Tropical Meteorology

Question 1

Explain the Hadley Circulation. What is the purpose?

Answer 1

The Hadley circulation is the primary circulation in the tropics and moves tropical energy toward the poles and is driven by the latent heat flux. The Hadley circulation creates a region of ascending air in the very low latitudes and descending air in the subtropics. Convective currents can extend through the entire depth of the troposphere. At the top of the troposphere (tropopause), the air moves laterally toward the poles. As the air moves toward the poles, the air loses energy through radiative release. Once the air density is greater than the surround air, then the air subsides until it encounters the earth’s surface – in the subtropics somewhere around 30 to 35 degrees N and S. When the air contacts the surface, the air once again spreads out laterally, with some headed back toward the equator and some headed toward the polar region.

Question 2

Where does the acsending branch exist and between which latitudes does it vary?

Answer 2

The ascending branch of the Hadley circulation occurs in the region of maximum surface heating because this is where the greatest vertical temperature gradient, and therefore instability, exists. The ascending branch of the Hadley cell oscillates between about 5 degrees S and 15 degrees N, with a mean position of about 5 degrees N.

Question 3

Why is the location of the Hadley cell skewd to the north?

Answer 3

Its location is skewed in the North direction because there is more land mass close to the equator in the northern hemisphere. Since land tends to heat much faster than water, the ascending branch is often drawn toward land areas. The subsiding branch of the Hadley circulation tends to limit convective activity and is associated with arid climatological regions on the earth’s surface.

Question 4

What type of ciruclation is the walker circulation (zonal or meridional) and why does it exist?

Answer 4

While meridional circulation is flow from in the North-South direction, zonal circulations flow in the East-West direction. Zonal circulations in each ocean basin set up due to land distributions (from east to west, land-ocean-land-ocean), the large differences in heat fluxes over land as compared to water which creates strong horizontal density gradients, and the fact that the Hadley cell causes an equatorward air flow in the low levels of the tropics which pulls ocean currents toward the equator.

Question 5

Explain the Walker Circulation.

Answer 5

In the northern hemisphere, warm waters tend to flow northward along the western side of the ocean basins and cold water tends to flow southward along the eastern side of the ocean basins. In the Pacific Ocean, warm waters tend to persist on the western side in the vicinity of Indonesia. These warm waters cause rising air, a region low pressure, and they fuel convection there. In the upper levels, the rising air eventually spreads eastward, cools and subsides on the east side of the Pacific near the coast of Central and South America. On the east, cooler waters tend to persist off the coast and these cool waters create enhance subsidence and tend to suppress convection there. In the low levels, air flows in the opposite direction, from the region of high pressure along the eastern Pacific to the region of low pressure near Indonesia. This sets up a zonal circulation known as the Walker circulation.

Question 6

What do the interactions between the Hadley and Walker Circulations create?

Answer 6

easterlies in the tropics and the westerlies in the sub-tropics.

Question 7

What is the hysometric equation?

Answer 7

Warm average temp leads to high heights and lw average temps leads to lower heights.

Question 8

Explain the Sea Breeze in terms of the hysometric equation.

Answer 8

Sea Breeze as described by the hypsometric equation: Since land surfaces tend to heat faster than water surfaces, the air column above the land surface heats and its thickness increases more so than over the water. At the top of the boundary layer, this causes higher pressure at a higher height over the land than over the water. As a result, in the upper levels, air flows from land to sea (Higher heights to lower heights) and sinks. The sinking air causes higher pressure to develop in the low levels of the boundary layer over the water surface, so air in the low levels flows from the water to the land. When viewed in this way, this circulation is driven by wind flow at the top of the boundary layer.

Question 9

Explain the Land Breeze in terms of the hysometeric equation.

Answer 9

Land Breeze as described by the hypsometric equation: Since land surface tend to cool faster than water surfaces, the air column above the land surface cools and its thickness decreases more so than over the water. At the top of the boundary layer, this causes high pressure at a higher height over the water than over the land. As a result, in the upper levels, air flows from the sea to the land and sinks. The sinking air causes high pressure to develop in the low levels of the boundary layer over the land surface, so ait in the low levels flows from the land to the water.

Question 10

Explain the Sea Breeze in terms of the development of a thermal low.

Answer 10

Sea Breeze as described by the development of a thermal low: Since land surfaces tend to increase the amount of sensible heat in the atmosphere more so than water surfaces, a thermal low develops over the land surface. This causes rising motion over the land surface in the low levels and convergence at the top of the boundary layer. At the top of the boundary layer above the land surface, the converging air must spread out, with some the air flowing back toward the water and sinking resulting in lower pressure at the top of the boundary layer over the water. The process of adding mass to the column over the water surface and the fact that low level temperatures are lower over the water than over the land causes high pressure in the low levels over the water surface. In this conceptual model, the circulation is driven by the thermal low at the surface

Question 11

Explain the Land Breeze in terms of the development of a thermal low.

Answer 11

Land Breeze as describe by the development of a thermal low: Since water surfaces tend to have add a greater amount of sensible heat flux to the atmosphere at night time, relatively lower pressure develops over the water surface. This causes rising motion over the water surface in the low levels and convergence at the top of the boundary layer. At the top of the boundary layer above the water surface, the converging air must spread out, with some air flowing back toward the land and sinking. This results in lower pressure at the top of the boundary layer over the land. The process of adding mass to the column over the land surface and the fact that low level temperatures are lower over the land than over the water causes high pressure in the low level over the land surface.

Question 12

Explain the Valley Breeze.

Answer 12

Valley Breeze: During the day, surface heating of mountain slopes causes them to increase in temperature relative to the surrounding air. This causes rising air in the valleys to cling to the mountain sides causing convergence and the most intense convection over the higher terrain.

Question 13

Explain the Mountain Breeze.

Answer 13

Mountain Breeze: At night, the mountain slopes decrease in temperature relative to the surrounding air. This causes the air to flow downslop and it subsides into the valley floors. When this happens, clouds and fog can form within the valleys themselves. There are two primary processes responsible for this. The cold air descending the valley walls can provide a forcing mechanism to lift the warmer surrounding air – much like a cold front. Also, as the cold air mixes and cools the air in the valley bottom,the resultant condensation can cause fog to form.

Question 14

Explain stress induced converegence.

Answer 14

Stressed induced convergence is a dynamical process which can either encourage or inhibit cloud formation along coastal areas, depending on the orientation of the pressure gradient force there. This is due to the increase in surface roughness and therefore friction over land surfaces. In a scenario with high pressure over the ocean and lower pressure over the land surface, winds which are initially flowing roughly parallel to the coastline experience an increase in friction as they approach land. This reduces the wind velocity and in turn reduces the magnitude of the coriolis force acting upon them. With a reduction in coriolis force, the winds are deflected toward the left - toward lower pressure. So, with winds flowing roughly parallel to coastline just off the coast and winds at the coast directed to the left, a region of low-level divergence is generated along the coast and the subsequent subsidence inhibits cloud formation there.
The opposite is true with high pressure over the land surface and low pressure over the ocean. When wind flowing roughly parallel to the coastline slows due to the effects of friction, it is redirected toward the region of low pressure over the ocean. This causes the wind to converge with the wind still flowing roughly parallel to the coastline just off shore. Low level convergence leads to dynamically produce rising motion and can encourage the development of cloud cover there.

Question 15

How does stress induced convergence play a role in making South America on of the top regions in the world to have the lowest annual precip.

Answer 15

Stress induced convergence is enhanced in the tropics since the magnitude of the coriolis force is already reduced there. That is why the west coast of South America tops the list of regions with lowest amount of annual precipitation. Here, the effects of stress induced convergence coupled with the land breeze severely inhibit could formation along the coast.

Question 16

Name the clouds discussed in class.

Answer 16

Trade Wind Cumulus, Doldrum Cumulus, Cumulonimbus, Stratocumulus, altocumulus, cirrus, cirostratus and cirrocumulus/

Question 17

Talk about Trade Wind cumulus.

Answer 17

Trade Wind cumulus tend to form within the band of trade winds near a low level wind maxima. This causes the clouds to appear as if they are tilting upstream. This is because the cloud base is advected faster, since it is located near the wind maxima, than the upper portions of the cloud. The bases of these clouds are typically around 2000 ft with an average cloud top near 7,400 ft. However, most of these clouds have tops near 4000 ft. This large discrepancy between the average cloud top height and the modal cloud top height results from the fact that the average is very sensitive to outliers and the potential for these clouds to grow to very high heights exists under the proper atmospheric conditions. The shape of these clouds offer a good way to diagnose the low level wind conditions and vertical shear. The direction the base of these clouds are moving indicates the direction of the low level winds and the amount of tilt indicates the amount of atmospheric shear.

Question 18

Talk about doldrum cumulus.

Answer 18

Doldrum cumulus are often present in areas that have little shear and wind. As a result, these clouds tend to form orientated straight up and down with little to no horizontal tilt. These clouds have a more triangular shape, with wide bases and slightly more narrow tops. The depth of these clouds generally ranges from 1500 feet to between 6000 – 12000 feet. These types of clouds indicate that synoptic scale forcing and lower level winds are weak. This would be a prime environment for a local sea/land breeze to set up given the absence of synoptic scale forcing to disrupt the circulation.

Question 19

Talk about cumulonumbus in the Tropics.

Answer 19

Cumulonimbus clouds in the Tropics are more common over land surfaces. This is because the differential heating of land surfaces tends to destabilize the atmosphere much more than waater surfaces. These clouds can grow as high as 30,000 to 40,000 feet, while tropical cyclones can support deep convection as high as 50,000 to 60,000 feet or greater. Since the atmosphere tends to more stable over ocean surfaces than land surfaces, cumulonimbus clouds over ocean waters are usually associated with some type of synoptic or mesoscale disturbance. Forcing gets the storms going and latent heat release keeps the storms developing. The anvil blow of from these storms can be advected for hundreds of miles away from the parent storm.

Question 20

Talk about stratocumuls in the topics.

Answer 20

Stratocumulus are commonly found along the eastern side of the ocean basins and east of subtropical high pressure. These types of clouds do not a lot of vertical instability to form. Along coastal regions, where there tends to be less instability than over the open ocean waters, stratus dominates. Over the open ocean, where there is a little more instability between the surface and the top of the maritime boundary layer, stratocumulus is often present. In mountainous regions, radiation fog often forms in mountain valleys, which is eventually lifted to form a stratus deck and then become stratocumulus as heating destabilizes the boundary layer.

Question 21

Talk about altocumulus in the Tropics.

Answer 21

Altocumulus and altostratus are often caused by some type of upper level dynamic al forcing. If you can see the sun through the cloud deck than it is not likely to be altocirrus or altostratus.

Question 22

Talk about the cirrifform type clouds in the tropics.

Answer 22

Because the tropopause height is much higher in the tropics as in the mid latitudes, cirriform clouds tend to be found at much higher heights in the tropics. These clouds can from anvil blow off, or independent from deep moist convection. Cirrostratus often form in association with an upper level disturbance and are often times located on the south side of the STJ. Turbulence associated with the descending branch of the Hadley circulation can cause cirrostratus to from. Cirrocumulus is not as common as cirrus of cirrostratus because there is generally very little upper level turbulence to create these clouds.

Question 23

When does the maximum in rainfall and cloudiness occur of ocean surfaces in the Tropics?

Answer 23

Late night and early morning.

Question 24

Explain the theory with regards to nightime destabalization as it relates to the late night and early morning precip/cloud maxima in the Tropics.

Answer 24

During the late night and early morning, maximum rainfall and cloudiness often occurs over the open ocean. The reasons for this are complicated and not well understood. There are a few competing theories. One theory is that as the atmosphere cools off over night, the boundary layer may be destabilized (warm ocean waters under colder atmospheric temperatures). Although the saturation vapor pressure should decrease with decreasing temperatures, vertical motion associated with the destabilization of the boundary layer could actually increase the amount of evaporation taking place by transporting the water vapor vertically. This theory hinges of the SSTs above the ambient air temperature. However, since the near surface atmospheric air temperature generally falls within about a degree of the SST, destabilization would be minimal. In addition, cooling of the near surface air temperatures would compete with the boundary layer destabilization to reduce the amount of evaporation taking place.

Question 25

Explain the theory with regards to the effects of cloud shields as it relates to late night and early moring precip/cloud maxima in the Tropics.

Answer 25

Another theory pertains to generation of horizontal temperature gradient due to the presence of convective cloud shields. The theory states that since air temp will cool lees under the cloud shield than beside it, a horizontal gradient and resulting circulation would develop that is similar to a land/sea breeze. In the low levels, wind would flow from the regions of higher pressure around the cloud shield, into the region of lower pressure under the cloud shield. This would lead to rising motion under the shield which would enhance cloud formation and precipitation there. In the upper levels, wind would flow away from the shield and subside around the shield. This theory only works for convection that initially began during the day. A third theory states that atmospheric tides generate a convergent/divergent pattern which would enhance convective activity over night.

Question 26

What type of diurnal cloud/precip oscillations occur at coastal locations and why? Do small islands fit with this?

Answer 26

Coastal locations and islands, in the absence of synoptic forcing, have precipitation and cloud patterns that are heavily influenced by local land/sea, land/lake, and mountain/valley breezes. This especially true for larger land masses because they tend to have a sensible heat flux sufficient to generate these types of circulations. These circulations typically cause an afternoon maximum in cloud coverage and precipitation. Cloud and rain patterns over small islands tend to resemble conditions over ocean surfaces (late-night and early morning max) since the sensible heat flux remains rather low there.

Question 27

Define a Tropical Cyclone.

Answer 27

In the United States, a tropical cyclone is defined as a warm core system (as you move toward the center surface temperatures do not decrease), that has no frontal boundaries associated with it(since a tropical cyclone exists within an airmass and not along a boundary between two air masses) , exists on a synoptic scale, and forms in the tropics or in the subtropical oceans. In addition, tropical cyclones contain organized deep convection (the storms within the cyclone are associated with each other much like storms share a common cold pool in mid-latitude MCS’s) and contains a closed surface circulation around a distinguishable point.

Question 28

List the steps in the Tropical Cyclone Lifecycle. (5)

Answer 28

Tropical Wave, Tropical Disturbance, Tropical Depression, Tropical Storm and Hurricane.

Question 29

Desribe a Tropical Wave.

Answer 29

Tropical cyclones begin their lives as tropical waves. A tropical wave is defined as a region of troughing or cyclonic motion existing within the trade winds or equatorial westerlies. These regions could be associated with a cold upper low, a line of shear, or a trough.

Question 30

Describe a Tropical Disturbance.

Answer 30

Tropical waves develop into tropical disturbances. A wave will be classified as a disturbance if it contains organized deep convection, has not frontal boundaries associated with it, and has existed for at least 24 hours. These tropical disturbances can form in either the tropics or the subtropics oceans. Often, the center of rotation is not clearly evident.

Question 31

Desrcribe a Tropical Depression.

Answer 31

A tropical disturbance develops into a tropical depression. A tropical depression will often begin to exhibit a noticeable low in which the system revolves. Maximum sustained surface wind speeds cannot exceed 34 knots.

Question 32

Describe a Tropical Storm.

Answer 32

A tropical depression becomes a tropical storm. A tropical storm will have a definite point in which the system revolves. Often an eyewall will develop and be accompanied by some rain banding. Maximum sustained surface wind speeds are greater than or equal to 34 knots but below 64 knots.

Question 33

Describe a Hurricane.

Answer 33

A tropical storm becomes a hurricane. A hurricane will be symmetrical, be highly organized, and contain an eye. Maximum sustained surface wind speeds are greater than or equal to 64 knots.A hurricane in the Northwest Pacific would be called a Typhoon as long as the maximum sustained wind speeds were greater than or equal to 64 knots, but less than 130 knots. When the maximum sustained wind speeds are greater than or equal to 130 knots, it becomes a super typhoon. A super typhoon would be a upper level category 4 or category 5 hurricane.

Question 34

Describe the Carnot Cycle.

Answer 34

The carnot cycle describes a thermodynamic engine which converts heat into work in a cyclic way (the initial point and final point have the same internal energy). This is done in 4 identifiable steps.
Step 1: At the surface, air from the outside of a hurricane, a region of high pressure, flow toward the eyewall, a region of the low pressure. As this occurs, the air parcel would expand and therefore cool adiabatically. As the parcel expands it produces work on the environment – work is positive. This is why the temperature drops. However, the temperature of the parcel remains constant because the low in temperature is offset by diabatic heating. The ocean provides energy to the parcel through latent and sensible heat. So, the parcel has gained heat and produced positive work on the environment.
Step 2: When the parcel encounters the eyewall and moves upward where it is lifted and cooled adiabatically, The internal energy of the parcel goes down because it expands and performs work on the environment.
Step 3: At the top of the hurricane, as the air parcel moves from the lower pressure at the hurricane center to the higher pressure at the periphery, the air parcel is compressed and warmed adiabatically. At this point, work is negative as the environment performs work on the parcel. However, this is an isothermal process whereby the increase in heat by work is offset by the emission of longwave radiation by the parcel.
Step 4: Air at the top of the hurricane sinks back down to the surface adiabatically. The parcel is compressed and heated by the environment. This causes the internal energy of the parcel to increase. Change in heat is zero.

Question 35

How does this Carnot Cycle relate to a heat engine.

Answer 35

The ultimate purpose of this engine is to take heat at the surface (heat source) and move into the upper levels (cold source). Work removes heat from the system, and the heat left over is deposited in the cold source. The remaining heat is the entropy.

Question 36

Describe a Rankine vortex

Answer 36

A Rankine vortex can be used to describe the circulation within a tropical storm. It is a 2 dimensional circular flow which consists of an inner and outer region. The inner region rotates as a solid body. In this case, the velocity of the fluid and the radius is equal to some constant: V/r = constant. This says that if velocity were to increase, the radius would also have to increase. Or, as we move further from the center of the circulation, the velocity must increase. The outer region behaves differently. Here, V x r = constant. So, velocity is inversely proportional to the radius. If the Velocity were to increase, the radius would decrease – and vice versa. So, as the distance from the system increases, velocity decreases. The Rankin Model can be used to understand the wind distribution within a tropical system.

Question 37

What is the equation with governs wind speed in the inner region of the Rankine vortex?

Answer 37

V/r = constant. This says that if velocity were to increase, the radius would also have to increase. Or, as we move further from the center of the circulation, the velocity must increase.

Question 38

What is the equation with governs wind speed in the outer region of the Rankine vortex?

Answer 38

The outer region behaves differently. Here, V x r = constant. So, velocity is inversely proportional to the radius. If the Velocity were to increase, the radius would decrease – and vice versa.

Question 39

For a real hurricane, is the equation governing wind speed different for the inner or outer region and what is the difference?

Answer 39

vr^a = C when the radius is greater than the radius of maximum winds. *a is an empirically derived value typically from .4 to .6. (Outer Region)

Question 40

Is angular momentum conserved within the hurricane structure? If not, what inhibits angular momentum not to be conserved?

Answer 40

Angular momentum is not conserved. Energy is lost from the parcel through turbulent dissipation. Energy is transferred to the surface of the Earth through turbulent processes which result in waves, storm surge, trees and building damage, etc… Friction. More surface friction means less wind speeds because a larger portion of parcels energy is transferred to the surface. In general, angular momentum decreases with decreasing distance to the center of circulation.

Question 41

What are the possible results of the equation used to estimate vortex stability?

Answer 41

If the equation is a positive value then we have an inertially stable parcel of air.This would indicate that when a parcel is displaced from the center of rotation, its angular momentum would be increased and would accelerate back toward the center of rotation.

If negative (become imaginary) then inertially unstable. This would indicate that as the parcel is displaced from the center of rotation its angular momentum would decrease so it would move away from the center of rotation.

If 0, then neutral. This would indicate that as a parcel is displaced from the center of rotation then it would move toward or away from the center of rotation as a constant dr.

Question 42

How does vortex stability relate to the tropical cyclone?

Answer 42

In general, if a vortex were inertially stable, then there would be very little interaction with the environment since each parcel would remain in place. This is important because a tropical cyclone needs to interact with the environment – there needs to be parcel acceleration taking place within the circulation. Energy needs to be moved horizontally and vertically.

Question 43

How can we describe the Rossby Radius of Deformation? That is, what does it represent in terms of tropical cyclone dynamics?

Answer 43

The Rossby Radius of Deformation can be described as the horizontal length scale for a disturbance that is contained inside a circulation.

Question 44

How do we use the Rossby Radius of Deformation?

Answer 44

If the disturbance is similar in size or much larger than the Rossby Radius of Deformation, then the wind will adjust to the mass field, or they will both adjust to each other. If the developing circulation is much smaller than the Rossby Radius of Deformation, then the mass field will adjust to the wind field and will not disturb the wind field. When this occurs, gravity waves propagate away from the mass field until the mass field is dissipated. Often times diabatic heating is what creates the mass field. In order for a circulation to develop in response to the mass field, the size of the region of heating must be on a comparable scale as the Rossby Radius of Deformation. A secondary circulation develops which means that they can be circulations inside of a larger circulation. So, within a circulation, diabatic effects can cause other circulations to form within it.

Question 45

Describe the components of the core region.

Answer 45

The warm core of the storm is narrow in the lower troposphere and widens with height. It resembles a funnel. The core has rainbands which are region of cloud cover and precipitation. These bands can either spiral into the center or exist as concentric rings around the center. Convective rings are regions of convective cloud cover and precipitation which exist in concentric rings around the center. There is some overlap between rainbands and convective rings, so they may oftentimes be categorized interchangeably. In essence, a convective ring is a type of rainband. Since there is subsidence between the convective rings, this region tends to contain less cloud, or be cloud free. This region is known as the moat (about 5 degrees from center). The convective ring closest to the center of rotation is called the eyewall. The eye is the relatively cloud free region at the center of rotation.

Question 46

How big is the core and what forces are involved to balance the circulation?

Answer 46

The core region tends to be between 3 to 6 times larger than the radius of the maximum winds. In the core, the Rossby number is very large. This implies that within the core the winds are in cyclostrophic balance – a balance between the pressure gradient force and the centrifugal force (no coriolis involved). The rotation is too small and intense for coriolis to be a factor.

Question 47

Describe the eyewall. What does the eyewall contain? What is it shaped like? How do winds change in the vertical? How does angular momentum change in the eyewall and why?

Answer 47

The eyewall usually contains the radius of maximum winds. In addition, since wind speeds increase rapidly from the center, there is typically a well-defined ring of maximum wind speeds. Unlike mid-latitude wind speeds, winds in the core decrease with height. Similar to the structure of the convective rings, the radius of the ring of maximum winds speeds increases with height – that is the funnel shape again. This is important because while the winds tend to decrease with height, the increase in radius may actually increase the angular momentum in the ring with height.

Question 48

In what quadrant are the radius of maximum winds often found?

Answer 48

The right front quadrant (or the northeast quadrant) of the cyclone usually contains the strongest wind speeds regardless of the motion of the storm.

Question 49

What is coincident with the radius of maximum winds? What are two possible explanations?

Answer 49

Finally, the strongest temperature gradient is often coincident with the radius of maximum wind speeds. This could be viewed in two ways. On one hand, an existing strong temperature gradient should create a strong pressure gradient and therefore increase wind speeds. On the other hand, strong winds flowing toward the storm center would concentrate heat and energy in the center portions of the storm which would generate a strong temperature gradient there.

Question 50

Is intertial stability high or low in the core? What does that mean for the core?

Answer 50

Due to the effects of vorticity in the core, inertial stability tends to be very high there. This means that a parcel in the core will tend to maintain its radius. In other words, if a parcel in the core were to be displaced from its initial radius, it would tend to move back toward its initial radius.

Question 51

Is the Rossby Radius of Deformation high or low in the core? What does that mean for the core?

Answer 51

Since the Rossby Radius of Deformation is very small within the core, the wind field will quickly adjust to changes in the mass field which could be caused by heating and cooling, or convergence and divergence. So, while a parcel in the core will tend to maintain its radius around the core, the wind field will rapidly adjust to energy or dynamic changes within the core.

Question 52

Is intertial stability high or low in the outer region? What does that mean for the outer region?

Answer 52

The outer region of a tropical storm has lower inertial stability and a much larger radius than the inner region. As a result, parcels in this region will tend not to return to their initial radius when displaced. This makes the outer portions of the storm less symmetric and more likely to undergo modification from environmental flow.

Question 53

Is the Rossby Radius of Deformation high or low in the outer region? What does that mean for the outer region?

Answer 53

However, since the outer region also has a larger Rossby Radius of Deformation than the inner region, the mass field will adjust to the wind field. Even though a parcel in the outer region may not return to its initial radius when displaced, the wind field has high angular momentum and is very stable. This makes the storm very efficient at absorbing energy in the outer region and transporting it inward.

Question 54

Explain the conceptual model of the TC eye. (7 items)

Answer 54

Step 1: The pressure gradient force causes air to travel toward the cyclone center. As the air spirals toward the center of the circulation, the velocity must increase since angular momentum is conserved. – increase in radius with a decrease in radius. At the same time however, some of this angular momentum is actually lost through processes such as convection and turbulence. The change in total absolute angular momentum is equal to the radial advection of momentum, the advection of vertical momentum, and the magnitude of friction. So, while the velocity of the air parcel is increasing due to the effect of convservation of angual momentum, friction is acting to slow the wind speeds.

Step 2: At some point, the increase in velocity by the conservation of angular momentum overwhelms the decrease in velocity due to friction. This causes a rapid increase in angular momentum beyond which the pressure gradient would allow.

Step 3: While the air was initially drawn toward the cyclone center due to an inward directed pressure gradient force, the increase in angular momentum allows the wind velocity to become super-gradient and the wind overcomes the effects of the PFG. The pressure gradient force can no longer cause this super-gradient wind to travel toward the center of the circulation and centrifugal force causes the winds to spiral away from the center. This forms a region of radial divergence inside of the radius of the super-gradient winds and convergence on the outside of the radius of super-gradient winds.

Step 4: Where convergence exists, there is rising motion and convection which forms the eyewall. This is because radial convergence has led to an abundance of angular momentum which cannot be dissipated horizontally so it is dissipated vertically. In other words, convection dissipated the angular momentum that friction cannot.

Step 5: Low level convergence and upward vertical motion at the eyewall leads to a region of higher pressure above the eyewall.

Step 6: The upper level mass above the eyewall diverges with a some flowing away from the circulation center and some flowing toward the center. The air that flows toward the center convergence near the center, when considered with the diverging super-gradient winds in the low levels, creates a region of subsidence within the eye.

Within the eye, the subsiding air adiabatically increases in temperature as it is forced toward the ground. At some point, the air actually becomes buoyant with respect to its surroundings, but is held in place by the subsiding air from above. Eventually the air spreads out horizontally and is reabsorbed into the eyewall. As long as the subsidence within the eyewall is enough to negate the buoyancy within the eye, the eye will be maintained.

Question 55

Explain the eyewall replacement cycle.

Answer 55

There are certain circumstances in which the eyewall is replaced by an outer ring. Since these rings represent regions of rising motion with mass divergence in the upper levels and subsiding air on the outskirts, subsidence associated with an outer ring can actually inhibit convection within an inner ring. In addition, the outer ring may actually absorb low level inflow and constrict the inner ring. That is, as an outer ring approaches the eyewall, convection along the eyewall can be suppressed by subsidence from the outer ring and through a decrease in low level inflow. This eventually causes the energy associated with the inner ring can be absorbed by the outer ring. This often leads to a decrease in tropical storm intensity as measured by wind speeds. This is because the outer ring will initially have a larger radius than the inner ring so angular momentum would be lower.

Question 56

Name and describe the three types of rainbands.

Answer 56

In general, rainbands exist in three types.

1. Moving spiral bands spiral away from the center of circulation and appear as though they rotate with the vortex.
2. Convective rings are concentric rings which wrap around the cyclone. The inner most convective ring is referred to as the eyewall. Over time, The eyewall may be replaced by other convective rings.
3. Principal spiral bands are like spiral bands in that they appear to spiral into the center of circulation, but they also appear stationary like a convective ring. Principal spiral bands tend to spiral into the eastern side of the tropical cyclone in the northern Hemisphere. The principal spiral band acts like a feeding tube in that it supplies warm moist lower latitude air directly into the core and may explain why convection tends to be more rigorous in the northeast quadrant of the storm. The principal spiral band typically wraps around the center of rotation several times in the inner core region. This can make it difficult to discern which is a principal band and which is a convective ring.

Question 57

What two forces must work in tandem to initiate tropical cyclogenesis?

Answer 57

Where you have a combination of existing low level convection and some form of upper-level dynamic forcing, tropical cyclogenesis can occur. In other words, deep convection is possible when we have strong thermodynamic and upper level dynamical forcing working in tandem.

Question 58

What are some sources of enhanced vorticity to initiate tropical cyclone formation?

Answer 58

A couple of sources of vorticity are monsoon troughs, tropical waves, an old frontal zone, or even an enhancement in the equatorial westerlies from an intense southward push of cold air in the wintertime hemisphere.

Question 59

How can an existing tropical cyclone both encourage the development and inhibit TC formation?

Answer 59

An existing tropical cyclone encourages the formation of other storms in its wake. This is due to processes such as gravity waves that add some type of cyclonic curvature to the environment. In other words, an existing tropical cyclone actually encourages convective development upstream because of regions of large scale vertical motion created by the tropical cyclone itself. Outflow from a large tropical cyclone may actually diminish the potential to form other storms to form due to the increase in shear these outflows tend to cause.

Question 60

What do you need to occur in the low levels and what do you need to occur in the upper levels in order for a TC to maintain itself?

Answer 60

Tropical cyclones can maintain themselves as long as there is sufficient inflow of warm, moist air into the cyclone and adequate outflow aloft. Ideally, there would be more mass evacuated from the top of the system than added in the low levels to encourage vertical motion. For a cloud cluster to develop into a tropical cyclone, the UL mass must transported well away from the cluster. If the susidence occurs in the vicinity of the cluster than there will be no overall reduction in mass because the subsiding air would be reingested by the system. Also, the adiabatically warming air would reduce the temperature defference between the rising air and its surroundings.

Question 61

What things can enhance the evacuation of mass in the upper-levels?

Answer 61

The formation of an anticyclone aloft through latent heat release aids in the upper-level mass divergence.
Necessary to sustain and intensify the cyclone.

There are a few different processes which can enhance the outflow jet in the north-east quadrant of a tropical system. These include linking the northeast quadrant with a subtropical jet, a tropical upper-tropospheric trough, a deep mid-latitude trough, or an upper level cold low. While the subtropical jet and upper level troughs evacuate mass from the storm, the upper-level cold low will lower heights and cause upper level mass and entropy to flow into the low. However, the position of these features is very important in assessing the effects that they will have on the existing convection. If too far away, there may be no response at all. If too close, the shear associated with these features may actually inhibit deep moist convection or may even remove midlevel energy from the system.

Question 62

What is the TC diurnal cycle and what do we think creates it?

Answer 62

Tropical cyclones display a noticeable diurnal cycle. While the cirrus shield tends to be at a maximum in the late afternoon, convection actually experiences a peak in the early morning. This is because over the night the surrounding environment cools rapidly while the greenhouse effect keeps regions under the storm relatively warm. The thermal gradient that sets up strengthens the storm updraft and encourages convection overnight. It is important to remember that the diurnal cycle is typically not sufficient to cause cyclogenesis, it can reinforce cyclogenic processes by increasing the magnitude of positive feedback in the system.

Question 63

Name some factors associated with TC intensity fluctuations. (Hint 4)

Answer 63

changes in sea surface temperatures, land interactions, changes in outflow conditions, loss of UL support, and vertical wind shear.

Question 64

How do the factors associated with TC intensity fluctuations work?

Answer 64

Since tropical cyclones thrive on warm, moist inflow, tropical cyclones moving over warm ocean waters tend to intensify while tropical cyclones moving over cooler sea surface temperatures tend to weaken through a reduction of the sensible and latent heat flux. Similarly, hurricanes which move over land will also experience a reduction in the low level latent heat flux. This can disrupt the Carnot cycle as parcel traversing the system in the would-be “isothermal” legs are no longer able to maintain their temperatures because of the reduction of latent heat added to the parcel. However, there are instances where a tropical cyclone can actually temporarily intensify upon making landfall due to a couple dynamic processes. Winds which move over land are subject to increased friction and subsequently slow. This may cause winds to converge near the coastline as high speed wind flow over the ocean impacts the lower wind flow at the coast. In addition, if the tropical cyclone makes landfall along the correct path, stress induced convergence along the coast can also encourage convective development there. Since a tropical cyclone needs persistent upper level divergence on an order of magnitude necessary to sustain pressure falls at the surface, if the system loses the upper level support, the updraft will weaken and the surface low pressure will fill. Increases in vertical wind shear can decrease tropical cyclone intensity by decoupling the necessary upper level exhaust mechanism from the surface low. This will cause subsidence to occur in close proximity to the storm and decrease the column net divergence. So, in general, tropical cyclone intensification is based upon both thermodynamic and dynamic conditions. This can make determining whether a system will intensify difficult as both processes may or not be working toward the same result.

Question 65

Explain Positive Feedback Mechanism 1.

Answer 65

Positive feedback 1: The latent and sensible heat flux carried by air parcels into the circulation center can help maintain the surface low. This process is enhanced by frictional convergence. Air parcels rotating around the circulation slowed by friction experience an increase in the ageostrophic flow component and will move radially toward the center. In addition, as the storm is strengthened and the wind flow around the circulation is increased, frictional convergence also increases which further increases the amount of frictional convergence into the core. The frictional convergence transfers sensible and latent heat into the core.

Question 66

Explain Positive Feedback Mechanism 2.

Answer 66

Positive feedback 2: The release of latent heat into the circulation increases the average temperature of the atmosphere there which causes an increase in atmospheric thickness – as governed by the hypsometric equation. This causes surface pressure to fall at the surface. The decrease in surface pressure causes more air to move radially into the circulation further enhancing latent heat release and convection, and decreasing the pressure even further. This type of positive feedback mechanism is known as CISK (Conditional Instability of the Second Kind).

Question 67

What are the issues of CISK as it applies to TC formation?

Answer 67

However, there are some issues with respect to using CISK to understand tropical cyclone formation. In general, in order for the region of latent heating to maintain itself and lead to tropical cyclone formation, it must be on the same order as the Rossby Radius of Deformation. In the tropics, the Rossby Radius of Deformation is quite large which would indicate that the region of latent heat release is not large enough to form a circulation. That is, the mass field would adjust to the wind field and the circulation would be torn apart. However, in the low levels of the atmosphere, where the period of oscillation is lower, it is possible that the region of latent heating could be on the same order of magnitude as the Rossby Radius of Deformation. But, most latent heat release occurs in the mid and upper levels. So, in general, the region of latent heat release is too small and the wind speeds are too high in the upper levels to support the formation of a circulation through CISK theory. There is some evidence to suggest that the smaller disturbance may be able to affect larger disturbances which are themselves on the same order of magnitude as the Rossby Radius of Deformation.