Big Drops

I would not quibble with anyone who says that this year's monsoon in Tucson got off to an early start on Tuesday (two days ago). Although most of yesterday (Wednesday) fit my picture of a pre-monsoon westerly trough, there were times on Tuesday when things were moving from the southeast. Then this morning a little after 4 AM I awoke to big drops falling on the skylight in the bathroom. The windows were still shut and the air conditioner was running, so it would have been hard for the thunder to wake me. But those big drops sounded like someone pounding on a saucepan with a spoon. (There were a bunch of damp, long-dead bees in my rain gage. It must have been not more than a few hundredths.)

I once remarked to John Marwitz that big drops were the most fascinating things to me about monsoon thunderstorms. John pooh-poohed the notion: A thunderstorm is a thunderstorm. He's right of course. Still I can't help thinking that there's something special about the distribution of big drops in storms west of 105W longitude.

When I first heard about dual-polarization radar about 25 years ago, I would be thinking about those big drops. The electrical engineer, V. Bringi, would fill the blackboard with a fog of phasors. But I would be picturing the old analog scopes on the finicky magnetron-based radar at Davis-Monthan, imagining those scopes magically transformed to show differential reflectivity. I wasn't aware until reading about it on Bob Maddox's blog that the National Weather Service radar in Phoenix (the radar is actually at the old Williams AFB) recently had an upgrade to dual-polarization. Since then I've been trying to figure out if there's a way to access the data. (My house is about 75 miles southeast of their radar; and I'll probably be disappointed with digital vs. old-time analog right next door. Still I think there will be interesting things to see.) In the meantime, I'll be excited to read about the first impressions in Phoenix. Here's an example from last night's Phoenix forecast discussion (FXUS65 KPSR 30035):

ALSO INTERESTING WAS OUR FIRST BETA TEST OF THE PHOENIX DUAL
POLARIZATION RADAR WITH THE NEARBY STORMS. OUR OBSERVATIONS
WERE...NOT A LOT OF PRECIP WAS DETECTED IN THE CLOUD CORES...
MEANING PRECIPITATION WAS IN THE FORM OF LARGE UNIFORM DROPS IN LOW
CONCENTRATIONS. OR VERY LITTLE MIXED PHASED PRECIPITATION
CONCENTRATIONS. SINCE VERY LITTLE PRECIP WAS DETECTED...THERE WAS
NOT A WHOLE LOT OF EVAPORATIVE COOLING ...AND THUS ACCOUNTED FOR
THE LACK OF STRONG DOWNBURST WINDS WE USUALLY GET WITH LARGE
TEMP/DEWPOINT SPREADS...WHICH AT THE TIME WAS 111 DEG OVER 50 F.

Obama In Pittsburgh

President Obama made an appearance in Pittsburgh last Friday. He traveled there to kick off a public-private partnership intended to promote the application of innovative technologies for use in manufacturing. It's nice that the President used the trip to remind everyone that innovation is not confined to the East and West Coasts. As for its effect politically, I think that the partnership will be a wash. Supporters will say that at least he is trying to do something to help with jobs in manufacturing. Opponents will say that he is doing it the wrong way.

As a supporter, I am perturbed by the part of his speech where President Obama cited Andrew Carnegie. For one thing, there is the incongruity of using Carnegie's role^* in the early development of the steel industry as the vision for an Advanced Manufacturing Partnership. The more important problem was Obama's pronounciation of the name. He used the New York pronunciation, CAR-nuh-Gee, instead of the way almost every voter in Southwest Pennsylvania says it, Car-NEIGH-gee. It's like making an effort to be seen in Philadelphia ordering a cheesesteak, then asking for it with Swiss cheese. And this was not the first visit to Pittsburgh. Getting legislation passed may be hard, but it isn't that hard to make an effort to connect with the local flavor. I hope that The President and his staff will do a better job of connecting with the voters he will need next year.

^*Before Carnegie built his own steel mill, he resisted the use of steel components in his iron bridges. His bridge engineers insisted that some steel parts were necessary for safety, but Carnegie then saw steel only as an extra cost that cut into profit. Carnegie did not invent the Bessemer process. He saw the innovative process applied successfully on a large scale in England, then scaled it up even more for his new mill in Pittsburgh. He made the huge capital investment only because he calculated that he could sell a high volume of a superior product for little additional production cost (analogous to China's big foot in global manufacturing). Carnegie was known to criticize innovations by his competitors, suggesting to customers that his competitors' products were dangerous. Then after his competitors went bankrupt he adopted those very same money-saving processes. Carnegie today would echo Senator Casey in criticizing the administration's trade policy.

The Monsoon Onset: What Has Changed?

Suppose that someone had lived in Tucson for a couple of decades, then fell into a Rip van Winkle sleep for another couple of decades, and finally awoke last week. Rip van Winkle would be bewildered to hear that the monsoon starts Wednesday, and that, alas, despite the approaching start date there's no rain in site (as if rain in site would be normal for this date in June). As terrible as the wildfires have been this year, and as good as it would be to get enough rain to relieve the situation, that's not a realistic hope this early. The monsoon itself of course has not changed. There is still much year-to-year and week-to-week variability. The variability would make it difficult to detect long-term changes in the monsoon anyway. But there have been other changes over the past few decades.

One of those changes is that meteorologists across the country recognize the notion of a monsoon in this part of the country. That recognition is thanks to various publications that have established the use of the terms Southwest Monsoon or North American Monsoon to describe the circulation and rainfall changes in the broader region around Arizona. But those broader changes have always been recognized from Arizona. Even before there was an Arizona there were Tucsonans who enjoyed sitting on their patios, peering far into the southern horizon (almost as far as a radar beam can reach), watching thunderstorm anvils and distant lightning, and wondering when the rains would get to Tucson.

Another change over the past few decades is the development of a wider variety of remote sensing techniques for better discerning the distribution of moisture in the atmosphere. The old satellite standby, an image of water vapor, happens to focus on moisture in the middle layers of the atmosphere. It has been joined by extra satellite sensors and by other indirect techniques. All of the remote techniques have their uncertainties. Instead of trying to weigh the uncertainties myself, I rely on depictions of precipitable water (PW) as assimilated into numerical models. Specifically, I like the presentations at COLA's wxmaps.org. Their plots clearly identify key contour levels of PW. When those contours are viewed alongside the old-standby satellite water vapor image, you get some feeling for the vertical distribution of the moisture: whether it is confined mostly to the lower layers of the atmosphere, as is often the case at the very beginning of the monsoon, or whether thunderstorms on previous days have left considerable remnant moisture in the middle layers. If the moisture is mostly confined to the lower layers of the atmosphere, then 25 mm (1 inch) of PW corresponds to a daytime surface dewpoint in the mid-50's. The bottom line is that 25-30 mm of PW is the transition to the monsoon.

Although they are still far from perfect, the model assimilations and forecasts of PW are more reliable than they were a few decades ago, providing a picture of the advance of the monsoon well before it reaches the southern horizon. But once it reaches the International Border, I revert to remotely sensing out on the patio and enjoying the show.

It's Monsoonal Season

The monsoonal season has arrived in Tucson. No, not the two months or so of near daily thunderstorms; that's still about a month away. What's already in full force is the silly season of talk about the monsoon. For the next month Tucsonans will endure media and meteorologists making pronouncements regarding the monsoon in general and the upcoming season in particular.

My peeve is not with the general public. Anyone who has lived in Tucson for awhile understands what the monsoon is about, and when it arrives. It's always fun to talk about the monsoon, and to speculate about the exact date when it will start. Most years the monsoon can be relied on to start within a week of, and usually just after the 4th of July. It's fun to plan on a dry 4th, anticipating a wet 6th, 8th or 10th.

More often than not at least once before the 4th it rains enough to wet the ground in Tucson. Obviously that will be the case when the monsoon arrives early. But there are rare rain days in June or very early July caused by mid-latitude westerly troughs. Traditionally meteorologists in Tucson would emphasize that those days were not the beginning of the monsoon. Their emphasis would be based not only on a diagnosis—that the winds aloft did not come close to meeting the definition of a seasonal wind shift, but also on a forecast—that the rains would not persist right on into the monsoon.

How does the National Weather Service currently define the beginning of the monsoon? I've lost track of the changes. I think that the NWS missed an opportunity a few years ago to reestablish a distinction between the monsoon, when thunderstorms occur almost every day, versus the encompassing summer thunderstorm season, the early part of which is mostly dry but with infrequent and especially hazardous days of high-based thunderstorms.

Although it goes against tradition, there is a case to be made for counting non-persistent June thunderstorms, perhaps racing swiftly off to the east-northeast, as part of the monsoon. The argument would go like this. The associated westerly trough managed to temporarily capture part of the moisture that was pooling south of the subtropical ridge. Climatologically that ridge is in the process of moving north to it's July-August position in Arizona. Parts of Sonora, Mexico, may already be experiencing the effects of a monsoonal flow. Thunderstorms in Tucson during the mid- to late-June period are less rare than earlier in June, suggesting that the northward advance of the subtropical ridge is more relevant than the obstinacy of the westerlies. The case against: those storms are followed by extended periods with no rain.

Tucson is very different from Phoenix in a number of ways. With regard to temporary surges of moisture before the monsoon, the difference lies in the trajectories of air flowing from the west-southwest toward each city. Phoenix is more susceptible to dry air from above the cold waters of the Pacific, say just south of San Diego. Tucson has more exposure to moist air channelled through the central Gulf of California. Temporary surges of high humidity at Phoenix get scoured away within a few days. In contrast, after the initial surge, high daily-average dewpoints may linger for several days in Tucson. It might be argued that persistently high daily-average dewpoints in June, albeit not quite high enough for afternoon/evening rain in the city, should define the beginning of the monsoon. But that definition clashes with what would have been the traditional call (based in part, I re-emphasize, on forecasters making forecasts) as well as my impression of what most Tucsonans think of as "the monsoon". If the monsoon starts this year on July 6, it will not be late. It will be right on time.