This web page is linked to by the main ROOF web page, at ROOF, and is devoted to brief comments on the "cloud liquid" solutions, Lz, determined from the WVR (Water Vapor Radiometer) instrument during the ROOF observation project.  The following figure is a repeat of the final solution for Lz versus time, and the following sections will refer to this data in zoom-expanded form.

Figure 1.  Cloud liquid burden for the ROOF period (red trace), based on TB1Z corrected for radome water effects and TB2ZE (which is unaffected by radome water effects), using equations for Lz that apply to the ROOF site, and corrected for Lz bias (due to water vapor altitude distribution changes) by subtracting interpolated Lz solutions for times when the sky was logged to be CLR.  Cloud cover recorded at the ROOF site is shown by the green trace.

When Does it Mist?

Mist precipitation was recorded on three occasions.  The first, and most misting occasion, was June 2.

Figure 2.  Zoom version of previous figure, showing a 1-day portion for June 2, with annotations from observing log.

On the June 2 "mist event" mist was first noted before valid observations with the WVR were obtained, but it appears that there was mist when Lz was lower than 100 microns.  The Lz values after June 2.10 are almost certainly valid, so the logged note of "V light mist" at June 2.402 shows that mist can be produced by a stratus cloud having as little as 200 microns of liquid burden!  This is much lower than the rule-of-thumb 700 micron value that was established any years ago from data taken at LA basin inland sites.  There's an anit-correlated pattern of Lz and mist between June 2.25 and June 2.6, which I don't understand.

The cloud event of June 7, during which Lz briefly reached 110 microns, did not produce mist.  I made the notation "Just wiped both radomes. Were dry, but WVR had light dust."

However, on the next day there was a mist episode.

Figure 3.  Lz and sky conditions for June 7, including observing log notations concerning mist.  The thick green lines indicate when sky conditions were being logged.

For this stratus cloud event the mist occurred when Lz was 130 microns and disappeared by the time Lz had decreased 105 microns.  It is possible that the mist stopped shortly after the 130 micron time.  The mist might have started several hours earlier, since no records were logged during the interval June 8.31 to 8.58 (when I slept).

Figure 4.  Lz, cloud cover and times when sky conditions were logged, for June 26, when a few big drops were noted.  The cloud cover trace is misleading during the un-logged period, as it is shown as changing linearly from CLR to OVC between the end of one logging period to the beginning of the second logging period.

This last "event" didn't produce "mist" per se; rather, a few large drops fell to ground level.  The cloud cover was just making a transition from OVC to BKN, and the radomes were found to be dry 14 minutes after noting the drops.  Nevertheless, it is interesting that any precipitation was produced when only 100 microns of cloud was present.

Based on the previous data I conclude that West Coast Stratus is capable of producing mist at ground level with as little as 130 microns!

What's Minimum Lz-Values for Overcast Sky Condition?

Referring to Fig. 3, at June 8.11, the cloud cover is logged as "OVC" yet Lz = 11 microns.  This is an incredibly low value for Lz to be associated with OVC conditions. The log also notes at this time that the stratus was at a low altitude, and an hour earlier it was noted that there was a thin cirrus layer.  The sky was SCT at June 8.08, just 44 minutes before it was OVC, when Lz is shown to be about 4 microns, so the procedure of subtracting interpolated Lz values is unlikely to have produced a spurious Lz result at the OVC time of June 8.12.

Figure 5.  Histogram of Lz values for times when sky was observed to be "overcast."

This figure shows that there were many times when Lz was below 50 microns when the sky was described as overcast, OVC.  The old 100 micron rule-of-thumb, which was derived from inland stratus observations many years ago, appears to not work close to the coast.  The required value for Lz for coastal stratus could easily be 30 microns, or even 10 microns.

Is this theoretically possible?  Rather, what kind of drop size distribution would be required to produce an optical depth of about 2, corresponding to "overcast," which simultaneously produces a liquid burden as small as 10 microns?

Consider a cloud layer that is 100 meter thick consisting of 1 micron diameter droplets and a number density of 100,000 per cubic centimeter.   This cloud has Lz = 10 microns.  What is its opacity?  The projected area of droplets, assuming they don't shade each other (OK for small opacities), equals the footprint at the base of a vertical column when the column is 3 meters thick.  Therefore, for a 100 meter column the optical depth, tau, would be something less than 30.  This is sufficient to block the sun and be percieved as "overcast."  If our criterion is tau = 3, then we may increase the drop diameter to 10 microns and decrease the number density by a million and still have a burden of 10 microns (since the ratio of Lz to tau varies as the first power of drop size).  This simple caluclation merely shows that it is physically possible to construct a cloud that is optically thick, yet has a liquid burden of only 10 microns, for reasonable estimates of cloud layer thickness and drop size.  A true model would incorporate a drop size distribution with a reasonable index value, but we can feel somewhat assured that such a distribution can be conjured that meets the burden and optical depth requirements and is reasonable for a stratus in formation or dissipation.

I reluctantly accept the observation that overcast conditions can be produced by a stratus that has liquid burdens as low as 10 microns.

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This site opened:  July 5, 2001.  Last Update:  July 25, 2001