Weather Radar and Automated Weather Observing System (AWOS) Information
Operation (How does the radar work?)
The current Bermuda Weather Service Doppler Weather Radar with dual polarization was purchased by the Bermuda Airport Authority and installed in April 2019. The radar is an S-Band Meteor 1700S with dual polarization, built by Leonardo Germany GmbH.
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 pulses of microwave radiation 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). Dual polarization adds a second frequency that the pulses are transmitted out on, enabling obstacles to be measured horizontally and vertically. This increases the accuracy of the data collected by the radar.
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 obstacle, or 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.
All of these measurements and computations allow the radar to measure size, concentration, state (frozen, liquid, or mix), and shape of the objects that reflect the beam back to the radar. Then, an accurate representation of the intensity, development, and movement of phenomena such as showers and thunderstorms can be created.
In order to ensure peak performance of the radar, planned preventative maintenance by qualified technicians is scheduled on a regular basis. These maintenance windows, when the radar will be temporarily shut down, will be scheduled in order to ensure as much as possible that they do not occur during periods of active weather.
There are many different products available from the Doppler Weather Radar with dual polarization. The three types of data available to generate these products 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 a reflector as it moves towards or away from the radar. Spectral Width data calculates the standard deviations of the velocity data, providing small-scale perturbations, giving an indication of turbulence. The following list details several of the products used by Meteorological Technicians and Meteorologists at the Bermuda Weather Service.
CAPPI Z is a constant altitude planned position indicator, returning reflectivity data. 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 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 user can determine the height of the base of (shower or thunderstorm) 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 user 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 from all the elevations. 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.
DPSRI is the surface rainfall intensity product. The radar uses a CAPPI reflectivity product, so that all data displayed is from the same altitude (constant height above orography). 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 millimetres 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 algorithm is able to determine the vertical profiles up a height of a few kilometres. This is because of the high sensitivity of the velocity channel of a radar, and the air pollution (e.g. dust, salt particles) all over the world.
Radar Image on Local Weather Channel
The radar image shown on our weather channel on television is a Surface Rainfall Intensity (DPSRI) product. This loop is updated every ten minutes.
AWOS is the Automated Weather Observing Systems. Along with the installation of the Doppler Weather Radar, the Bermuda Weather Service has and has use of several automated weather stations around the Island and Marine area. Some are maintained by other entities and some are maintained by the Airport Authority. Present and future stations include three offshore and four on Island. The current land-based stations are located in St. Georges and on the Causeway with future stations planned at Fort Prospect, and at Commissioner's Point. The offshore stations are located at Pearl Island and the Crescent Channel Marker with a future station planned at Chub Heads.
The public can view the data from a link on our website to the graphics page. There is the AWOS Composite image, which is a map of Bermuda, with each station overlaid on the map, with text data collected from the stations. This image is updated every ten minutes. Only a few of the stations currently have individual graphs of the data, which is also updated every ten minutes.