Uday,

yes, it is possible to calculate dew point
temperature (TDEW) from dry bulb temperature
and relative humidity. The underlying principles
are described in a number of books on environmental
physics such as Campbell, G. S. and Norman, J. M.
(1998) An introduction to environmental biophysics,
Springer, Berlin.

I constructed following formular from a table, which
you can find in the appendix from an older version
(Campbell, 1977):

ln(RH*exp(0.05892*T+1.62179)-1.62179)/0.05892

where RH is the relative humidity in percent (i.e. hum/100)
and T the dry bulb-temperature in degrees celsius.

The constants orginate from a regression of temperature
versus saturation vapor density :
rho(vs)=f(T°C)=exp(0.05892*T+1.62179)

Another formular is the following (I am sorry not
to recall the source) :

T -
(
(14.55+0.114*T)*(1-RH)
+((2.5+0.007*T)*(1-RH))^3
+(15.9+0.117*T)*(1-RH)^14
)

where, again, RH is the relative humidity in percent
(i.e. hum/100) and T the dry bulb-temperature in degrees
celsius.

Check, which formular works best under your conditions
(differences get large at low temperature conditions).


You also asked about PAR : PAR is photosyntheticaly
radiation and is measured in quantum units. The energy
of one quant (it carries energy, but is no particle)
is very small. The energy of a quant varies with its wavelength.
For example, a "green" quant carries 3.6 * 10^-19 Joule.
A better imaginable unit is the photon mole. By definition,
one photon mole (also called Einstein) contains
6.03 * 10^23 quants (Avogadro Constant). A photochemical
reaction which uses one light quant per molecule, needs
one Einstein per mole.

It is generally accepted that photosynthetically active
radiation covers the solar radiation waveband between
400 and 700 nm (Interesting to note is, that the East
and West used different definitions for the PAR spectrum
in former times).

Converion of solar radiation into PAR (and visa versa) is quite
simple using the relation (i.e. Varlet-Grancher et al. 1982)

1 Watt Rglob = 2.02 umol s-1 PAR

Corrections are probably necessary, when solar radiation
elevation is shallow or predominantly diffuse. However,
the impact on daily integrated PAR is small.

Wish you success in your modelling studies.

Matthias



--
Matthias Langensiepen, PhD
Hannover, Germany

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