Spring Weather - Holding Pattern

Printer-friendly versionPrinter-friendly version

Evergreen Soaring is blessed with a number of intrepid meteorologists that help to the general membership to get a better understanding of our local weather.  Here is Ed's analysis of the strange weather this year that have made XC soaring such a challenge so far in 2016 (until May 3).

Enjoy the article and chime in if you want to add to the discussion.

- editor

Holding Pattern by Ed Walker

So, why has our soaring season failed to start this year? Like me, you’ve probably spent a good bit of time over the past two months looking out the window wondering why the atmosphere isn’t firing up the way it usually does in the spring. By this time last year we had multiple 100+ km flights and the CUs were popping. After all, warm days in the spring generally give us good conditions, and we’ve just had one of the warmest springs on record. So what’s up? I have a hunch about what might be happening. It’s really a number of factors coming together in perfect storm.

The seasonal move from winter to summer normally creates a transition in our climate where sun angles, length of day and the resultant daytime insolation heating give us a relatively warmer surface temperature to trigger convection. Unlike September, however, where we can also get comparable daytime heating, the upper air in April is cooler and more receptive to sustaining growing thermals. That made me wonder about whether the upper atmosphere was warmer than usual this year, leading to more stability.

So let’s back up a second and think about how to make an ideal soaring day. First of all we need instability which is related to the lapse rate. We want the environmental lapse rate to equal or exceed the adiabatic lapse rate of 2 C per 1000 ft. That means that on a standard atmosphere 15 C day at sea level the standard adiabatic lapse rate results in temps of -25 C at 18,000 ft, roughly the 500 mb level. So, in an idealized atmosphere with perfect temperature linearity we need something close to a 40 C temperature difference from sea level to 500 mb to get sustainable, uniform convection.

We know that this never actually happens – the skew-T doesn’t show a lapse rate that is a perfectly straight line. Nevertheless, this idealized model does provide a mental framework for rough guesses. This 500 mb level is important because it is one of the mandatory reporting levels in forecasting, and we can use sounding data and model forecasts to get upper air data for analysis.

To examine this rule of thumb I looked over the 2015 OLC flights from some of our ESI superstars (you know who you are) who were able to make multiple 100+ km flights between March 1 and May 15 of last year. There were about 20 exceptional days that stood out during that roughly 2-month period. I then compared the actual and forecast soundings from March to May of 2015 with the comparable period in 2016 to see if there were any differences.

First of all, below are the data for 2015. The table shows a sample of the best flights from last year with distance flown (km in ascending order), 500 mb heights in DM, 500 mb and sea level temps in deg C and temp difference between SL and 500 mb.  Note that in nearly every case flights of over 100 km were associated with temp differences of close to 40 C, with the larger distances adhering more closely to the rule of thumb. Other days during the period had much lower temperature spreads.

So why is this important for soaring forecasting? The problem is that in the mild climate of the Pac NW the Skew-T environmental temperature line rarely approaches this 40C slope. We know intuitively that warmer days pull the lower end of the line to the right which is why we usually see springtime trigger temps in the 60s and 70s. But the key to really good days is to also pull the upper end of the line to the left (lower 500 mb temps) as well. When you do both the slope can be closer to the 40C slope we need for higher convection.  Given that our spring temps at sea level are usually around 15C that means that we need 500 mb temps near -25 C to get a good day.

But that isn’t enough. So far we’ve only talked about lowering upper level temps. The next problem is the air also has to be dry enough down low not to form low level clouds. The issue here is that our maritime polar (mP) air arrives close to saturation from the beginning, so not only so we need it to warm up, we also need it to dry out. One way to think about this is a variation of the frontal passage rule we all know.

Generally we’ve all read that frontal passage results in unstable drier air replacing warmer moist air with cool, clear unstable conditions. Just warm it up with some sun and you’re good. The problem with that rule of thumb is that it works better outside the west coast in places where the cooler air is continental polar (cP) which is drier. Our situation often uses mP air which is not. So we get lots of clouds, convergence zones and days of MVFR conditions instead. The lift is there, we just can’t use it without an IFR clearance. Our problem is that the beneficial trough of cold air aloft is unfortunately frequently coupled with a surface trough of mP air that is close to saturation, so a simple lapse rate temperature difference isn’t enough. The sweet spot occurs when the upper and lower atmospheric troughs uncouple and we just the right conditions – persistent cold air aloft with drying conditions below.

Putting this all together, we can deduce some principles for springtime soaring meteorology.

  1. To get excellent spring soaring we need cold air aloft, usually an upper trough of -25 C air at 500 mb with warmer air below, close to about 15 C.

  2. The surface has to be relatively dry, so if there is an associated surface tough it must be replaced by a ridge of drier high pressure, and the sweet spot is the short period between the arrival of the upper low and the arrival of the surface high. Not often a very wide window, since our surface ridges frequently warm the 500 mb levels as well.

  3. Cold upper air but with a coupled surface low will yield MVFR and little soaring.

  4. A warm spring day may not be enough unless there is cold air aloft.

  5. Warm upper air irrespective of what’s underneath will probably not yield much good flying.

This also explains why the warmer temps in the summer yield a stable atmosphere. Soaring pilots are conditioned to associate warm days with trigger temps and good soaring. But that’s only true with cold air aloft. Although ground temps are high in the summer, so are temps aloft since our ridges warm the higher air quickly.

I think this is one of the reasons why we have not had a great 2016 spring soaring season in the Pac NW. The temps aloft have been generally higher, and when that happens it really doesn’t matter what happens at the surface. On the days where there have been colder temperatures aloft, a surface low often obliterated the soaring, and the transition to the next ridge was accompanied by warmer air aloft.  This pattern seems to have been locked in for quite some time by the long wave patterns that are the main drivers for jet stream steering.  For example, we have had two rather persistent omega blocks in the past two months, relatively immovable airstream configurations shaped like the Greek letter omega, with low pressure in the twist of the two southern tails and a high pressure bulge in the northern center resulting in higher temps both at the surface and aloft. These tend to be persistent and not conducive to soaring conditions if you’re sitting in the high pressure center.

Just to validate this theory, here’s an applied example from this year of how it can work when it all comes together. On April 25, 2016 Ron made the first truly long flight of the year. The 500 mb temp was -32 C, a ridge was replacing the surface low, surface temp was 14 C, dewpoint 6 C. That’s a 45 C temp difference with a surface temp-dewpoint spread of 8 C. Voila… an OLC flight of 300 points! Fred and Ron also flew on March 29 and 31 in similar conditions (37-40 C temp differences, high pressure, high surface dewpoint spread) with 110+ km flights. The 500 mb air was only -20 but the ground temp was near 21 C both days. To those of us who valiantly tried on other dates – maybe it wasn’t our fault – the atmosphere was just not ready for us.

One more thing. Remember that the environmental lapse rate is never a straight line, so sometimes in a stable atmosphere you can take long tows into the mountains and take advantage of a local, limited superadiabatic lapse rate aloft above an inversion. In the summer, for example, you sometimes have a low level inversion below 5000 ft dampening thermals at sea level, with higher mountain ridges triggering lift in the relatively cooler air above the inversion which you can see as midlevel clouds near the Cascade ridges. But as many of you have found out in the past 2 months, even that strategy has its limits. In the summer the truly stable atmosphere of mid August is warm quite a way up due to the easterly flow of deep, warm air from eastern WA. No mountain tow will get above that. The warm air aloft this spring is considerably deeper than last year, and it’s hard to get a decent lapse rate even with a mountaintop start. It’s all about lapse rates, and given our spring temps, if the 500 mb temps don’t get down below about -25 C it’s harder to make it happen.

So, in summary, this year we have had generally warmer upper air with cooler, moist maritime influences at the surface, very different from our 2015 spring.  Unfortunately, until there is a major change in the long wave patterns it’s unlikely we’re going to return to our usual epic springtime soaring conditions.