Sensing of the atmospheric water vapor with millimeter wave spectrometer — KUMODeS

Forecasting and nowcasting of severe weather conditions, such as tornadoes, local heavy rainfall, thunderstorms, and hailstorms, are important subjects in meteorology. Cumulonimbi such as supercells and multicell convective storms locally cause these natural hazards in a short period of time. To mitigate their damage, technology to detect a “sign” prior to the event should be developed. One of the most significant “signs” is the rapid increase of the atmospheric water vapor in the low-level troposphere. High-resolution and high-precision monitoring of the water vapor provides knowledge of the short-term variation of the atmospheric thermodynamic structure before the occurrence of severe weather. We have been developing such a monitoring system based on the technology of millimeter waves spectroscopy. It is named “KUMODeS” (KEK Universal Moisture and Oxygen Detection System). KUMODeS measures spectra at two frequency bands: 20-30 GHz and 50-60 GHz. Characteristic absorption peaks of water vapor (~22 GHz) and Oxygen (~60 GHz) exist at each frequency range. The spectroscopy around the water vapor peak allows us to separate the signal from the water vapor and the signal from the cloud (i.e., dew condensed water). We are able to calculate the quantity of the atmospheric water vapor in the instrument's field of view. The spectrum in the Oxygen band contains information about the physical temperature of the atmospheric molecules and its perspective profiles. A combination of the two spectra provides us precise information of the water vapor. KUMODeS employs technologies developed for radio-astronomy. Its high sensitivity provides us quick and all-sky observation. The incoming atmospheric radiation signal is separated into two optical paths by using a wire grid: the transmitted path for the 20-30 GHz band system and the reflected path for the 50-60 GHz band system. The 20-30 GHz band system employs a cold receiver; an amplifier inside the receiver is maintained at 10 K. This cold receiver approach allows us to achieve noise temperatures of 50K which is sufficiently low compared to the atmospheric temperature during Japanese summer (≳ 50 K). A cold calibration source is also implemented inside the cryostat. We can quickly select the optical path for the atmospheric observation or calibration by using a mechanical drive of the wire grid. Test observations are on-going in Tsukuba, Japan, and we have observed the increase of the water vapor before cloud formation prior to rainfall.