Satellite Weather Radar
Weather satellites collect a large range of environmental data. They monitor rain, snow, wind, city lights, pollution, fires, auroras, and more.
In addition, a few radar types are designed to examine the weather in high detail at short range. These kinds of radars have a major drawback in that echoes from features lying far from the radar are often underestimated by a process called “range folding” or “aliasing”.
Base Reflectivity
Base reflectivity is a useful tool for examining the amount of precipitation within a radar’s range. However, it does not show precipitation that is much more distant from the radar.
When the radar beam refracts less than the standard (the amount that would bend the beam under standard atmospheric conditions), it is called “subrefraction.” Subrefraction means that the radar pulses may overshoot areas of interest, which can increase the distance that the radar can examine precipitation.
Subrefraction may also cause the radar beam to intercept and become trapped on ground targets, which is commonly referred to as “ducting.” This can produce confusion on radar images by interfering with the echoes that the radar produces.
Some radars at low elevations detect what are commonly referred to as “sea clutter.” These echoes originate from sea spray and are more common during times of rough seas.
Another feature to watch for is a “cone of silence.” This cone of silence is the volume that is left unscanned above the highest inclination angle of the beam. During a thunderstorm, rotating updrafts can often produce this feature at lower levels of the troposphere.
This area of lowered radar reflectivity can be difficult to interpret, and there are several possible causes. The most likely reason is that the lowered values are associated with attenuation, which occurs when a heavy portion of precipitation passes overhead and reduces the radar’s energy available to scan precipitation.
The second possible explanation is that the lowered values are associated with sea clutter, which results from radar beams intersecting with waves and sea spray on lakes and oceans. This phenomenon is more common during times of rough seas and can be compounded by superrefraction, which happens when the radar beam refracts more than the standard.
Finally, the lowered values can be due to ground clutter, which is also quite common and may occur at certain sites situated at low elevations on coastlines. These echoes are often very weak and extend for long periods of time in a beamlike shape.
In addition, dust storms can also occur in arid regions and can accompany thunderstorm outflow boundaries or strong synoptic frontal passages. The amount of dust that is transported is dependent on a number of factors, including temperature and moisture gradients.
Short Range
Satellite weather radars can be operated in a number of different modes depending on the situation. For example, a WSR-88D is usually operated in Precipitation Mode when precipitation is present, although it can also be operated in Normal Mode for non-precipitation echoes.
These modes can change the appearance of the images displayed on the radar screen, which helps to make them more informative. They also allow the radar to rotate faster, which can help to update radar images more frequently.
Another useful feature of these types of radars is that they can produce a wind profile, displaying snapshots of the horizontal winds blowing at various altitudes above the radar. This is especially useful when assessing thunderstorms and the strength of a storm’s horizontal winds.
The VAD Wind Profile (VWP) product presents snapshots of the horizontal winds blowing at different altitudes above the radar, spaced 6 to 10 minutes apart in time. This information can be used to estimate the wind speed and direction at different upper levels of the atmosphere, which is helpful in determining whether or not a thunderstorm has sufficient wind shear to cause downdrafts or microbursts.
In addition to wind shear, the VAD Wind Profile image also shows a vertical cross-section of precipitation. This is useful in determining the tops of stratiform precipitation, as well as the tops of low-topped convection.
This feature is available for both the 0.5o Base Reflectivity and Composite Reflectivity products, as shown in Figure 4-2. For the 0.5o Base Reflectivity product, the maximum range of the image is 124 NM from the radar location. The image will not display echoes that are more than 124 NM from the radar location, even though precipitation may be occurring at these greater distances.
Similarly, the Composite Reflectivity image will not show echoes that are more than 143 NM from the radar location, even though the rain may be occurring at these greater distances. This is because the tops of the precipitation that are visible on the radar are generally at least 18,000 feet above the radar.
These features are all impacted by a supplemental issue called ground echoes. These are echoes of the ground that are not actually from precipitation but instead reflect off of the radar’s antenna or other components. The ground echoes can be confused with real echoes of precipitation and are therefore not a good basis for interpreting radar data. These echoes can impact radar data products including CAPPIs and PPIs, as well as the Doppler velocity of precipitation areas.
Composite Reflectivity
Weather radars are used for the detection of precipitation and other weather conditions. There are many types of radars, from the National Weather Service’s WSR-88D pulse radars to less common but much more sophisticated phased array radars. In general, radars produce a series of beams that can be scanned in different directions and at different angles. Because of this, the images they produce inherently contain areas of varying resolution, which is a good thing when it comes to weather observations.
The most common type of radar used in the United States is the NEXRAD system, which uses pulse radars to measure weather. These are generally used by the National Weather Service to monitor thunderstorms, but can also be used to detect snow and wind.
While most rain goes undetected by a radar, fog and heavy cloud cover can make a storm appear on the radar. This is due to the fact that fog and clouds tend to be low, layered, and have small droplets.
These factors cause the radar’s beams to bend towards the ground, forming the image that you see. This means that the angle of the radar’s beam is very important to determine the location of a storm.
This can be a problem when the radar is far from the storm’s center or when it’s scanning a relatively narrow angle beam. As a result, some of the features that are important to understanding the storm (such as its rotation) can be overlooked.
Similarly, some of the storm’s heavy precipitation may evaporate before it reaches the ground. The resulting decrease in reflectivity can make it difficult to see the heaviest part of the storm.
Composite reflectivity is a common display on satellite weather radars that shows the maximum dBZ (strongest reflected energy) in a given vertical column for all columns in the radar’s range, giving a more accurate view of the most intense portion of a storm. It can be useful in noting the areal extent of heavy precipitation at middle and upper levels, but it’s important to keep in mind that some of this dBZ is lost as the radar’s beam moves away from the storm.
Wind Barbs
Wind barbs are a familiar format for meteorologists and other people looking at weather maps. They display both the wind direction and speed at a station and are easy to interpret.
The first step in interpreting wind barbs is to understand the convention used to denote speed. The barbs at the outer end of the direction line indicate wind speeds in knots (kt).
Each long bar represents 10 kt, each half barb (half-lines) represent 5 kt, and each flag indicates increments of 50 kt. The number and size of the barbs and flags are defined on a key provided by the radar operator.
This is a familiar format for all meteorologists, yachtmen and others who are familiar with synoptic maps. The key also describes how to read the symbols.
For example, the barbs on this station plot indicate winds blowing from the northeast at 25 knots. This is the same as a barbed line on the bottom right quadrant of the plot.
A station plot also shows cloud cover by shading the center dot of the plot. A hollow circle is a sign of clear skies, while a solid circle indicates overcast conditions that could be dangerous for a backpacking trip. The amount of shading inside the dot conveys the fractional cloud coverage, from 25 percent filled to 75 percent filled.
These plots also report temperature in degrees Fahrenheit, as well as current weather reported at the radar location. Each cloud symbol has an H, M or L that identifies the level (high, middle or low) it lives in the atmosphere.
The symbols are often color coded, as well. For instance, a blue H over a high pressure system indicates a strong, steady wind, while a red L over a low-pressure system suggests that the winds are blowing from an area of lower air pressure.
Because wind strength varies greatly across a range of locations, it’s important to know how to decipher the wind speed reported on wind barbs. This information can be crucial for evaluating boating safety. In addition, it can help identify potential hazards on land, such as fires and wind surge. It’s also a valuable tool for mariners when it comes to navigation.