Diode Laser Hygrometer (DLH)


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The DLH has been successfully flown during many previous field campaigns on several aircraft, most recently ATTREX (Global Hawk), DISCOVER-AQ (P-3), MACPEX (WB-57), SPartICus (Learjet), and Operation IceBridge (DC-8). This sensor measures water vapor (H2O(v)) via absorption of a strong, isolated line in the (101) combination band near 1.4 μm and is comprised of a compact laser transceiver mounted to a DC-­8 window plate and a sheet of high grade retroflecting road sign material applied to an outboard DC‐8 engine housing to complete the optical path. Using differential absorption detection techniques, H2O(v) is sensed along the 28.5m external path negating any potential wall or inlet effects inherent in extractive sampling techniques. A laser power normalization scheme enables the sensor to accurately measure water vapor even when flying through clouds. An algorithm calculates H2O(v) concentration based on the differential absorption signal magnitude, ambient pressure, and temperature, and spectroscopic parameters that are measured in the laboratory. Water vapor mixing ratio is provided in real-time to investigators aboard the DC-8, and will be transmitted to scientists on the ground during SEAC4RS.

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