A Method to Retrieve Precipitable Water Content Using a Microwave Spectro-Radiometer

Abstract

A new method, utilizing a 22.235 GHz-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 ) 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:where a=0.2135+0.0021, b=-0.0420+0.0251, 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 df=f2-f1.dTb is a correction factor for the term correcting cloud and water vapor continuum emission. It is best for dTb 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).

Deriving the tropospheric integrated water vapor from tipping curve–derived opacity near 22 GHz

“…Therefore the coefficients need to be modified if the instrument is applied in regions with largely different atmospheric conditions such as polar or tropical regions. The study by Takamura [1996] showed that the relation Figure 7. MIAWARA IWV derived using the t + tT trop regression approach (asterisks) and that calculated from the NRL method (crosses) compared to GPS, TROWARA, and ASMUWARA IWV values.…”

“…Therefore the coefficients need to be modified if the instrument is applied in regions with largely different atmospheric conditions such as polar or tropical regions. The study by Takamura [1996] showed that the relation between the brightness temperature in the frequency range of 18 to 26.5 GHz and the IWV is marginally dependent on the measurement location in all regions from subarctic to tropical conditions.…”

“…A literature search for other algorithms used for the estimation of integrated water vapor using the opacity near 22.235 GHz showed no results, where the measured opacity at 22.235 GHz is directly related to the integrated water vapor. Takamura [1996] presented a method to calculate the IWV content from the measured mean brightness temperature between 18 and 26.5 GHz.…”