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 ).

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Three two-equation turbulence models developed specifically to improve prediction of jet flowfields are investigated. These models are the Tam-Ganesan k-" formulation, a standard k-" model with modification for heated jets referred to as the PAB temperature correction, and a standard k-" model employing variable diffusion for the k and " equations. Two standard two-equation models are also investigated for comparison with the modified formulations. The standard models are the Chien k-" and Menter shear stress transport formulations. All of the models were investigated for a reference nozzle producing heated and unheated jets at a low acoustic Mach number of 0.5 to avoid complications of large compressibility effects. The primary deficiency of the standard models was the delayed initial jet mixing rate. All of the modified turbulence model formulations provided improved mean flow predictions relative to the standard models. The improved mixing rate enabled by the Tam-Ganesan model and the variable diffusion correction resulted from increased turbulent diffusion enabled by both models. The TamGanesan model and PAB temperature correction improved predictions of mean axial velocities for the heated jet, but did not improve prediction of the calculated turbulent kinetic energy fields.Nomenclature a = speed of sound D = jet exit diameter k = turbulent kinetic energy L t = turbulent length scale M a = acoustic Mach number M t = turbulent Mach number Pr t = turbulent Prandtl number r = distance to centerline S ij = normalized rate-of-strain tensor T g = normalized stagnation temperature gradient T t = stagnation temperature t = time U jet = jet exit velocity u j = velocity tensor X = vortex stretching invariant x j = position tensor " = turbulent dissipation rate " s = solenoidal turbulent dissipation rate = dynamic viscosity = density contribution to the eddy viscosity T = total eddy viscosity, Tam-Ganesan model t = eddy viscosity = density ij = turbulent shear stress tensor = vorticity magnitude ! = specific turbulent dissipation rate ! ij = normalized rotation tensor

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