Monday, September 16, 2013

Converting the Flooding Colorado Rain to Snow

The rainfall totals coming out of Colorado over the week have been incredible--upwards of 15" in many places.  One common comparison to "put this into perspective" that I've seen a lot of on TV, online and social media is converting these rainfall totals to equivalent snow depth.  In another blog post, I talked about computing snow ratios--the ratio of snow depth to liquid water precipitation.  One common snow ratio we use for quick, back-of-the-envelope calculations is 10:1--10 inches of snow for every 1 inch of liquid water precipitation.  Thus I've seen a lot of people commenting that the 15" of rain they saw in Boulder would have been 12.5 feet of snow (150")!  While that sounds incredible, could that really happen?  Had this event happened in winter, would this storm really have produced 10 feet of snow in Boulder?

It's extremely unlikely.  There are many factors working against such an event.  First, let's start by looking at the tremendous amounts of moisture associated with this storm.  In my last blog, I talked about how the precipitable water (PWAT) associated with this storm is the highest ever observed in the month of September over Denver.  Here's the annual climatology of average precipitable water values, with the PWAT observed on Thursday evening highlighted.  You can see it was at the maximum observed PWAT value for September.

But what about PWAT values during the winter?  Notice that in December through February, the PWAT values on average are only about 0.25 inches with all-time maxima around 0.5-0.6 inches.  We're currently just about 200% of the normal PWAT for September, so even with the same anomaly in winter that would still work out to be only 0.5 inches.  Certainly a lot for winter, but less than half of the current PWAT.

So why are the PWAT values so much lower in the winter?  Remember that as the temperatures get lower, so do the saturation vapor pressures for water vapor in air.  This means that, by mass, at colder temperatures far less water vapor can be present in air before it starts condensing out.  We often describe this as the air not being able to "hold" as much water at lower temperatures.  You've felt this--even with the relative humidity at 80% on both a hot day and a cold day, the hot day feels far muggier than the cold day.  There's just more water vapor present when the air is warmer.  For snow, we want the temperatures through a good depth of the lower atmosphere to be below freezing, putting more limits on just how high the PWAT values can go.  It's just too cold to have this much water.

Another thing to consider is the matter of what is called precipitation efficiency.  One distinguishing feature of the rain that has been affecting Colorado is that it has had a high precipitation efficiency.  What does precipitation efficiency mean?  It's basically the ratio of how much water is raining out of a storm to how much water is being brought into a storm through advection and evaporation.  If the precipiation efficiency (PE) is 100%, then as much rain is falling out of a storm as is entering it.  If PE is at 0%, then the storm is growing--the cloud is getting bigger--but no precipitation is falling out of it.  PE can theoretically go all the way up to infinite, for a storm that is raining but no longer has any inflow.  Here's a figure from Market et al (2003) showing these different PEs:
These storms in Colorado have had a very high precipitation efficiency--probably close to 100%.  We've had 12+" of rain over 36 hour periods or so in many places with precipitable water values staying very steady around 1.2-1.3 inches over the same period of time.  To get so many inches of rain with these PWAT values the storms have to be very efficient.  One rule of thumb to estimate precipitation efficiency (Scofield et al 2000 and Market et al 2003) is to multiply the average relative humidity from the surface to 500mb by the total precipitable water.  Well, let's look at our Denver sounding from Wednesday evening:
Our dewpoint and temperature are virtually identical up to ~600mb and pretty close above that (aside...we drop below freezing at that point so we have to consider relative humidity with respect to ice...) we are more or less saturated (at 100% relative humidity) all the way up through 500 mb.  With precipitable water around 1.3 inches, the rule of thumb would suggest precipitation efficiency of  130%--probably a bit of an overestimate, but still--very efficient.

This high efficiency is typical of warm rain processes--storms where the rain spends little to no time frozen. Throw in cold processes--including ice and snow--and efficiency tends to drop.  There are a lot of microphysical reasons why this is so, but snowing just is not as efficient of a process.  A study by Hindman et al. (1981) (cited in the Market study mentioned above) showed that for mountain winter storms, typical precipitation efficiencies average between 7%-49%.  Much less efficient than warm rain storms.

So, in summary, if it were cold enough to snow we wouldn't have nearly as much water vapor as this week's Colorado rain storms have had.  And even if we did have anomalously high water vapor to work with, ice and snow precipitation does not occur nearly as efficiently as warm rain precipitation.  We wouldn't be able to capitalize on this moisture.  Unfortunately for Colorado, all this rain had to be rain...

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