experiments has been designed to understand the physics at the land surface–atmosphere interface inthe Sahel and to find the major parameters and parameterizations that are crucial to simulate arid climate processes.Observational data sets from the Sahelian Energy Balance Experiment (SEBEX) and the Hydrological Atmospheric PilotExperiment (HAPEX-Sahel) were used to help interpret the results. The observational errors in these two data sets are examinedfirst. In the control simulations, the standard type-8 (shrubs with ground-cover) data typically used by general circulationmodels for vegetation in the Sahel produce more than 50% underestimation of average latent heat flux and 100% overestimationof average sensible heat flux. The diurnal cycle of surface temperature may be overestimated during the day and underestimatedat night. Sensitivity experiments were conducted toidentify the most important parameters that affect Sahel simulation and theirrole at the surface/atmosphere interface. It was found that proper parameter estimation of leaf area index, stomatal resistance,and hydraulic conductivity allow enough evaporation to occur in land surface simulations. Sensible heat flux is thus reduced bygreater partitioning of energy to latent heat flux. Proper parameter estimation of thermal diffusivity also impacts sensible heatflux and diurnal variation of surface temperature. In addition, the water balance may be misrepresented with standard parametervalues by maintaining an unnecessarily high amount of soil moisture due to less evaporation. In general, the error existingbetween observations and simulation is reduced substantially by proper calibration of the vegetation/soil parameters.

Dear Marcel,

if net radiation is negative and LE is close to zero Bowen ratio should be negative and it going to infinity. For example: Rn = -100 W/m2, LE = 0.2 W/m2, thus C = Rn - LE = -100.2 W/m2 and beta = -100.2/0.2 = -501. You can get different values if condensation of water occurs: Rn = -100 W/m2, LE = -10 W/m2, thus C = -90 W/m2 and beta = -90/-10 = 9. It is real scenario (with fictive values) in both cases.

Regards,

Jakub

Hi Marcel,

what do you really want to know? The latent heat flux of the vegetation, so the plants surfaces only or do you mean the latent heat flux of an area with vegetation on it? I.e. including soil, plants, maybe open water also.

Well, the Bowen ratio can be positive or negative by its mathematical formulation and the sign depends on the signs of the energies. These are usually linked to the gain or loss of energy.

About the "negative" stomatal resistance, I think you can explain it by yourself. The flow of water over a leaf surface can be described by Fick's law as

f = 1/R * deltaC

with deltaC describing a concentration gradient and R the resistance value. Assume a constant deltaC, what value R need to be to get f=0 and what value to get no limit for f? This is, of course, an ideal world case ;) Does a "negative" resistance makes sense at all?

Regards,

Steffen