An expression which estimates the total precipitable water from the infrared observed data of VTIR of MOS-1 is developed :
W= α [1-I2/B2 (Ts)] +β [1-I4/B4 (Ts)]
where I2 and I4 are respectively observed radiation value of the second band (67 μm band) and that of the fourth band (11.512.5 μm band) of VTIR, B2 (Ts) and B4 (Ts) are respectively the black body radiation values for a surface temperature Ts of the second and the fourth bands
I2, I4 B2 (Ts) and B4 (Ts) are expressed by the observed brightness temperatures T2 and T4 of the second and the fourth bands of VTIR and surface temperatures Ts. And the surface temperature Ts can be also estimated from observed values of brightness temperature T3 of the third band and T4 of the fourth band of VTIR.
The coefficients α and β in the above expression are given by the function of surface water vapor pressure Pv (0) :
For Pv (0)≤2.91 mb
α =0.3671, β = 26.95
For Pv (0)≥26.66 mb
α =1.288, β= 32.38
For 2.91 < Pv (0) <26.66 mb
α =0.3671+0.9209 [ (Pv(0) - 2.91)/19.75]4
β= 26.95 + 5.430 [ (Pv (0) - 2.91)/19.75]4
and for the case in which the proper surface vapor pressure is not available, the following expressions of α and β in terms of surface temperature Ts are given :
For Ts ≤257.1°K
α =0.3671, β= 26.95
For Ts ≥300.0°K
α =1.288, β = 32.38
For 257.1 <Ts <300.0°K
α =0.3671+0.9207 [(Ts-257.1)/42.9]6
β = 26.95+5.430 [(Ts-257.1)/42.9]6
The root mean square residuals between the precipitable waters from VTIR data and those from radiosonde data are 0.39 g/cm2/ (for α and β of functions of Pv (0)) and 0.51 g/cm2 (for α and β of functions of Ts)
