Why do we need warm sea surface temperatures?
A pre-existing disturbance is always needed to initiate development, whether it is a tropical wave, tropical low, surface trough, upper low, or sometimes a frontal boundary and extratropical cyclone. We often here that cyclones need warm sea surface temperatures but why? I mean water is water regardless of the temperature. The main reason why we need warm sea surface temperature lies in the fact that warm water is more energetic than colder waters. Tropical cyclones need warm moist air and the only source of such air is from the ocean. Temperature is simply the degree to which molecules move and evaporation can only occur if water molecules can escape the sea surface and enter the air and in order for that to occur; you need the temperature to be high.
How does a tropical cyclone cools the sea surface?
A tropical cyclone's winds causes upwelling of colder sub-surface waters but there is another process by which tropical cyclones remove heat from the ocean – evaporation. Evaporation is the process by which water molecules change from a liquid state (water) to a gaseous state (water vapour). Now the molecules in the liquid state are bonded closer than in the gaseous form and this pretty obvious if you ever boiled water. Now remember when I said warmer temperatures cause molecules to move faster, well to get those molecules spread out from the liquid form to the gaseous form you need energy (heat of evaporation). The heat comes from the surrounding ocean, which is referred to as latent heat.
In summary, heat needs to be added to a group of water molecules to get it change from a liquid state to a gaseous state. The heat needed is called latent heat and comes from the surrounding ocean, which causes it to cool.
Latent heat versus sensible heat
Latent heat is the heat absorbed or given off by a substance while it is changing its physical state. Sensible heat can be sensed, or measured, with a thermometer, and the addition or removal of sensible heat will always cause a change in the temperature of the substance. In other words, sensible heat is the heat that affects the temperature of things; latent heat is the heat that affects the physical state of things. The summer time warming of the sea surface is related to sensible heat, while the heat removed from the ocean needed for evaporation is latent heat.
Figure 1. A schematic diagram showing the simply flow of energy from the sun to the ocean to the air aloft, illustrating the process of evaporation and condensation and the types of energy that are associated.
Why is moist air lighter than drier air at the same temperature?
Water vapour is a relatively light gas when compared to oxygen and nitrogen, the two other abundant gases in the atmosphere. If we take a parcel of moist air, the pressure of the vapour molecules will be greater than the heavier gases that share the parcel so in effect it will be lighter. Oppositely, a dry air parcel will have lesser vapour molecules and more of the other heavier gases so in effect will be heavier.
Another reason why moist air is lighter, is because of its temperature. Moist air is warmer because the tendency for condensation is greater and condensation releases heat. Dry air is cooler because the tendency for evaporation is greater and evaporation absorbs heat.
This is the reason why moist air (above 70%) is often a requirement for tropical cyclogenesis and why dry air interferes with tropical cyclone development.
How does dry air affect tropical cyclone development?
As mentioned before, dry air is heavier and cooler than moist air. If the air is dry in the mid levels of a tropical cyclone (between 500 mb-700 mb) then evaporation of raindrops or condensed water will occur. This causes the mid-levels to cool and become even denser. Dense air will sink which retards both lower pressure and rising air needed for convection. When it hits the surface it spreads out and the leading edge is what we called arc clouds or outflow boundaries.
Figure 2. Visible image of Tropical Storm Chris in August of 2006 showing a series of arc clouds emanating from within the central overcast, indicating that the tropical cyclone struggled with some level mid-level dry air. Sometimes you cannot use water vapor imagery to detect this dry since water vapor mainly shows the upper level moisture fields. For this you might use model soundings or total precipitable water loops.
What causes a tropical cyclone's warm-core?
The simple cause of a tropical cyclone's warm-core is heat of condensation. When condensation occurs, it releases heat because you are trying to bond the molecules from a gaseous form to a liquid form. The greatest condensation occurs near the center of the cyclone relative to the surroundings so there is where the greatest heat will be found, thus it fulfills the definition of a warm core.
What causes the pressure heights to fall in a tropical cyclone?
The main cause of the pressure falls associated with tropical cyclone or any low-pressure system is rising air. As air rise, it exerts less force on the earth surface and you have lower pressure. There is another process by which rising air cause pressure heights to fall – opposing the force of gravity. As air rises, it opposes the downward force of gravity. In order to compensate for this imbalance, pressure heights must be lowered. One person told me if this did not occurred, the troposphere would rise into the stratosphere but I have always oppose this by stating the tropopause, which is a seal, would not allow this.
Figure 3. A systematic diagram showing the formation of a tropical cyclone's warm-core and the associated pressure height falls.
What are the different types of energy associated with tropical cyclones?
You first have heat and potential energy stored in the ocean. Tropical cyclones obtain this heat energy through evaporation, which rises and condenses. Water vapour retains the heat and carries it from the sea surface to the air aloft. Now the heat ends back into the atmosphere through condensation. The air aloft warms, and becomes more buoyant and rises even further. The pressure heights fall in response and lower pressure is created. The pressure gradient increase and air begins to move and now we have kinetic energy. The moving air picks up more moisture from the sea surface and rises again to begin the cycle over again. As long as conditions remain favourable, this cycle intensifies with each round – positive feedback loop.
Now the moving air exerts a force upon the water surface to create and cause it to move in waves, this is now called mechanical energy. Mechanical energy is also responsible for physical damage and storm surges. The wind exerts a force on objects which causes them to move.
The heat and kinetic energy released by tropical cyclones is equivalent to 200 times the world-wide electrical generating capacity - an incredible amount of energy produced!
I hope after reading this blog you are left with a little bit more knowledge of how tropical cyclones work, and rather than just looking at one on satellite imagery, you can appreciate the energy, thermodynamics and physics that go in to making one.
Other News
I will also issue my October Summary next Sunday and Winter Outlook for the Western Atlantic sometime in early November.
Erika was really a forecasting challenge for me and probably one of my personal best forecasts in predicting her to go left of the forecast track but here is Erika’s model verification and I LMFAO last night when I saw how poor the models did with her.
Figure 4. Model tracks from 00 UTC 2 September and Erika's best track [actual (black)]. You can clearly see the large forecast error made by the models but this is no surprise because as an invest Erika never did follow the forecast models.
We often hear radar imagery is confined to land-based areas but wouldn't it be cool if numerical model-based radar imagery was available - which has no spatial constraints. The animation below is of Hurricane Fred from the COAMPS model of Fred that shows 1-hr increments radar based reflectivity from the 120-hr initial forecast – September 8 2009 at 12Z. This model had 95% accuracy in the forecasting of Fred's track and intensity but this accuracy was the same for most forecasters and other models.
Figure 5. COAMPS radar based reflectivity of Fred +120 hrs from September 8 2009 at 12Z. You can clearly see the pre-banding features that wrapped all the way around, the formation of the eye and the maturity and then the demise due to vertical shear.
Now, the season is winding down with only 5 weeks left and I do not expect much in November since this is an El Nino year and for various other factors. Now, I will still be here updating my blog on topics just like this one but for many others, they will be leaving, never to be seen again until next hurricane season, so for those that won't be here - Seasons Greeting and a blessed New Year.
Weather456
