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Bowen ratio (BR) is the ratio of sensible heat flux to latent heat flux, i.e.,

$$Q=Q_{s}/Q_{l}$$ where $Q_{s}$ is the sensible heat flux and $Q_{l}$ is the latent heat flux. It is an indicator of the amount of heat lost from the surface to the atmosphere. In water bodies, latent heat of vaporization is higher and hence the bowen ratio is less than one. In dry urban surfaces (e.g., buildings and asphalt roads), evaporation is lower and the bowen ratio is greater than one. The bowen ratio over different surfaces have different values.

Can we then identify the underlying surface roughness type based on the bowen ratio alone?

I am thinking of the latitudinal and altitudinal dependence of the heat flux regime. For example, assume that there are the same vegetation distribution at 1km above sea level (a.s.l) and 4 km a.s.l at different but relatively closer latitudes, then can they have the same bowen ratio?

My idea is that BR is not the right way of quantifying the type of surface roughness because the lower pressure and temperature at 4 km in the example above favors more sensible heat flux than latent heat flux which increases the bowen ratio and gives wrong signal that the surface type is bare soil or sand. Can someone explain why BR is used as a means of quantifying the surface roughness type?

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A very simple answer is that it is related to the demands of the vegetation types and their ability to maximize the net radiation. A "perfect" plant that covered the entire surface would theoretically result in a BR of 0 as 100% of Rn is used for photosynthesis and released as ET, such that H=0 and LE=RN and B=H/L. It obviously doesn't work that way though.

Since $B=y\frac{\partial T}{\partial e}$ where y is the psych constant (~0.4 and really not a constant) it can be seen that the gradients of temperature and moisture, not the actual values, are the only two factors that matter in determining the Bowen ratio. Each plant essentially has a "specific Bowen Ratio" range where they are able to maximize their use of RN for the environment they are in. If the climate changes in that area the plant would not be able to survive.

That is a very very basic and highly simplified explanation, but hopefully it helps

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    $\begingroup$ For the ones that are not that familiar with the subject (like me) it would be great if you can include the meaning of all the letters and acronyms you use. Thanks! $\endgroup$ – Camilo Rada Mar 18 '18 at 20:06
  • $\begingroup$ Then I would assume the basic definition of BR above has a problem. In the formula, B=ydt/de, y needs to be determined by experiment? $\endgroup$ – Gemechu Fanta Garuma Mar 18 '18 at 20:57

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