NOAA Weather Radio sings Deck the Halls
NOAA Weather Radio sings Deck the Halls
NOAA Weather Radio sings Deck the Halls
This all-day seminar will be organized and run by my friend Mark Thornton, one of the original graduates of the Penn State Weather Forecasting program. In addition to being a meteorologist, Mark is an active sailor in Lake Erie and is well-versed in Great Lakes weather.
The seminar is unfortunately restricted to CYC members only and their guests, but I would encourage you to track down that CYC friend if you are in Chicago and interested in attending. At $49, it is a great deal for such a great educational opportunity.
I enjoyed learning about tropical weather, about which I really knew very little before the beginning of the course, beyond that which I picked up watching TV. I feel I have a much better understanding of tropical weather now. I also am beginning to feel competent about the forecasting part of the Penn State program. After Meteo 101 in the spring, I had developed a good base in general meteorology and how to collect data and interpret forecast models. However, I didn’t feeel even remotely competent in actually forecasting anything. After 241, I think I could make a reasonable forecast of future intensity and track of a hurricane. Certainly nothing on the level of an NHC forecast, but I did walk away with the skills needed to estimate track and strength.
I’m looking forward to 361 in the spring, Fundamentals of Meseoscale Weather Forecasting.
Hurricane Earl was a Cape Verde Islands hurricane that swept a path across the Atlantic in late August and early September 2010. It brushed the Leeward Islands before curving north, tracking the Atlantic coastline and making landfall in Nova Scotia. It reached peak strength as a Category 4 storm.
In this article, I will be discussing Earl’s steering environment, its seedling easterly wave, the conditions on August 28-29 just before attaining hurricane status, and an eye wall replacement cycle. I will conclude with a discussion of the self-development processes of hurricanes.
The image below shows Earl’s general track during its life in the late summer of 2010.
Earl followed a relatively classic pattern for an Atlantic hurricane, sweeping west-northwest towards the Caribbean before veering north and then recurving northeast, before hitting Canada’s Maritimes. The vector wind map below will show why this track occurred.
The steering winds guided Earl; the easterlies in the mid-Atlantic pushed it westward. The lighter southwesterlies over the Caribbean curved it eastward, while the stronger southwesterlies and westerly winds off the mid-Atlantic seaboard caused the recurving towards a more northeasterly direction. Earl first became a tropical storm at about 15N, 35W, where it was being pushed along by fairly strong easterlies. By the time it reached about 16N, 62W, it had reached hurricane force. Except for a brief period of weakening off the mid-Atlantic coast, it maintained hurricane strength as the steering winds guided it up the seaboard towards landfall in Canada.
Earl had its genesis in a tropical wave which left the African coast on August 23, 2010.
This image shows the convective cluster associated with the wave; the wave itself was likely located just to the east of this cluster, as upward motion in waves near Africa tends to precede the wave itself. This pattern reverses itself in the central Atlantic and Caribbean. Once the wave reaches the eastern flank of the mid-Atlantic trough, the coupling of upward motion from the wave and the eastern flank of the trough reinforces upward motion, encouraging development.
For a tropical cyclone to develop and strengthen, there are six fundamental criteria: (a) sufficient sea surface temperatures (SST); (b) a preexisting disturbance; © at least 5 degrees of latitude away from the equator; (d) relatively low mid-tropopsheric wind shear; (e) a relatively moist mid troposphere; and (f) a troposphere that is unstable with respect to moist ascent.
In the easterly wave, we have (b) and © accounted for. Let’s now turn to some of the other necessary ingredients, looking at the time period (August 28) just before Earl reached hurricane strength. Earl was found to be at hurricane strenght by 1200Z on August 29 by a USAFR reconnaissance aircraft.
First, sea surface temperatures must be in excess of 80F/26.5C/299K to sustain further development, ideally with a deep, warm layer beneath the surface. A SST image (below) shows that Earl’s path took it over water with SSTs sufficient to sustain development, and it maintained such SSTs all the way until it reached the waters off the coast of New Jersey.
TCHP imagery below also shows that the waters surrounding Earl’s location on August 28 provided sufficient deep warm water to sustain further development but not so much as to encourage rapid intensification.
Another criterion favoring development is sufficient moisture in the mid-trophosphere. The water vapor image below shows Earl shortly before attaining hurricane status.
Earl at this time is embedded in an area of the atmosphere which is indicated (via gray shading) to have sufficient moisture in the mid-troposphere, to sustain further development. Indeed, the future path of Earl will keep it in this area of relatively moist air; the dark black patch to the north, off the Atlantic seaboard, is the nearest area of dryer air.
A third criterion for further development is benign vertical wind shear.
This image shows that on August 28, earl was in an area of minimal deep layer vertical wind shear, with strengths in the range of 5 to 10 m/s. Shear values of below 10 m/s are considered low enough for tropical storms to intensify. Earl was thus in an area favorable for further strengthening, shortly before it reached hurricane strength on August 29.
On August 28 (shortly before attaining hurricane strength on August 29), Earl was in an area that supported further strengthening, using our ‘recipe’ for tropical storms.
The MIMIC imagery shown above as an animated GIF file is the result of a digital manipulation of microwave imagery. Microwave imagery is well suited to showing the thunderstorms that exist in the eyewall of a hurricane as well as those in the spiral bands. In areas without sufficient radar coverage, microwave imagery can help serve as a substitute, and combining multiple images as done by CIMSS above helps us study the dynamics of eye wall replacement cycles.
The image above documents the eye wall replacement cycle which occurred in the early morning hours (GMT) on August 31. The dark red returns of the eye wall thunderstorms that show at the beginning of the loop can be seen to fade away as the replacement cycle occurs that morning. By 0800Z the inner eye wall thunderstorms disappear; meanwhile a new eye wall, further away from the center, can be seen to be organizing.
An eye wall replacement has the effect of initially weakening the storm, as the radius of maximum winds increases. After the new eye wall organizes and contracts, the storm can again gain strength, as the radius of maximum wind decreases as the eye wall contracts around the center.
Tropical cyclones strengthen through a process of positive feedback. The ‘ingredients’ of tropical cyclone development point to conditions which encourage deep convection to form and persist. We looked at three of these criteria. Warm SSTs encourage a high rate of evaporation of moisture from the sea surface; additionally, by warming the atmosphere above the relatively warm water, it encourages upwards motion and thus convection. The warm-SST fed evaporation feeds moisture to the middle troposphere; dry air entraining into a thunderstorm causes downdrafts to develop as a result of evaporational cooling, which tends to disrupt convection. Therefore a moist middle troposphere, free of dryer air, encourages convection. Finally, relatively strong vertical wind shear disrupts the convection around the center of the storm; weak vertical wind shear allows this convection to persist.
In such conditions, a positive feedback cycle can exist for the cloud clusters, known as conditional instability of the second kind (CISK). As a convective cluster develops, a large amount of latent heat is released, promoting deep convection. This encourages compressional warming in the descending air surrounding the convective cluster. This descending air feeds moisture and air necessary to promote the low-level convergence needed to cause further convection. As long as conditions such as SST, shear and moisture permit this cycle to persist uninterrupted, further convection will occur.
El Ñino is a climate pattern that occurs periodically in the Pacific Ocean that is is characterized by abnormally-warm sea surface temperatures (SST) in the Eastern Pacific Ocean, defined as when the three-month average of sea-surface temperatures in a strip between latitudes five degrees north and south and longitudes 170 degrees west and 120 degrees west exceeds the long-term mean by 0.5 degrees Celsius.
Typically building in strength from March through June, El Ñino has the greatest effect on weather in North America in the December – February time frame. The abnormally-warm SSTs cause a disruption in the normal Walker circulation, bringing more convection to the eastern Pacific and, through teleconnection, affecting weather throughout North America. Although there are several theories as to the cause of El Ñino, there is no consensus yet. Regardless of cause, its effect is clear.
I will be examining the El Ñino that occurred in 1982-1983 and the impacts it had on the subtropical jet stream (STJ) and on seasonal temperature and precipitation in the Gulf Coast States in the United States. The 1982-1983 El Ñino was the strongest since records on these patterns have been kept (1958). This strong alteration in the SST had a significant impact on the STJ and on weather in the US Gulf Coast.
The subtropical jet stream is a narrow ribbon of high speed winds, found on the equatorward side of the subtropical front. It is characterized by high speed winds increasing with altitude from about 400mb to 200mb or so, where they reach their maximum.
Note that the typical STJ tends to fade over the central and eastern Pacific before regaining some strength over Mexico and the southern US. The STJ’s root can be traced to temperature gradients at about 400mb. The tropics are generally devoid of significant horizontal temperature gradients at the surface, but at 400mb a relatively strong north-south gradient forms as a result of Hadley circulation. It is this gradient that is the root of the STJ, which itself is strongest at 200mb.
Now let’s examine the STJ in the winter of the 1982-83 El Ñino event.
In the winter of 1982-83, the band of strong winds extending off of Japan extended much further into the Pacific, across the date line and almost to California. Additionally, the “gap” in the STJ that normally exists over the eastern Pacific has nearly disappeared, with a larger-than-normal pocket of higher-speed STJ from south of Baja California extending to off the coast of the Carolinas. What caused this enhanced STJ during the winter of 1982-83?
During the 1982-83 El Ñino, there was a pool of significantly-warmer-than-normal air over the eastern equatorial Pacific, off of South America, and a commensurately-cool pocket of air over the North Pacific. This sets up an anomalously-strong horizontal temperature gradient along the path of the STJ. The temperature anomaly contributed to a significant height anomaly. Rembering that the root of the STJ is at about 400mb, we can see why this resulted in increased STJ at 200mb extending further east than normal climatology would suggest.
What caused these anomalies? The abnormally-warm SST during an El Ñino causes the overlying atmosphere to also become unusually warm in the central and eastern Pacific. As this warm air rises, forming the Hadley circulation, the north-south temperature gradient at 400 mb becomes stronger than in normal years. The 1982-1983 El Ñino was particularly strong, and thus SSTs were quite high. The effect of this was enhanced baroclinicity, starting at the surface and extending all the way up to 200mb, resulting in a strengthening of the STJ.
Now that we understand how El Ñino serves to enhance the STJ, we turn to the regional effects of it on weather in the US.
The extended El Ñino-influenced STJ over North America affects weather patterns over the Gulf states of the US, including precipitation and temperature.
Precipitation in the Gulf states is significantly higher in our El Ñino event than normal. The STJ plays an important role in cyclogenesis and transporting moisture. The STJ and jet streaks enhance upper-air divergence, which helps intensify surface lows.
The divergence in the left-exit regions of these jet streaks can be seen to contribute to the precipitation in the Pacific Northwest of the US as well as the area off coastal Carolina. These divergence-rich area are rich grounds for cyclogenesis and accompanying precipitation. Furthermore, the fast and strong STJ serves to transport moisture from the maritime environment in the eastern Pacific and the Gulf into the adjoining Gulf states of the US.
We can see from this that the 1982-1983 El Ñino did not have significant effects on temperature anomalies in the southeast. However, a pattern of warmer-than-normal surface temperatures did exist over the North Central, Great Lakes and Northeastern states. This is typical of an El Ñino year.
Why was it that the Gulf Coast states were, if anything, warmer than typical in an El Ñino? I suggest that this is because the strong El Ñino of 1982-83 had a strong STJ, which transported warmer air from the Pacific across Mexico into the region. This would have the effect of moderating temperatures in the Gulf area, and perhaps moving those states back towards climatological norms. Let’s compare this to the map showing strong El Ñino’s.
We can look to a specific day as an example, with the caveat that ENSO cannot predict specific weather on specific days. Looking at 0600Z on December 12, 1982, we can see that a significant trough existed in the central part of the country.
This jet streak is part of the anomalous STJ that existed during the 1982-83 El Ñino. Going back to our earlier conversation, the normal STJ during non-ENSO years doesn’t pick up steam until further east, towards the eastern seaboard. In El Ñino years, the STJ’s strength over Mexico and Texas is stronger, and the setup on December 12, 1982, is consistent with this. This positioned the divergence-rich left exit region of the jet streak over Mississippi, Alabama and Georgia.
The result of this was precipitation ‘downstream’ towards the Carolinas.
Additionally, we should note that the STJ is further north than the average for the 1982-83 DJF time period. As was noted in lesson 4, when the STJ is carried north, particularly when there is a strong jet streak over the eastern half of the US, there is a favorable breeding ground for storms in the coastal Atlantic. That is what seems to have happened on December 12, 1982. Indeed, on the map below, we can see the polar jet stream and STJ interacting:
Although the precise causes of El Ñino are not known, it is fairly clear that the seasonal changes to the STJ in the 1982-83 cold season are an effect of El Ñino, not the cause.
El Ñino is a warming of the SSTs of the central and eastern equatorial Pacific. As we demonstrated above, the warming of the SSTs causes a warming of the atmosphere. The enhanced baroclinicity related to this warming is the primary cause of the enhancement to the STJ during El Ñino episodes. There is anomalously-large north-south temperature gradient, which in turn contributes to similar height anomalies. This is the root cause of the extension of the STJ eastward from it’s normal location as shown my climatology.
Of course, general patterns such as the enhanced STJ during an El Ñino cannot be said to cause specific storms or specific localized weather conditions. Instead, the large-scale patterns established can help us establish conditional probabilities on specific weather patterns such as local or regional temperatures or precipitation. The teleconnections established by El Ñino in strong episodes, such as 1982-83, are fairly strong themselves and thus the probabilities can be forecast with greater definition. However, even the effects of strong ENSO episodes interact with many other atmospheric conditions and forces, and thus the exact impact cannot be predicted far in advance.