cdcollura
EF5
Good day all,
We always talk about the words "Vertical Wind Shear" with great excitement and anticipation while out chasing in May or Early June in the Central USA. But turn attention to the tropics, and the other form of chasing - Hurricane Chasing, and the same words bring MAJOR disappointment.
With supercells and tornadoes, shear is one of the most important ingredients, akin to instability. Even more preferred, is directional shear, where winds also change direction with height. Such vertical changes with wind speed and / or direction with increasing height cause an updraft (given the correct instability) reaching the free convection will become stronger and even rotate (supercells). Tilting of a storm updraft, caused by the shear, also keeps the downdrafts and updrafts of the storm separate, and the storm continues to strengthen without collapsing immediately because of the outflow.
Meanwhile, in the tropics, the OPPOSITE is true. You still need instability, of course, but for tropical cyclone formation, development, and sustenance, very light winds aloft are REQUIRED in the air column. This means there cannot be excessive wind shear, and what I mean by "excessive" is like under 15 knots of total "bulk" shear from the surface to 6 km (500 MB / 18,500 feet)!
A developing tropical cyclone requires that the convection, responsible for "removing" the air from near the sea surface, to high altitudes, remains OVER the same area of the tropical oceans, this creating a low pressure area. If ANY shear is present, the convection is "blown away" from the developing low, and it elongates into a weaker trough, and dissipates. In 2006, tropical storm "Chris" in the NE Caribbean was DESTROYED in less than one day from a 20-30 Knot NW flow aloft.
Also, tropical cyclones favor a weak HIGH PRESSURE area aloft, much different than the preferred conditions of supercells, which require LOW PRESSURE aloft. The upper-level high provides a "venting mechanism" to remove the blow-off and cloud "debris" from the thunderstorms near the core of the developing tropical low.
There also must be nearly constant dew points around the tropical system. If a pool of dry air, such as a Saharan Air Layer (SAL) is present, it will become entrained into the tropical system, and reduce convection through evaporative cooling and drying of the storm core. This can even erode a "hole" or dry slot into the eyewall of a hurricane itself!
The key to a tropical cyclone is a sea surface temperature of at least 78.8 deg F, weak high pressure aloft, moist air all around the storm, little subsidence, weak "trough-iness" (such as a tropical wave) at the surface, and most-importantly, low bulk wind shear.
What I mean by BULK shear is the sum of the deep-layer shear. For example: A west wind of 10 knots at sea level and west wind of 40 knots at 6 km is the SAME BULK SHEAR as an east wind of 20 knots at sea level and a west wind of 10 knots at 6 km. Both cases will have a sum of 30 knots of shear, the first example being speed shear and the latter being owned to directional shear - 30 knots of bulk shear will DESTROY a tropical cyclone.
Ever see those hurricanes that recurve after struggling for days barely maintaining their convection and fighting shear when moving NW at 10 MPH, then, when moving NE at 25 MPH, they rapidly intensify? Well, such examples (such as Hurricane Erin in 2001), are because the storm is moving NW at 10 MPH, but there may be a 30 MPH wind at 6 km (bulk shear is over 35 MPH) ... But when the same storm moves NE at 25 MPH (after recurverature), the 35 MPH SW flow aloft does not "feel" so bad relative to the storm, which now is bulk shear of only 10 MPH. Also, a little shear is actually good for a tropical system (provides cloud-top "venting").
Hurricane Wilma striking Florida in 2005 was also a clear example of a very intense hurricane despite a high shear environment. Wilma was ahead of a deep trough, with nearly 40 MPH winds at 6 km from the SW, but the storm moved NE at nearly 30 MPH, so shear (according to Wilma) was only 10 MPH! Had Wilma not recurved, and continued N or NW into the Gulf, the shear would have ripped it to shreds. Remember, a little shear is GOOD, so the 10 MPH relative wind aloft at the back of "Wilma's head" actually was more of a benefit than a detriment, so Wilma intensified and struck FL as a strong Cat-3 storm with devastating effects.
We always talk about the words "Vertical Wind Shear" with great excitement and anticipation while out chasing in May or Early June in the Central USA. But turn attention to the tropics, and the other form of chasing - Hurricane Chasing, and the same words bring MAJOR disappointment.
With supercells and tornadoes, shear is one of the most important ingredients, akin to instability. Even more preferred, is directional shear, where winds also change direction with height. Such vertical changes with wind speed and / or direction with increasing height cause an updraft (given the correct instability) reaching the free convection will become stronger and even rotate (supercells). Tilting of a storm updraft, caused by the shear, also keeps the downdrafts and updrafts of the storm separate, and the storm continues to strengthen without collapsing immediately because of the outflow.
Meanwhile, in the tropics, the OPPOSITE is true. You still need instability, of course, but for tropical cyclone formation, development, and sustenance, very light winds aloft are REQUIRED in the air column. This means there cannot be excessive wind shear, and what I mean by "excessive" is like under 15 knots of total "bulk" shear from the surface to 6 km (500 MB / 18,500 feet)!
A developing tropical cyclone requires that the convection, responsible for "removing" the air from near the sea surface, to high altitudes, remains OVER the same area of the tropical oceans, this creating a low pressure area. If ANY shear is present, the convection is "blown away" from the developing low, and it elongates into a weaker trough, and dissipates. In 2006, tropical storm "Chris" in the NE Caribbean was DESTROYED in less than one day from a 20-30 Knot NW flow aloft.
Also, tropical cyclones favor a weak HIGH PRESSURE area aloft, much different than the preferred conditions of supercells, which require LOW PRESSURE aloft. The upper-level high provides a "venting mechanism" to remove the blow-off and cloud "debris" from the thunderstorms near the core of the developing tropical low.
There also must be nearly constant dew points around the tropical system. If a pool of dry air, such as a Saharan Air Layer (SAL) is present, it will become entrained into the tropical system, and reduce convection through evaporative cooling and drying of the storm core. This can even erode a "hole" or dry slot into the eyewall of a hurricane itself!
The key to a tropical cyclone is a sea surface temperature of at least 78.8 deg F, weak high pressure aloft, moist air all around the storm, little subsidence, weak "trough-iness" (such as a tropical wave) at the surface, and most-importantly, low bulk wind shear.
What I mean by BULK shear is the sum of the deep-layer shear. For example: A west wind of 10 knots at sea level and west wind of 40 knots at 6 km is the SAME BULK SHEAR as an east wind of 20 knots at sea level and a west wind of 10 knots at 6 km. Both cases will have a sum of 30 knots of shear, the first example being speed shear and the latter being owned to directional shear - 30 knots of bulk shear will DESTROY a tropical cyclone.
Ever see those hurricanes that recurve after struggling for days barely maintaining their convection and fighting shear when moving NW at 10 MPH, then, when moving NE at 25 MPH, they rapidly intensify? Well, such examples (such as Hurricane Erin in 2001), are because the storm is moving NW at 10 MPH, but there may be a 30 MPH wind at 6 km (bulk shear is over 35 MPH) ... But when the same storm moves NE at 25 MPH (after recurverature), the 35 MPH SW flow aloft does not "feel" so bad relative to the storm, which now is bulk shear of only 10 MPH. Also, a little shear is actually good for a tropical system (provides cloud-top "venting").
Hurricane Wilma striking Florida in 2005 was also a clear example of a very intense hurricane despite a high shear environment. Wilma was ahead of a deep trough, with nearly 40 MPH winds at 6 km from the SW, but the storm moved NE at nearly 30 MPH, so shear (according to Wilma) was only 10 MPH! Had Wilma not recurved, and continued N or NW into the Gulf, the shear would have ripped it to shreds. Remember, a little shear is GOOD, so the 10 MPH relative wind aloft at the back of "Wilma's head" actually was more of a benefit than a detriment, so Wilma intensified and struck FL as a strong Cat-3 storm with devastating effects.
