From the Lee Side

Quoting georgevandenberghe:
What is the difference between a Rex block and an Omega block? I thought a Rex block had a cut off closed low to the south of the high which is a common pattern.

Perhaps we should start naming blocking highs??

Perhaps I should delete previous sentence fast before someone takes off with it??

I don't keep anything throwable near the TV when politics is being discussed. Sports I don't follow and weather mistakes I just sigh.

The subtropical highs do sometimes appear to have a blocking effect on cold fronts. This is very different from what blocks in the westerlies do however. Also at some time I'd like to examine why the boreal winter midlatitude westerlies sometimes penetrate to 20N while at other times the boundary is near 30N. It doesn't seem to be just due to available cold air or midlatitude cyclone activity but I haven't pursued it.

Before the snowstorm dumped 18" on MA it developed to the southeast of the DC area. But our boundary layer was too warm and upward motion, hence dynamic cooling was less than forecast by all models. Even with the expected dynamic cooling I told my cub scout troop to expect a slop storm, not a snow storm because of the BL temps. Something else of note was convective robbing over southern DE which cut off our moisture reducing our total precipitation. But this bust does not have a simple explanation and what I describe above doesn't entirely capture it.

I was about to jump on

e) the pattern must exist with recognizable continuity for at least ten days."

as restrictive before noting you already had.

As long as an omega block meets the requirements of Rex, there is no difference between the two. Just a cutesy way to describe the shape. I believe the example I showed from February 25, 2004 is a good example.

Subtropical highs don't really block cold fronts. Rather, the subtropical ridge can be part of a longer-wave pattern characterized by upper-level winds (especially at 850 mb and 700 mb) to the east of the upper-level trough that blow from the southwest nearly parallel to the surface cold front. When 850-mb to 700-mb southwesterly winds parallel the cold front, the front moves slowly or stalls. Such a result often gets misconstrued as "blocking."

Quoting bappit:
Chris Burt's current blog ties this week's blizzard in France to a blocking high. Stealing from his blog:



His comment: "The unusually cold air that allowed the snowstorm to happen was facilitated by a ridge at the higher latitudes of the northern hemisphere that squeezed the cold air to the south and west over England and France."

Seems like the blocking high up there is persistent.

Wondering, does this qualify as a blizzard by your lights? (the snowstorm in France)

"Squeezed" is probably not the correct word to use in the context of blocking. Two closed, stagnant cold 500-mb lows are typically part of a blocking pattern.

These days, the term, blizzard, has colloquial meaning...any snowstorm is often characterized as a blizzard if there's notable wind. In my view, only the National Weather Service follows the true meaning of a blizzard...

Even the AMS has dropped the requisite three hours (here's their definition, and I just threw a virtual bad-call brick at the AMS Web site).

To be honest, if I don't see protracted heavy snow and strong winds, I think using the term, blizzard, is just hype. Take a look at the meteogram at Paris below. I don't see a blizzard. Really, not even close, colloquially speaking or otherwise.



Thanks for your comment.

Quoting georgevandenberghe:
I'm with you on watered down blizzards. The definition describes falling or blowing snow obscuring visibility. To get the true <2 meter visibility whiteouts and total disorientation experienced in bad ones in the midwest and great plains, it has to blow, it can't fall that fast.

Agreed, George!

And thanks!

Quoting 1900hurricane:
Speaking of blocks, what do you think about this one forecasted by the ECMWF? It's already starting to balloon itself up now, but by 72-96 hours, it looks absolutely ridiculous! Is it as anomalous as it looks to me, or is this just my lack of experience in dealing with blocks showing up?

Good catch! That's a classic!!!

Blocking highs in this region are favored because there is constructive interference among the longest planetary waves.

Perhaps I'll do another blog on blocks, explaining why blocks are favored in this region (I'll elaborate on this post).

Thanks for posting!

Quoting 1900hurricane:
One more thing I just noticed. The 12Z GFS even shows the block partially merging with another cell of high pressure aloft very near the pole, which diverts a good portion of the flow to the other side of the pole and back. Once again, despite following meteorology for a good while now, I've only recently begun looking at the bigger picture like this, so how unusual is this?

I tend to follow hemispheric flow quite regularly, so these kinds of amplified patterns are not rare, especially during the cold season.

You're making good observations, the first key step in becoming a better scientist.

Thanks again for posting.

Quoting georgevandenberghe:
This blocking will have noticeable upstream consequences in the eastern U.S. and it looks like the second two weeks of March will be cooler than seasonal averages. No hurry to get the DC garden in this year. The soils are too cold and wet even for greens and I'm still worried about -5C or colder in my frost hollow located rental garden. That's the threshold where even hardened pea and lettuce transplants can be damaged or killed.

What a contrast from last year but that said, this year is much more typical of March in the East.

Agreed, George. I've got an inch of snow on the ground and it's still snowing here in central Pa.

Thanks!

Quoting bappit:

Is this constructive interference related to what was discussed in the study that discounted the North Atlantic Drift (misnomer alert: Gulf Stream) as the cause of the warm climate up that aways?

The Gulf Stream Myth They ran models and concluded that the Rocky Mountains tended to induce a trough over eastern North America that brought warm air to the North Atlantic. This transported more heat than the North Atlantic Drift in their calculations. They also attributed the mildness of the climate to it being maritime.

I realize that this viewpoint has some controversy attached, but if this blocking pattern is not all that unusual then ... I dunno, but it sure is interesting.

I think you have the right idea. The planetary waves I'm talking about (wavenumbers 1, 2 and 3) are associated with the distribution of oceans and continents (uneven heating), mountain ranges, etc.

I would think they have an impact on climate...the relatively warm ocean current also contributes to the milder-than-expected climate for such relatively high latitudes (regardless of any so-called myth).

Quoting bappit:

I think the question might be where does the heat in the relatively warm water come from and how much is the ocean temperature pattern an artifact of cold currents coming from the north. The Gulf Stream peters out off of Nova Scotia, much further south and west. If that is the source of the heat, it has to be transported across in the North Atlantic Drift. The author, Seager, argues that heat stored in the water during the summer (sunlight) is more significant, i.e., the effect of a maritime climate.

That seems reasonable, but, assuming it's true, I can't "fathom" (get it, :-) that the weak, warm current is a complete non-factor. I mean, just look at the long-term average D-J-F SSTs. The North Atlantic drift is in there somewhere. Agreed?

I confess I haven't read the paper. All I know for certain is data, and the SSTs are higher there than at most similar maritime latitudes. So I can't help but believe that the North Atlantic Drift plays a role.

Quoting georgevandenberghe:
The other side of the myth is more quickly disproven. People (but not meteorologists) say the warm Gulf Stream moderates the climate of the U.S. East Coast.
This just doesn't happen except during those occasional short periods when we get southeast flow. If for some reason we had a cold, perhaps ice choked Atlantic Ocean (sans western boundary current) to 35N (in violation of numerous principles of Oceanography by the way) we'd be colder during those occasionswhen we had southeast flow and probably have a lot fewer and weaker coastal storms,but otherwise not much effect
Bottom line, a warm ocean to your east doesn't warm you much in the midlatitudes.

It's an interesting idea to speculate what the climate would be like if the topography were different. I used to gripe about cold air damming before realizing what the Appalachians took out of the Arctic outbreaks that come out of the midwest.
When I was in high school and always complaining about cold (three years in Florida cured me of that),
I wondered what would happen if we had an east west mountain range in North America running along 50N.

One effect I think is understated about the Gulf Stream remnants effect on European climate, is what it does to the north Atlantic between Iceland and Scandinavia. This region does not freeze over in winter and so does not become a source of Arctic air. Arctic air from the ice cap further north has 1000 miles of ocean to traverse before reaching Britain, France and Germany. The pattern that occasionally deep freezes Germany and France and chills Britain is persistant high pressure east of or over Scandinavia which pushes Continental Arctic air west from Russia. Veterans of the Battle of The Bulge in 1944 experienced this.

George,

I'm with you on this. Really like the historical references.

Quoting bappit:

I think you put your finger on it in that last phrase of "the midlatitudes would be colder". The (ahem) radiational cooling of the Arctic night is so efficient there would have to be a really strong mechanism of moving the cold air out and warm air in. It gets unbelievably cold up there. Those youtube videos of hot water thrown into frigid air are an awesome demonstration.

I'm up at an odd hour and a bit whacked (as usual) myself, but one thought is maybe if the ice free summer ocean gradually gained heat over time. The ice insulates it in winter. Hmmmm. It would be paradoxical if the winter ice allowed the ocean to collect enough heat to then become ice free to some extent in winter. I've seen the charts showing the high insolation values at summer solstice in the Arctic--equal or greater than at the equator--but I'd be really surprised if the ice covered ocean did not continue during winter.

LOL!!! ...the "ahem" was meant for my stickler approach to language and physics. If you say "net radiational cooling" at night, then I'm all smiles. It's "large radiational cooling" at night that pushes my button. That's because radiational cooling is governed by the Stefan-Boltzmann Law, which states that radiational cooling (emitted radiation) is directly proportional to the fourth power of absolute temperature. Low temperatures at night mean weak radiational cooling. But there's large net radiational cooling. I hope you can understand the distinction I'm trying to make here.

I think I was misunderstood when I wrote an earlier blog, which purported that the greatest radiational cooling occurs around the time of maximum temperature. According to Stefan-Boltzman, I'm correct. But there is also a lot of solar energy also absorbed by the ground around the time of max daytime temperature, so, even though radiational cooling proceeds like gangbusters around the time of maximum daytime temperature, there is still a net warming because incoming radiation exceeds outgoing energy.

So radiational cooling at night is typically much, much lower than radiational cooling during the daytime. Temperatures are higher during the daytime because incoming solar radiation exceeds outgoing infrared energy (radiational cooling) and temperature of the ground (object) increases until it reaches a maximum (incoming equals outgoing).

The bottom line here is that radiational cooling at night is relatively small (relatively low temperatures at night), but net radiational cooling at night can be rather large (clear sky, light wind and low dew points).

By the way, it's possible for there to be a clear sky, calm wind and low dew point at night, but the surface air temperature does NOT continue to decrease...outgoing equals incoming radiation.

So I think we mean the same thing. We just say it differently.

I think one of the obstacles to learning that lay people and some professionals must hurdle is that there is large radiational cooling during the daytime (that's because they think radiational cooling means the temperature has to decrease...it does not!!!).

Indeed, there's typically relatively large radiational cooling of the ground during the daytime, yet the ground's temperature continues to increase because it's gaining more solar radiation than the infrared radiation it's emitting (i.e., radiational cooling).

After I wrote my my first few blogs at WU, I know that I lost my fellow professional meteorologist who believed that the atmosphere is just a "medium" through which radiation simply passes. He apparently does not believe that the atmosphere radiates at all times, which renders any discussion of the greenhouse effect a waste of time.

My students often recoiled in disbelief when I told them that the earth's surface receives almost twice as much radiation from the atmosphere as it does from the sun. That's why I have such strong feeling regarding anthropogenic global warming...my button gets pushed when folks try to obfuscate the argument by introducing the sun into the equation. It's not unimportant, but the source of twice as much radiation to the earth's surface (the atmosphere) should be the real focus of discussion.

Hope that makes sense to you. My goal in this blog, From the Lee Side, is to get one convert a month on the topic of radiational cooling.

May I count you as my latest convert? :-)

Many thanks!

Quoting bappit:
On the subject of posts 17, 19, etc., I did see a story publicizing research someone did on northerly winds blowing surface water south through the Denmark strait. The idea was that sinking brine would not be needed to pull water from the south to bring warmer water northward to ameliorate the NW Europe climate. It moves north to replace the colder water being pushed south. Arctic air approaching Europe then has to go over relatively warm water--relative to all the land masses up there to say the least. Either way, it is hardly justified to call either of these sorts of circulation the Gulf Stream.

If you stretch the Gulf Stream to include the North Atlantic Drift I think you still remain well south of the Norwegian and Greenland Seas. Of course, it is just so much easier to attribute the ice free waters to "The Gulf Stream" when there is such an aura around that current in the public imagination. It is an awesome (that word again) phenomenon, the higher sea levels built up in the Carribean and Gulf of Mexico, the Loop Current, the energetic Florida Current passing between Cuba and the Keys, the turn north for thousands of miles as a distinct river of water in the ocean before dissipating off the coast of Canada. It even was the title of a painting by Winslow Homer in 1899.



Romantic stuff that's sure to capture the imagination.

I can't disagree with anything you said. Good post. Great discussion.

At any rate, a good post and a very interesting discussion.

Quoting redseatrough:
Hi Professor Grenci,
i am from Iran and a weather enthusiast.i am very interested in your blog and follow it regularly.would you please help me explain this situation on Iran and south asia?is it a blocking high?
thanks a lot

I would say that it is not a block. Although there is split flow, it does not extend over a sufficient span of longitude. And I'm sure the pattern did not persist for at least five days. There also has to be more of a pronounced meridional flow. The 500-mb ridge seems too small to me.



Larger image.