TC Motion SO 442 – Tropical Meteorology - PowerPoint Presentation

TC Motion SO 442 – Tropical Meteorology

Learning objectives for today Understand the concept of deep-layer steering flow and its impact on TC motion Apply deep-layer steering flow to a real-world example Understand the Beta effect and how it influences TC motion Get a feel for the complex interaction between steering flow & the Beta effect

Tropical Cyclone Motion As a general rule, TCs tend to move in the direction of the “mass-weighted” average of the mean flow We may consider, for instance, the average flow that exists in the layer from say 850-300 hPa (the choice of 850 hPa as a lower boundary ensures we are considering flow in the “free” atmosphere and away from boundary layer effects) The “mass-weighted” part means that the wind in the lower portion of the layer (e.g., from 850-600 hPa) contributes more strongly to direction of motion than the wind in the upper portion of the layer (greater density/mass at lower altitude) The result of the mass-weighted averaging produces a “steering current” which we would expect to generally drive TC motionReturning to the example from the start of class today…

We’ll look at two locations (red stars) at heights of 850, 700, 500, and 300 hPa to get a qualitative sense of the steering flow that would exist there 850 hPa Beginning at 850 hPa…

At 700 hPa… Overlaying the 850 hPa winds… 700 hPa 850 hPa

At 500 hPa… Overlaying the 850 and 700 hPa winds… 5 00 hPa 700 hPa 850 hPa

And finally at 300 hPa… Overlaying the 850, 700, and 500 hPa winds… The “mass-weighted” average of the winds at the four different pressure levels may be approximated by averaging the four vectors These mass-weighted vectors (in black) would be the approximate steering currents that exist as these locations For instance, a TC in the vicinity of the NW red star might be expected to be moving approximately toward the NW at around 10 kts 300 hPa 5 00 hPa 700 hPa 850 hPa

For instance, a TC in the vicinity of the NW red star might be expected to be moving approximately toward the NW at around 10 kts Steering flow is of 1 st order importance to TC motion. There is, however, another effect (having to do with vorticity) which governs TC motion that we must consider… 300 hPa500 hPa 700 hPa 850 hPa Let’s see how that compares to model forecasts for TD 15 in this region:

Tropical Cyclone Motion Consider: In the absence of a strong steering flow, what will be the motion of a TC?We’ll approach answering this question by considering how the vorticity of the TC interacts with the “background” vorticity (i.e., planetary vorticity ) of the ambient environment We’ll apply the concept of potential vorticity (PV) conservation here as we have for other situations throughout the course So we’ll treat when following parcels through the circulation of the TC noting that parcels tend to evolve so that PV remains constant (in the absence of other forcing)     relative vorticity   vertical depth  

decreased   increased   Tropical Cyclone Motion We begin by considering a Northern Hemisphere hurricane at some latitude corresponding to a value of planetary vorticity  We’ll follow the path of a parcel beginning at the 12 o’clock position due north of the hurricane’s center As the parcel begins to rotate southward in the western semicircle of the cyclone, it loses planetary vorticity as it loses latitude and so, by conservation of , it experiences a gain in relative vorticity       As the parcel passes the 6 o’clock position due south of the center, it begins to rotate northward in the eastern semicircle of the cyclone and gains planetary vorticity as it gains latitude…and so, by conservation of , it experiences a loss in relative vorticity       As a result of these effects, the wind field evolves a mean cyclonic spin on the west side of the TC and an mean anticyclonic spin on the east side   (latitude)    

Tropical Cyclone Motion Because of our use of the -plane approximation, the secondary circulations indicated by the dashed circles are known as beta-gyres     (latitude) - gyre   The beta-gyres not only induce the hurricane to move westerly, but they also induce a northward flow over the cyclone as indicated The two competing motions result in the storm moving somewhat northwestward In other words: Beta-gyres induce westward cyclonic development (propagation) Beta-gyres induce northward advection of the cyclone Leads to a northwestward TC motion in the absence of strong steering currents (poleward drift)            

In reality, the “Beta effect” exists all the time , not only in the absence of strong steering currents…thus the steering currents from before may be modified qualitatively to account Note that the Beta effect not only has an impact on a storm’s direction of motion, but also will tend to cause a storm to move slightly faster than the mean flow for westward accelerations (additive) and slightly slower than the mean flow for eastward accelerations (subtractive)Steering current without Beta effectSteering current with Beta effect?

Tropical Cyclone Motion Note that the Beta effect not only has an impact on a storm’s direction of motion, but also will tend to cause a storm to move slightly faster than the mean flow for westward accelerations (additive) and slightly slower than the mean flow for eastward accelerations (subtractive)A few other factors to consider:Storm size is related to the significance of the Beta effect…storms with larger wind fields tend to have a larger westward component of propagation and also tend to have a faster speed of propagation compared to storms with smaller wind fields Storm intensity does not appear to have a significant bearing on the Beta effectStorm intensity may, however, have bearing on what depth of the atmosphere should be considered when diagnosing steering flow for the stormAll else being equal, more intense storms (characteristically deeper convection) would have a steering flow that constitutes the average wind over a deeper layer of the atmosphere (deep layer flow)Weaker storms (shallower circulations) would tend to be steered by the average flow over a shallower (and closer to the surface) layer of the atmosphereStorm circulations devoid of deep convection tend to simply advect with the low-level winds (e.g., the trade wind flow)

Tropical Cyclone Motion H As a quick application, consider a hurricane in the central Atlantic located south of the subtropical high The upper-level anticyclonic flow around the high combined with the low-level easterly trade wind flow creates a steering current which advects the storm westward The Beta effect results in a slightly faster westward motion than what the steering current alone would produce As the storm approaches the westward extent of the high pressure ridge, steering flow weakens and the Beta effect begins to induce a poleward component of motion (toward the NW) As the storm gains latitude, it eventually comes under the influence of increasingly westerly steering flow which causes its path to turn toward the north and then northeast in a process known as recurvature (at slightly slower speeds than the steering alone)

Tropical Cyclone Motion H Occasionally, a trough in the westerlies will cause a weakness in the subtropical high at a certain longitude The resultant weaker steering flow at that longitude causes a reduction in the storm’s forward speed and allows the Beta effect to advect the circulation northward a bit As the storm passes the longitude of the weakness and the high restrengthens to the north, a more westerly track resumes H

Tropical Cyclone Motion H H If the trough is sufficiently strong, it may cause a complete break in the subtropical high (ridge) In this case, the Beta effect might advect the storm due northward, following the break in the ridge, where it eventually encounters westerly steering flow east of the trough axis and begins to recurve

Summary: learning objectives Understand the concept of deep-layer steering flow and its impact on TC motion Apply deep-layer steering flow to a real-world example Understand the Beta effect and how it influences TC motion Get a feel for the complex interaction between steering flow & the Beta effect