AVHRR/MVISR Composite Image of Earl on September 2, 2010 as it tracked north off the coast of the Carolinas. Image courtesy CIMSS.

Introduction

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.

Body

Steering Environment

The image below shows Earl’s general track during its life in the late summer of 2010.

Track of Hurricane Earl. Map courtesy The Weather Underground.

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. 

500mb vector winds from August 25 – September 4, 2010. The approximate track of Earl is indicated in red. It is quite apparent how the steering winds affected Earl’s path, with 500mb vector winds serving as a proxy for the steering wind layer. Image courtesy ESRL.

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.

Easterly Wave

Earl had its genesis in a tropical wave which left the African coast on August 23, 2010. 

 This image from 1800Z on August 23, 2010, shows the convective cluster associated with the easterly wave that would provide the seed for Earl. The red ellipse highlights the convection; in and just offshore of Africa, convergence and convection tends to occur ahead of the easterly wave, so in this image the wave itself would be just east of the convective cluster. Channel 10 image from Meteosat SEVRI archive courtesy of Dundee Satellite Receiving Station.

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. 

Favorable conditions for Strengthening

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.

Sea Surface Temperatures

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. 

SST map for August 28, 2010, the day before Earl reached hurricane status. SSTs of greater than 301.5K (28.5C) supported development in the area where Earl was located as well as along its future track parallel to the Atlantic seaboard. SSTs of greater than about 299K (80F/26C) support strengthening, and these existed as far north as the New Jersey coast. Image courtesy ESRL.

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.

 TCHP image from August 28, 2010, with Earl’s approximate position noted in red. TCHP values in the range of 60-70. This levels do not suggest the likelihood of sudden intensification, but when combined with the SST data, do suggest that there is sufficient warm water to sustain development. Image courtesy NOAA/AOML.

Midtropospheric Moisture

Another criterion favoring development is sufficient moisture in the mid-trophosphere. The water vapor image below shows Earl shortly before attaining hurricane status.

Water vapor image from GOES East on August 29, at 1200Z, shortly before Earl became a hurricane. Earl is embedded in an area of relatively moist middle and upper tropospheric water vapor, as shown by the gray shading. Image courtesy Dundee Satellite Receiving Station.

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.

Vertical Wind Shear

A third criterion for further development is benign vertical wind shear.

Deep layer shear image from 1200Z on August 29, just as Ear was becoming a hurricane. Earl was embedded in an area of relatively low shear. Image courtesy CIMSS.

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.

Eye-wall replacement cycle

 

MIMIC imagery from August 31, 2010, showing an eye wall replacement cycle. Imagery courtesy CIMSS.

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.

Conclusion

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.