It sits, stalking you from the Arctic Circle, waiting for the perfect time to pounce. What is it? It's the dreaded Icelandic low. The Icelandic low is the greatest producer of weather for Royal Air Field (RAF) Mildenhall, located in East Anglia, United Kingdom. Year round, the Icelandic low creates problems for forecasters at RAF Mildenhall. During the summer and spring it can cramp a general's golf game, and during the winter it will just plain make everyone miserable. First, let us take a look at some of the characteristics of the Icelandic low. "The Icelandic Low is one of six centers of action in the northern hemisphere (Barry et al. 1997)." These centers of action include the Azores, Pacific, and Siberian Highs as well as the Aleutian, Asian, and Icelandic Lows. The Icelandic Low is the northern part of the North Atlantic Oscillation, also known as the NAO. The Azores High makes up the southern portion. The Icelandic Low is also known as a semi-permanent low. This means that it is not always in the same location or sometime does not exist at all. For the most part, it migrates from just east of Iceland over to southern Greenland. In between, it can usually be found around sixty degrees north to sixty-five degrees north. These movements from winter to summer and back again are what cause the Icelandic Low to be such a nuisance to RAF Mildenhall. The spring and summer are probably the least impacting time for the Icelandic Low, but it can still cause a few significant problems. In the summer the low center moves to the eastern side if Iceland, sometime a little bit south as well. When this happens, the jet stream also moves over the United Kingdom. This provides for an excellent exhaust for the development of thunderstorms at RAF Mildenhall. What is worse is that this jet can often split into two separate jets. One will be over Scotland and the other over the English Channel or northern France area. The shared energy from the two jets can dramatically increase the chance for severe weather. Large hail can occur at RAF Mildenhall during this situation and there is a good possibility for a microburst. This could cause extensive damage to aircraft and equipment at RAF Mildenhall. On the positive side, the Icelandic Low moving east of Iceland does bring about some good weather. When the low center gets east of Iceland, it brings about a northwest to north predominate wind flow for East Anglia. This northwest to north flow is first of all a dry wind flow pattern for RAF Mildenhall. Second, it is a down slope flow for the region of East Anglia. These two factors greatly reduce the chance for dense fog for the area. Fog events become more easily predictable during the summer because of this, usually occurring the morning after showers or thunderstorms due to residual moisture. However, all of this can change if the Icelandic Low becomes displaced. On occasion, the low center can be forced by a ridge from the Azores High to a position over the United Kingdom. This can cause numerous problems in the summer for RAF Mildenhall. First, the increase in vorticity nearer to the center of the low will increase the chance for thunderstorms on station as well as the area covered by these storms. Then there is a constant threat of winds, which gust to more than sixty knots due to the quickly tightening gradient. If the low center then moves out into the North Sea it will decrease the winds significantly, but it will also switch the winds to a northeasterly flow. This is the flow with the greatest amount of moisture advection, and will lead to dense fog within a day or two. Those are the main impacts for the spring and summer regime of the Icelandic low, now let us take a look at what happens during the autumn and winter. In the fall and winter months the Icelandic Low moves to the northwest of Iceland and as far as southern Greenland. This movement opens the flow for constant system passage through RAF Mildenhall. Cold fronts move over East Anglia on average every three days or so. A warm frontal passage occurs at the same rate, but its effects are less predominate due to the weakening warm air advection. The systems during this time of year are the largest contributor of precipitation for RAF Mildenhall. The added moisture will quickly add up to a dense fog event about two days after frontal passage. The systems that move over the United Kingdom also have a tendency to bring winds in excess of forty-five knots to East Anglia. These winds make it difficult for aircraft to take-off and land at RAF Mildenhall. They also often cause moderate turbulence from the surface to 5000 ft due to velocity wind shear. At times, these systems can stall out and create a stationary boundary over East Anglia. This is due to the occasional loss of cold air advection to support the movement of fronts, because the Icelandic Low steering system is so far to the west. Another impact from the Icelandic Low during winter is the switch in predominate wind flow. Starting in fall and into winter the surface wind flow changes from north to northwest and becomes southwesterly. Due to the terrain of the southern coast of the England, this cause an upslope effect to the southwest of RAF Mildenhall. During this time of year the south coast of the England is notoriously foggy place. This fog is pushed over the upslope and becomes a 1000ft ceiling. As the ceiling progresses northeastward, the now down-slope flow can cause the bases to sink. The southwest flow continues to push this ceiling over East Anglia. At times, this ceiling can sink to below 500ft and completely restrict aircraft from take-offs or landings. Now, the most significant weather caused by the Icelandic low during winter is when the low extends a trough all the way south to the Azores. This only happens during the fall and winter due to the weakened state of the Azores High. This trough extending over such a large area allows for constant cyclogenisis whenever an upper level trough moves over it. These systems quickly develop and usually track directly towards East Anglia. These systems are also stronger than the average system that passes RAF Mildenhall during this time of year. This can lead to unexpected thunderstorm development. In the winter the chance for thunderstorms drops dramatically for RAF Mildenhall. These stronger systems producing winter thunderstorms can quickly stop all operations on the base. In addition to unexpected thunderstorms, this setup of the Icelandic low also brings high winds. The intrusion of low pressure, far to the south, causes the gradient to tighten drastically. Winds at RAF Mildenhall will often gust to greater than forty-five knots for several days in a row. The high winds also cause moderate turbulence over East Anglia. This time though it is not a change in velocity but as a result of directional wind shear. The flow at the surface is moved to the southeast while the air flow above the surface remains southwesterly. As with the Icelandic Low during summer, the low center can get displaced in the winter. When this situation occurs, the weather affects resemble that of when the Icelandic Low extends to the Azores with one exception, there is a lot more cold air advection. When the low center moves down from the Arctic during the winter it brings a lot of very cold maritime polar air with it. Temperatures at RAF Mildenhall quickly drop below freezing. The possibility for freezing drizzle and rain becomes a real concern until the low modifies and the temperatures begin to rise again. If the low moves quickly east over the North Sea, it will open a cold northeast flow into East Anglia. This setup provides one of the main situations for a snow event at RAF Mildenhall. It can be compared to lake effect snow and can quickly accumulate several inches in a few hours. The Icelandic Low is a semi-permanent pressure system that is one of six "centers of action" in the northern hemisphere. Located between Iceland and Greenland, it affects the United Kingdom year round. The spring and summer can bring and increase in thunderstorms with the possibility of severe hail and winds. It is also the time of year with good visibility and ceilings so it makes for great flying. In the winter the Icelandic low strengthens and brings a more drastic change in the weather. An increase in the number of weather systems bring in more moisture and increase the occurrence of dense fog. When the low displaces over the North Sea temperatures drop and there becomes a good chance for snow. All of these reasons are why the Icelandic Low is truly the main weather producer for Royal Air Field Mildenhall in East Anglia, United Kingdom. References Icelandic Low Training Material; 21st Operational Weather Squadron Training Page Website: https://ows.sembach.af.mil Icelandic Low Cyclone Activity: Climatological Features, Linkages with the NAO, and Relationships with Recent Changes in the Northern Hemisphere Circulation from Volume 10, Issue 3 (March 1997) of American Meteorological Society Journal Website: http://ams.allenpress.com/perlserv/?request=get-document&doi= 10.1175%2F1520-0442(1997)010%3C0453:ILCACF%3E2.0.CO%3B2&ct=1 |