Rainy days and Saturdays

Nice sunset yesterday….as some Stratocumulus spread over the sky underneath a pesky Cirrus cloud cover, clouds that announced the beginning of our next rain spell, now underway.

Light rain is falling this morning at 4:07 AM, and has been for hours, amounting to 0.01 inches.  However, some places in Pima Land have gotten much nicer rains, around a third of an inch in the Cat Mountains overnight, for example; check here.

Progress of the real monsoon, since June 1st, can be checked back there at the beginning of this sentence.  The coastal state of Karnataka has an average rainfall of about 70 inches since June 1st, a below normal amount, believe it or not.  However, being a statewide average, that 70 inches doesn’t reflect the hill stations in the western Ghats, surely to have about twice that amount.

Now, as a further aside, Karnataka, Kerala, two Indian west coastal states  would be a great place to go for a vacation now!  There you could REALLY absorb a REAL monsoon, where passing rains, heavy, pounding, thick with drops, visibility down to less than a mile, go on hour after hour with brief interruptions.  Its really pretty amazing and worth experiencing, at least once.

But, not much lightning there, like we have, because the rain develops mainly through a process not requiring ice, much like the rains in Hawaii where lightning is also rare.  The rain develops largely through the collisions of drops, ones that stick together after they collide, and get bigger on the way down through the cloud, sometimes called the “warm rain process” because ice is not involved, and that causes most of the rain in that Indian coastal region.  Cloud bases are right on the deck, and are typically 20-25 deg C, very, very warm.

In contrast, to continue a pedantic stream, “warm rain” is rare here in Arizony because cloud bases are relatively cool (less than 10 deg C in the summer as a rule), and droplet concentration are moderate to high (hundreds per cc).  Higher cloud droplet concentrations make it harder to grow cloud droplets big enough to collide and stick together inside our clouds.

But, we do get that kind of rain, “warm rain” here once in a great while in Arizona as part of the rain that forms in our Cumulonimbus clouds when their bottoms are especially warm, higher than 10 deg C.  Seems to happen about once or twice a summer in my experience so far.

What’s ahead?

Now that afternoon and evening rains around the area are back for the foreseeable future (5 days), what’s way ahead, beyond the foreseeable future?

There, as you know, when we start thinking about beyond the foreseeable future we start thinking about spaghetti! What do those crazy northern hemisphere-wide plots produced by NOAA with their dizzying numbers of lines mean for us here in Arizona?

First, I present a map of the 500 millibar contours as produced in the Haight-Asbury hippie district by San Francisco State–I mention this because the lines on this 500 mb map look a little nervous and maybe it has something to do with that map origin, being from a cultural area whose norms are “anomalous.”  I have pointed out  on this map, “Our Big Fat Anticyclone”, one whose position is critical for decent summer rains here.  In this map, as you can see, its not really OUR “BFA”, but rather belongs to Amarillo, TX, as of last evening.

Nevertheless, it is well positioned to fan humid air from the southeast into Arizona, as is happening now.  Remember, the circulation around a big fat anticyclone is clockwise.  When it sits on top of us, things are not so good; upper level temperatures are high, humidities are low up there, stifling convection and preventing tall Cumulus clouds.

But when the high is away on holiday, temperatures are lower above us, its more humid up there, and those factors allow for deep convection; huge Cumulonimbus clouds.  It only takes a few degrees difference to go from those dry days we just had with their Cumulus pancakus, to the kinds of days ahead for us now, where clouds stand tall!

Continuing, finally, Here’s is today’s plot for 15 days from now, the afternoon of August 11th, based on global data taken at 5 PM AST yesterday.  What do you see?  You see an arrow pointing to something of a void in all the “spaghetti.”  That void represents the most likely position of our BFA some two weeks from now, and that position is pretty darn good for summer rains here.  And it is in that region, to the north of us, almost the whole time from now!

So, based on this “most likely” position, one would venture that the rich summer rain season we have had thus far, will continue to be active.  Of course, this doesn’t mean rain everyday, but that breaks will likely be short through almost the first two weeks of August.

Can you imagine how tall those desert grasses and weeds will be by then if this is the case?

The last couple of photos document our fabulous re-greening now in progress.  If you haven’t been out in the desert, you should get out there and experience this wonderful event.  Doesn’t happen every year, as we know!

By Art Rangno

Retiree from a group specializing in airborne measurements of clouds and aerosols at the University of Washington (Cloud and Aerosol Research Group). The projects in which I participated were in many countries; from the Arctic to Brazil, from the Marshall Islands to South Africa.