Flashcards in Tropical Meteorology Deck (67)
Explain the Hadley Circulation. What is the purpose?
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.
Where does the acsending branch exist and between which latitudes does it vary?
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.
Why is the location of the Hadley cell skewd to the north?
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.
What type of ciruclation is the walker circulation (zonal or meridional) and why does it exist?
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.
Explain the Walker Circulation.
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.
What do the interactions between the Hadley and Walker Circulations create?
easterlies in the tropics and the westerlies in the sub-tropics.
What is the hysometric equation?
Warm average temp leads to high heights and lw average temps leads to lower heights.
Explain the Sea Breeze in terms of the hysometric equation.
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.
Explain the Land Breeze in terms of the hysometeric equation.
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.
Explain the Sea Breeze in terms of the development of a thermal low.
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
Explain the Land Breeze in terms of the development of a thermal low.
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.
Explain the Valley Breeze.
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.
Explain the Mountain Breeze.
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.
Explain stress induced converegence.
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.
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.
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.
Name the clouds discussed in class.
Trade Wind Cumulus, Doldrum Cumulus, Cumulonimbus, Stratocumulus, altocumulus, cirrus, cirostratus and cirrocumulus/
Talk about Trade Wind cumulus.
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.
Talk about doldrum cumulus.
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.
Talk about cumulonumbus in the Tropics.
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.
Talk about stratocumuls in the topics.
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.
Talk about altocumulus in the Tropics.
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.
Talk about the cirrifform type clouds in the tropics.
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.
When does the maximum in rainfall and cloudiness occur of ocean surfaces in the Tropics?
Late night and early morning.
Explain the theory with regards to nightime destabalization as it relates to the late night and early morning precip/cloud maxima in the Tropics.
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.
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.
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.
What type of diurnal cloud/precip oscillations occur at coastal locations and why? Do small islands fit with this?
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.
Define a Tropical Cyclone.
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.
List the steps in the Tropical Cyclone Lifecycle. (5)
Tropical Wave, Tropical Disturbance, Tropical Depression, Tropical Storm and Hurricane.
Desribe a Tropical Wave.
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.