20-4 Figure 20-5. Precipitation Attenuation Versus Wavelength
As the wavelength of the radar decreases, the amount of precipitation attenuation increases.
The WSR-88D’s 10-centimeter wavelength is not significantly attenuated by precipitation. However, aircraft radars, which typically have 3-centimeter wavelengths, have a significant precipitation attenuation problem. As a result, aircraft weather radar typically only shows the leading edge of extreme intensity echoes.
20.1.5.2 Range Attenuation. Range attenuation is the decrease of the intensity of energy within the radar beam as the beam gets farther away from the antenna. If not compensated for, a target that is farther away from the radar will appear less intense than an identical target closer to the radar.
Range attenuation is automatically compensated for by the WSR-88D.
However, most airborne radars only compensate for range attenuation out to a distance of 50 to 75 nautical miles (NM). Targets beyond these ranges will appear less intense than they actually are.
20.1.6 Resolution. Resolution is the ability of the radar to show targets separately.
20.1.6.1 Beam Resolution. Beam resolution is the ability of the radar to identify targets separately at the same range, but different azimuths (see Figure 20-6).
20-5 Figure 20-6. Beam Resolution
Two targets must be separated by at least one beam width (diameter) in order to be displayed as two separate echoes on a radar image.
The WSR-88D has a beam width of 0.95°. Therefore, at a range of 60 NM, targets separated by at least 1 NM will be displayed separately. At a range of 120 NM, targets separated by at least 2 NM will be displayed separately.
Aircraft radar have beam widths that vary between 3 and 10°. Assuming an average beam width of 5° at a range of 60 NM, targets separated by at least
5.5 NM will be displayed separately. At a range of 120 NM, targets separated by at least 10 NM will be displayed separately.
The beam resolution is better for the WSR-88D than aircraft radar (see Figure 20-7).
20-6 Figure 20-7. Beam Resolution Comparison Between WSR-88D and Aircraft Weather Radar
In the example above, the targets (thunderstorms) are at the same range in azimuths for both the aircraft and WSR-88D radar. At 10 NM, the beam width is small enough for both the WSR-88D and aircraft radar to display the thunderstorms separately. At 60 NM, the WSR-88D beam width is still small enough to display both thunderstorms separately. However, the aircraft radar beam width is larger, which results in the two thunderstorms being displayed as one echo.
Note that the beam becomes wider at greater distances from the radar.
Therefore, the beam resolution decreases with increasing range from the radar.
As a result, lines of precipitation may appear to break up as they move closer to the radar. In reality, the breaks in the precipitation were most likely always there.
20-7
20.1.7 Wave Propagation. Radar beams do not travel in a straight line. The beam is bent due to differences in atmospheric density. These density differences, caused by variations in temperature, moisture, and pressure, occur in both the vertical and horizontal directions, and affect the speed and direction of the radar beam.
In a denser atmosphere, the beam travels slower. Conversely, in the less dense atmosphere, the beam travels faster. Changes in density can occur over very small distances, so it is common for the beam to be in areas of different densities at the same time as it gets larger. The beam will bend in the direction of the slower portion of the wave.
20.1.7.1 Normal (Standard) Refraction. Under normal (i.e., standard) conditions, the atmosphere’s density gradually decreases with increasing height. As a result, the upper portion of a radar beam travels faster than the lower portion of the beam. This causes the beam to bend downward (see Figure 20-8).
Figure 20-8. Normal Refraction
The radar beam curvature is less than the curvature of the Earth. Therefore, the height of the radar beam above the Earth’s surface increases with an increasing range.
20.1.7.2 Subrefraction. Atmospheric conditions are never normal or standard.
Sometimes, the density of the atmosphere decreases with height at a more than normal rate. When this occurs, the radar beam bends less than normal.
This phenomenon is known as subrefraction (see Figure 20-9).
20-8 Figure 20-9. Subrefraction
Subrefraction may cause the radar beam to overshoot objects that would normally be detected. For example, distant thunderstorms may not be detected with subrefraction. Subrefraction may also cause radar to underestimate the true strength of a thunderstorm. Thunderstorms may appear weaker on radar because subrefraction causes the radar beam to strike the thunderstorm near the top of the cumulonimbus cloud, where the precipitation particles tend to be smaller.
20.1.7.3 Superrefraction. Conversely, sometimes the density of the atmosphere decreases with height at a less than normal rate, or even increases with height.
When this occurs, the radar beam will bend more than normal. This phenomenon is called superrefraction (see Figure 20-10).
Figure 20-10. Superrefraction
20-9 Superrefraction causes the radar beam to travel closer to the Earth’s surface than what would occur in a normal atmosphere. This can lead to overestimating the strength of a thunderstorm, as the beam would detect the stronger core of the storm, where precipitation-sized particles are larger.
If the atmospheric condition that causes superrefraction bends the beam equal to, or greater than, the Earth’s curvature then a condition called ducting, or trapping, occurs (see Figure 20-11).
Figure 20-11. Ducting
When ducting occurs, the radar beam will hit the surface of the Earth, causing some of the beam’s energy to backscatter. This often leads to false echoes, also known as anomalous propagation (AP), to appear in the radar display.
20.1.8 Intensity of Precipitation. The intensity of precipitation is determined from the amount of energy backscattered by precipitation, also known as reflectivity. Reflectivity is determined by:
• The size of precipitation particles;
• The precipitation state (liquid or solid);
• The concentration of precipitation (particles per volume); and
• The shape of the precipitation.
20.1.8.1 Intensity of Liquid Precipitation. The most significant factor in determining the reflectivity of liquid particles is the size of the precipitation particle (see Figure 20-12).
20-10 Figure 20-12. Reflectivity Associated with Liquid Targets
Larger particles have greater reflectivity than smaller particles. For example, a particle with a 1/4-inch diameter backscatters the same amount of energy as 64 particles that each have a 1/8-inch diameter.
Radar images/intensity scales are associated with reflectivities that are measured in decibels of Z (dBZ). The dBZ values increase based on the strength of the return signal from targets in the atmosphere.
Typically, liquid precipitation-sized particle reflectivities are associated with values that are 15 dBZ or greater. Values less than 15 dBZ are typically associated with liquid cloud-sized particles. However, these lower values can also be associated with dust, pollen, insects, or other small particles in the atmosphere.
20.1.8.2 Intensity of Snow. A radar image cannot reliably be used to determine the intensity of snowfall. However, in general, snowfall rates generally increase with increasing reflectivity.
20.1.8.3 Bright Band. Bright band is a distinct feature observed by radar that denotes the freezing (melting) level. The term originates from a band of enhanced reflectivity that can result when a radar antenna scans through precipitation.
The freezing level in a cloud contains ice particles that are coated with liquid water. These particles reflect significantly more energy (appearing to the radar as large raindrops) than the portions of the cloud above and below the freezing layer.
21-1 CHAPTER 21. TROPICAL WEATHER
21.1 Circulation. Technically, the Tropics lie between latitudes 23½° N and 23½° S.
However, weather typical of this region sometimes extends as much as 45° from the Equator. One may think of the Tropics as uniformly rainy, warm, and humid. The facts are, however, that the Tropics contain both the wettest and driest regions of the world.
This chapter describes the basic circulation over the Tropics, terrain influences that determine arid and wet regions, and transitory systems that invade or disturb the basic tropical circulation.
Chapter 4 stated that wind blowing out of the subtropical high pressure belts toward the Equator form the northeast and southeast trade winds of the two hemispheres. These trade winds converge in the vicinity of the Equator where air rises. This convergence zone is the Intertropical Convergence Zone (ITCZ). In some areas of the world, seasonal temperature differences between land and water areas generate rather large circulation patterns that overpower the trade wind circulation; these areas are monsoon regions.
Tropical weather discussed here includes the subtropical high pressure belts, the trade wind belts, the ITCZ, and monsoon regions.
21.1.1 Subtropical High Pressure Belts. If the surface under the subtropical high pressure belts were all water of uniform temperature, the high pressure belts would be continuous highs around the globe. The belts would be areas of descending or subsiding air, and would be characterized by strong temperature inversions and very little precipitation. However, land surfaces at the latitudes of the high pressure belts are generally warmer throughout the year than are water surfaces. Thus, the high pressure belts are broken into semi-permanent high pressure areas over oceans with troughs or lows over continents, as shown in Figure 21-1 and Figure 21-2. The subtropical highs shift southward during the Northern Hemisphere winter and northward during summer. The seasonal shift, the height and strength of the inversion, and terrain features determine the weather in the subtropical high pressure belts.
21-2 Figure 21-1. Mean Worldwide Surface Pressure Distribution and Prevailing Winds Throughout the World in July
In the warm Northern Hemisphere, warm land areas tend to have low pressure, and cool oceanic areas tend to have high pressure. In the cool Southern Hemisphere, the pattern is reversed: cool land areas tend to have high pressure, and water surfaces have low pressure. However, the relationship is not so evident in the Southern Hemisphere because of relatively small amounts of land. The subtropical high pressure belts are clearly evident at about 30° latitude in both hemispheres. The red dashed line shows the ITCZ.
Figure 21-2. Mean Worldwide Surface Pressure Distribution and Prevailing Winds Throughout the World in January
In this season, the pattern from Figure 21-1 is reversed. In the cool Northern Hemisphere, cold continental areas are predominantly areas of high pressure, while warm oceans tend to be low pressure areas. In the warm Southern Hemisphere, land areas tend to have low pressure, and oceans have high pressure.
The subtropical high pressure belts are evident in both hemispheres. Note that the pressure belts shift southward in January and northward in July with the shift in the zone of maximum heating. The red dashed line shows the ITCZ.
21-3
21.1.1.1 Continental Weather. Along the west coasts of continents under a subtropical high, the air is stable. The inversion is strongest and lowest where the east side of the subtropical high pressure area overlies the west side of a continent. Moisture is trapped under the inversion; fog and low stratus occur frequently. However, precipitation is rare since the moist layer is shallow and the air is stable. Heavily populated areas also add contaminants to the air which, when trapped under the inversion, add to the visibility problem.
The extreme southwestern United States, for example, is dominated in summer by a subtropical high. We are all familiar with the semiarid summer climate of southern California. Rainfall is infrequent, but fog is common along the coast.
In winter, the subtropical high pressure belts shift southward. Consider southern California as an example. In winter, the area comes under the influence of mid-latitude circulation, which increases frequency of rain. Also, an occasional wintertime outbreak of polar air brings clear skies with excellent visibility.
The situation on eastern continental coasts is just the opposite. The inversion is weakest and highest where the west side of the subtropical high pressure area overlies the eastern coast of a continent. Convection can penetrate the inversion, and showers and thunderstorms often develop. Precipitation is generally sufficient to support considerable vegetation. For example, in the United States, Atlantic coastal areas at the same latitude as southern California are far from arid in summer.
Low ceiling and fog often prevent landing at a west coast destination, but a suitable alternate generally is available a few miles inland. Alternate selection may be more critical for an east coast destination because of widespread instability and associated hazards.
21.1.1.2 Weather over Open Sea. Under a subtropical high over the open sea, cloudiness is scant. The few clouds that do develop have tops from 3,000 to 6,000 feet, depending on height of the inversion. Ceiling and visibility are generally sufficient for visual flight rules (VFR) flight.
21.1.1.3 Island Weather. An island under a subtropical high receives very little rainfall because of the persistent temperature inversion. Surface heating over some larger islands causes light convective showers. Cloud tops are only slightly higher than over open water. Temperatures are mild, showing small seasonal and diurnal changes. A good example is the pleasant, balmy climate of Bermuda.
21.1.2 Trade Wind Belts. Figure 21-1 and Figure 21-2 show prevailing winds throughout the Tropics for July and January. Note that trade winds blowing out of the subtropical highs
21-4 over ocean areas are predominantly northeasterly in the Northern Hemisphere and southeasterly in the Southern Hemisphere. The inversion from the subtropical highs is carried into the trade winds and is known as the trade wind inversion.
As in a subtropical high, the inversion is strongest where the trades blow away from the west coast of a continent and weakest where they blow onto an eastern continental shore.
Daily variations from these prevailing directions are small, except during tropical storms.
As a result, weather at any specific location in a trade wind belt varies little from day to day.
21.1.2.1 Weather over Open Sea. In the trade wind belt, skies over open water are about one-half covered by clouds on the average. Tops range from 3,000 to 8,000 feet, depending on height of the inversion. Showers, although more common than under a subtropical high, are still light with comparatively little rainfall. Flying weather generally is quite good.
21.1.2.2 Continental Weather. Where trade winds blow offshore along the west coasts of continents, skies are generally clear and the area is quite arid.
The Baja Peninsula of Baja California is a well-known example. Where trade winds blow onshore on the east sides of continents, rainfall is generally abundant in showers and occasional thunderstorms. The east coast of Mexico is a good example. Rainfall may be carried a considerable distance inland where the winds are not blocked by a mountain barrier. Inland areas blocked by a mountain barrier are deserts; examples include the Sahara Desert and the arid regions of the southwestern United States. Afternoon convective currents are common over arid regions due to strong surface heating. Cumulus and cumulonimbus clouds can develop, but cloud bases are high, and rainfall is scant because of the low moisture content.
Flying weather along eastern coasts and mountains is subject to the usual hazards of showers and thunderstorms. Flying over arid regions is good most of the time, but can be turbulent in afternoon convective currents;
be especially aware of dust devils. Blowing sand or dust sometimes restricts visibility.
21.1.2.3 Island Weather. Mountainous islands have the most dramatic effect on trade wind weather. Since trade winds are consistently from approximately the same direction, they always strike the same side of the island; this side is the windward side. The opposite side is the leeward side. Winds blowing up the windward side produce copious and frequent rainfall, although cloud tops rarely exceed 10,000 feet. Thunderstorms are rare. Downslope winds on the leeward slopes dry the air leaving relatively clear skies and much less rainfall.
Many islands in the trade wind belt have lush vegetation and even rain forests on the windward side while the leeward is semiarid. For example, the island of Oahu, Hawaii, is about 24 miles wide in the direction of the trade winds.
Annual rainfall averages from about 60 inches on the windward coast to 200 inches at the mountaintops, decreasing to 10 inches on the leeward shore.
21-5 The greatest flying hazard near these islands is obscured mountaintops.
Ceiling and visibility occasionally restrict VFR flight on the windward side in showers. Instrument flight rules (IFR) weather is virtually nonexistent on leeward slopes.
Islands without mountains have little effect on cloudiness and rainfall.
Afternoon surface heating increases convective cloudiness slightly, but shower activity is light. However, any island in either the subtropical high pressure belt or trade wind belt enhances cumulus development even though tops do not reach great heights. Therefore, a cumulus top higher than the average tops of surrounding cumulus usually marks the approximate location of an island. If it becomes necessary for a pilot to ditch in the ocean, he or she should look for, and head toward, a tall cumulus. It probably marks a land surface, increasing chances of survival.
21.1.3 The Intertropical Convergence Zone (ITCZ). Converging winds in the ITCZ force air upward. The ITCZ appears as a band of clouds consisting of showers, with occasional thunderstorms, that encircles the globe near the Equator. The solid band of clouds may extend for many hundreds of miles and is sometimes broken into smaller line segments.
It exists because of the convergence of the trade winds. In the Northern Hemisphere, the trade winds move in a southwesterly direction, while in the Southern Hemisphere they move northwesterly. The tendency for convective storms in the tropics is to be short in their duration, usually on a small scale, but they can produce intense rainfall.
It is estimated that 40 percent of all tropical rainfall rates exceed 1 inch per hour. Greatest rainfall typically occurs during midday. On the Equator, this occurs twice a year in March and September, and consequently there are two wet and two dry seasons.
Figure 21-1 and Figure 21-2 show the ITCZ and its seasonal shift. The ITCZ is well-marked over tropical oceans, but is weak and ill-defined over large continental areas.
Convection in the ITCZ carries huge quantities of moisture to great heights. Showers and thunderstorms frequent the ITCZ, and tops to 40,000 feet or higher are common, as shown in Figure 21-1. Precipitation is copious. Since convection dominates the ITCZ, there is little difference in weather over islands and open sea under the ITCZ.
Flying through the ITCZ usually presents no great problem if one follows the usual practice of avoiding cumulonimbus clouds and any thunderstorms.
Since the ITCZ is ill-defined over continents, we will not attempt to describe ITCZ continental weather as such. Continental weather ranges from arid to rain forests and is more closely related to the monsoon than to the ITCZ.
21.1.4 Monsoon. As shown in Figure 21-1 and Figure 21-2, over the large land mass of Asia, the subtropical high pressure breaks down completely. Asia is covered by an intense high during the winter and a well-developed low during the summer. The same occurs over