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Our Instrument Paper… Accepted!!!

July 6th, 2006 by Sean Davis · No Comments

Last year, my group submitted an instrument paper on the University of Colorado closed path laser hygrometer (CLH), which has been the main focus of my research over the past couple of years. Just last week, I received word that it was accepted for publication into the Journal of Atmospheric and Oceanic Technology. Yay!

The paper is entitled “Measurement of Total Water with a Tunable Diode Laser Hygrometer: Inlet Analysis, Calibration Procedure, and Ice Water Content Determination”, and the authors are myself, Gannet Hallar, Linnea Avallone, and William Engblom. It is available for download here.

In the paper, we discuss the nitty gritty details that go in to how our instrument measures the “enhanced” total water content of cirrus clouds from aboard NASA’s WB-57 aircraft (shown below).

wb57.jpgEnhanced total water content is the sum of ambient water vapor plus water vapor resulting from the evaporation of solid/liquid particles that have been “enhanced” above their ambient concentrations within a special subisokinetic inlet. Subisokinetic inlets are those in which the ambient flow of air over an aircraft wing is slowed as it enters and flows through the inlet. The resulting enhancement of particles within such an inlet allows for a lower detection threshold. In contrast, isokinetic inlets maintain flow through the inlet and instrument at the same speed as that of the free stream.

inlet(the CLH inlet)

In this paper, we discuss how the cloud ice crystal sampling and enhancement characteristics are affected by flow around and through our instruments inlet. Ice crystal evaporation and heat transfer calculations for our inlet are also presented.

We also discuss how the CLH’s algorithm for obtaining water vapor compares to laboratory calibration mixtures of water vapor, which ends up being to within about 5-10%.

Near the end, we talk about how our enhanced total water measurement is used to determine the more relevant parameter, cirrus ice water content (IWC). IWC is simply the density (g m-3) of condensed water in a cloud, and in conjunction with the ice crystal effective radius, determines to a good degree of accuracy the optical extinction within the cloud. The paper also has a detailed error propagation analysis for the IWC calculation, which leads to the conclusion that the uncertainty in our IWC is around 20% (2 sigma), and increases to around 50% for the case of extremely low IWC values (< 5 mg m-3).

What is not addressed in this paper, of course, is how accurate do in situ measurements of IWC need to be for modeling and satellite validation studies? My thoughts from preliminary work I’ve done are that for satellite validation, there is the potential for significantly larger errors from the spatial/temporal sampling differences between the plane and satellites. Also, from what I’ve seen from field campaign data, uncertainties in other instruments such as particle probes is much more likely to be an issue than uncertainties in IWC measurements.

As for modeling, well, I’m not sure about that one. Perhaps someone with a modeling background would be willing to comment on this one?

Tags: instruments

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