Abstract
A new method, utilizing a 22.235GHz-water vapor emission line, to retrieve precipitable water content (PWC) is proposed. To obtain PWC, the method includes estimating the portion of the water vapor contribution to the brightness temperature, integrated over the microwave K-band (18-26.5GHz) range. It is appropriate to use the K-band range in order to retrieve PWC because the emission (or absorption) in this frequency range is weak and the line strength is little dependent on atmospheric temperature and pressure.
By using several atmospheric models with typical cloud types, the following approximate relation to retrieve PWC is introduced:
PWC=aTb.H2O+b(g/cm2)
where a=0.2135±0.0021, b=-0.0420±0.0251,
Tb.H2O=1/Δf∫f2f1Tb.obs(f)df-[Tb.obs(f2)+Tb.obs(f1)]/2+ΔTb.
Tb.obs (f) is a brightness temperature observed at a frequency of f GHz, and f1 and f2 are lower and upper boundary frequencies for integration, which are 18 and 26GHz, respectively, and Δf=f2-f1. ΔTb is a correction factor for the term correcting cloud and water vapor continuum emission. It is best for ΔTb to be set at 0.2K empirically.
By using actual radiosonde data along with clouds introduced depending on atmospheric conditions, an error is determined from the above approximate method for retrieval of PWC. The error for PWC is less than 5% under the average atmospheric condition (PWC>1g/cm2 and liquid water path (LWP)<0.1g/m2), and is about 6% for rather cool atmospheric conditions (PWC<1g/cm2 and LWP<0.01g/m2).
