Measurement of Total Water with a Tunable Diode Laser Hygrometer: Inlet Analysis, Calibration Procedure, and Ice Water Content Determination

Abstract

The University of Colorado closed-path tunable diode laser hygrometer (CLH), a new instrument for the in situ measurement of enhanced total water (eTW, the sum of water vapor and condensed water enhanced by a subisokinetic inlet), has recently been flown aboard the NASA DC-8 and WB-57F aircrafts. The CLH has the sensitivity necessary to quantify the ice water content (IWC) of extremely thin subvisual cirrus clouds (ϳ0.1 mg m Ϫ3 ), while still providing measurements over a large range of conditions typical of upper-tropospheric cirrus (up to 1 g m Ϫ3 ). A key feature of the CLH is its subisokinetic inlet system, which is described in detail in this paper. The enhancement and evaporation of ice particles that results from the heated subisokinetic inlet is described both analytically and based on computational fluid dynamical simulations of the flow around the aircraft. Laboratory mixtures of water vapor with an accuracy of 2%-10% (2) were used to calibrate the CLH over a wide range of water vapor mixing ratios (ϳ50-50 000 ppm) and pressures (ϳ100-1000 mb). The water vapor retrieval algorithm, which is based on the CLH instrument properties as well as on the spectroscopic properties of the water absorption line, accurately fits the calibration data to within the uncertainty of the calibration mixtures and instrument signal-to-noise ratio. A method for calculating cirrus IWC from the CLH enhanced total water measurement is presented. In this method, the particle enhancement factor is determined from an independent particle size distribution measurement and the size-dependent CLH inlet efficiency. It is shown that despite the potentially large uncertainty in particle size measurements, the error introduced by this method adds ϳ5% error to the IWC calculation. IWC accuracy ranges from 20% at the largest IWC to 50% at small IWC (Ͻ5 mg m Ϫ3 ).

The Size Distribution and Mass-Weighted Terminal Velocity of Low-Latitude Tropopause Cirrus Crystal Populations

“…For the CRYSTAL-FACE time periods, the CLH measurements were used for the r e calculations. Even though the CLH has been reported to have a higher uncertainty (Davis et al 2007) than the HT (Weinstock et al 2006), it was noted that the relationship between the CLH measured IWC and the IWC calculated from the forward scattering spectrometer probe (FSSP) was more consistent than for the HT measurements at very low values. As the CLH instrument was not on the aircraft for Pre-AVE, the HT measurement was used for that portion of the dataset.…”

A statistical comparison of cirrus particle size distributions measured using the 2-D stereo probe during the TC<sup>4</sup>, SPARTICUS, and MACPEX flight campaigns with historical cirrus datasets

“…Bulk IWC measurements are available for MACPEX from the Closed-path tunable diode Laser Hygrometer (CLH) probe (Davis et al, 2007). Condensed water that enters the CLH is evaporated so that a measurement of total water can be made.…”

A fiber-coupled laser hygrometer for airborne total water measurement

“…The second-generation University of Colorado closed-path tunable-diode laser hygrometer (CLH-2) is a total water instrument based on the measurement principles of the firstgeneration instrument, or CLH (Hallar et al, 2004;Davis et al, 2007b). The principal innovations that distinguish the CLH-2 from its predecessor are the former's compact and standardized form factor, redesigned electronics and optical system, operation without consumable gas, and Ethernet control.…”

“…For example, the hot-wire Nevzorov probe has a sensor with conical geometry designed to measure the liquid and/or ice water content of clouds (Korolev et al, 1998); the measurements made with this sensor are often referred to as "total water content" (e.g., Cober et al, 2001;Strapp et al, 2003;Boudala et al, 2004, Baumgardner et al, 2011, although these measurements by design exclude water vapor. Alternately, authors working with evaporative hygrometers refer to the measured sum of condensed water plus ambient water vapor as the "total water" (e.g., Brown and Francis, 1995;Wood and Field, 2000;Davis et al, 2007b). Also complicating interpretation of the literature is the diversity of terms used to denote the sum of ice plus liquid water excluding water vapor; these terms include "condensed water content" (e.g., Twohy et al, 1997), "total condensed water content" (e.g., Hogan et al, 2002), "cloud water content" (Strom and Heintzenberg, 1994), and "total condensate content" (Gultepe and Isaac, 1997).…”