Table of Contents
The Bermuda Weather Service Doppler Weather Radar was purchased by the Department of Airport Operations for the L.F.Wade International Airport. The radar is an S-Band Meteor 1500S, built by AMS Gematronik.
The radar antenna looks like a very large satellite dish, enclosed by a radome. The radome is the large white dome that protects the antenna from high winds and debris.
A radar works by transmitting an energy wave out from the antenna at a specified frequency. If the radar beam encounters an obstacle after leaving the antenna, the signal will be reflected back to the antenna. The return signal is often called an echo. Depending on the return signal strength, the radar can then determine the reflectivity value of the obstacle (or reflector).
The meteorologists at the Bermuda Weather Service have programmed the radar to make several different scans at different elevation angles. Knowing the elevation angle, and the time it takes for signal to return to the antenna, the radar computer is then able to determine how far away and how high above sea level the reflector is.
By using the Doppler Theory, the radar is able to detect the Doppler shift of a reflector. Doppler shift is the term for determining the change in frequency of the radar beam, due to the movement of the reflector. By detecting the Doppler shift, the radar is able to calculate the velocity of the reflector, as long as it is moving towards or away from the radar beam. By applying further computations, it is possible to display wind shear and turbulence.
With the installation of the state-of-the-art radar, a lot of weather data and information is now available to the public. However, with the new technology, there is much to learn and understand. There are now many new products available, however there are some phenomena that must be mentioned and understood, to avoid misinterpreting the radar images.
Ground clutter occurs when the radar beam encounters fixed obstacles on the Earth's surface, as it extends out from the antenna (see Figure 1). These obstacles can be buildings, trees, hills, etc. The unique location of Bermuda also lends itself to Sea Clutter. Waves and salt spray, especially along the reefs, can cause clutter. You will notice this especially when conditions are windy, as the sea clutter becomes slightly more pronounced. The Clutter is generally relatively close to the radar, as the further out the radar beam goes, the higher it's elevation is. Figure 2 shows an example of local clutter.
Figure 1. Courtesy of Environment Canada's Weatheroffice website.
Figure 2. Sea Clutter is located near the radar, off the South Shore. This image was created during strong southerly winds.
Anomalous Propagation (AP) occurs when a temperature inversion (temperature rising with height) occurs (see Figure 3). The radar beam can be bent by the inversion, causing it to hit the Earth's surface. This will return a strong reflectivity value that is not true. Temperature inversions generally occur on clear nights, and generally dissipate by mid morning. The best way to determine of you have AP is to compare what you are looking at on the radar with satellite imagery.
Velocity Folding (see Figure 4) occurs when measured velocities exceed the scale of values. Velocity data is displayed in meters/second, and is negative when the velocities are approaching the radar, and positive when moving away from the radar. To compensate for exceeding the highest values of the scale, the scale will "fold over" to the opposite scale. For example, if inbound velocities exceeded the scaled limit, they would change over to the outbound velocity colours.
In pulsed radar, a second-trip echo is an echo from a given pulse that is not received until after the transmission of the next pulse. If a second pulse has been sent out before the first one returns, the first pulse may return with the radar thinking it is the second pulse. This results in the false second pulse returning sooner than it should, placing the reflector closer to the radar than it actually is. These usually return as slightly weaker echoes. A good way of determining if you are looking at a second-trip echo is to compare the radar to the satellite. The area within the orange lines in Figure 5 show the second-trip echoes, resulting from the actual echoes which are within the red lines.
Figure 5. Second-Trip Echoes
You may notice that the time on the radar images and the AWOS images on the radar and automated weather systems network website are different from the time on your watch. The time used on our radar is Universal Coordinated Time (UTC). During daylight savings time, the conversion to Local Time is minus 3 hours, during the rest of the year it is minus 4 hours.
With the installation of the new Doppler Weather Radar, there are many different products available. The three main types of data available are Reflectivity Data (Z), Velocity Data (V), and Spectral Width Data (W). Reflectivity data is the value returned from the radar beam, indicating the strength of reflection in decibels of reflectivity factors (dBZ). The radar computer uses the measured Doppler Shift to compute the velocity of the reflector as it move towards or away from the radar providingvelocity data. Spectral Width data calculates the standard deviations of the velocity data, providing small-scale perturbations, giving an indication of turbulence.
CAPPI Z is a constant altitude planned position indicator, returning reflectivity data. With the new radar, several scans are made at different angles of elevation, allowing the user to obtain what is called a volume scan. From the data collected, it is then possible to extract data at a specified altitude. Thus, all of the data on this display is at a constant altitude, which is displayed in the legend of the image.
EHT comes from a volume scan, where several scans at different elevation angles are taken, and from this the Echo Heights are returned. The operator is able to determine the height of the base of cells, as well as the tops of the cells. The base of the cells gives the user an idea of how low the clouds are, while the tops give the meteorologist an idea of the vertical extent of the clouds, and if thunderstorms are expected, how strong these may be.
MAX Z is a maximum display of reflectivity data. After the radar has made a series of scans at different elevations, known as a volume scan, the maximum reflectivity values are returned to create this product. Thus, it shows us, the maximum reflectivity value for each pixel coordinate. To determine what height these are values are from, the user can compare the cross sections included in the image. The cross section at the top of the image is taken is on a west to east axis, and the cross section on the right is on a north to south axis.
PPI V is planned position indicator velocity data. Using the Doppler Theory, the radar uses a Doppler Shift to determine the velocity of a reflector. In this product, a single elevation angle is scanned, and the velocity of the reflectors is returned. Negative values indicate reflectors moving towards the radar, while positive values indicate reflectors moving away from the radar. It should be noted that the further the radar beam extends from the radar, the higher it's elevation, thus it should report higher velocity values as winds are generally stronger at higher altitudes.
PPI V HWIND is a PPI V image with an HWIND overlay. HWIND is a horizontal wind field. One of the nice features of this radar is that it is able to apply different algorithms to the data collected. In this case, the radar takes the velocity data from a horizontal slice of the atmosphere, and displays wind speed and direction in the form of wind barbs. Although the PPI V data is in meters per second, the wind barbs will give you the speed in knots.
SRI is the surface rainfall intensity product. The radar uses a CAPPI reflectivity product, so that all data displayed is from the same altitude. It then applies an algorithm to the data to estimate the rate of rainfall occurring at the surface at the time of the image. The data displayed is the rainfall rate in millimeters per hour.
VVP is the volume velocity processing product. This product comes from a volume scan of velocity data. VVP is a wind barb display of the horizontal wind velocity and direction in a vertical column above the radar site. In this product, north is oriented directly to the top of the image.
The radar image shown on our weather channel on television is a Surface Rainfall Intensity (SRI) product. This image is updated every ten minutes. We are planning on upgrading the still image to a loop, however we need to upgrade our communication link first.
AWOS is the Automated Weather Observing Systems. Along with the installation of the Doppler Weather Radar, the Bermuda Weather Service had six automated weather stations installed by Ground Electronics Services. Three of the stations are located on the Island, and three are located offshore out on the reefs. The land-based stations are located in St. David's, Fort Prospect, and at Commissioner's Point. The offshore stations are located at Chub Heads, Eastern Blue Cut, and North East Breakers.
We have two ways available for the public to view the data collected at the sites. Each AWOS location has a page that displays a graph of its data, which is updated every ten minutes. There is also the AWOS Composite image, which is a map of Bermuda, with each station overlayed on the map, with text data collected from the stations. This image is also updated every ten minutes.
The process of installing and configuring our six AWOSs is still ongoing. Table 1 below shows the status of each station.
Each site has slightly different instrumentation. (See Table 2 below).