By Irene Crisologo
In analyzing rainfall from thunderstorms or typhoons, we must know how much rain fell in different areas of the country. For this, we have what we call rain gauges, the most common is the tipping bucket type. For this type of rain gauge, a funnel collects the rain and directs it to a seesaw-like container which tips when it reaches a certain amount of precipitation, sending electrical signals to the receiver. The number of tips is converted to rain rate, and these data are then transmitted to a remote collection station every 10 minutes.
Tipping bucket rain gauge. (http://0.tqn.com/d/weather/1/0/w/0/-/-/Rain-gauge-animation.gif)
An ideal rain gauge network would be dense and spaced at least 20 kilometers apart. However, this is difficult to achieve due to financial and topography constraints. Currently, we have around 220 automated rain gauges (ARG) from the Department of Science and Technology- Advanced Science and Technology Institute (DOST-ASTI) that cover the whole country. Some areas have denser rain gauge networks than others, some areas have none.
The DOST-ASTI Philippine Rain Gage Network consists of about 200 automated rain gauges.
Since rain gauges are considered as point data, small scale rain such as convective currents or thunderstorms can be easily missed. For example, in the image below, the rain gauges say that there is no rain in the area, but the person in the middle would say otherwise.
And these isolated rain showers and thunderstorms are a common occurrence in our country. They have limited horizontal extent – they are not very widespread. It can be raining over San Juan but not Mandaluyong or Quezon City.
Isolated rainshowers and thunderstorms.
In the recent years, the PAGASA acquired several Doppler Weather Radars. These instruments send out electromagnetic signals which hit objects that are in the way. These objects reflect the electromagnetic signals, and the receiver for the radar listens for these reflections.
Electromagnetic signals that hit objects get reflected back to the radar transmitter. (http://www.srh.noaa.gov/jetstream/doppler/images/radarops.gif)
The most important objects that the radar signal detects are the clouds. With Doppler radars, not only do the signals tell the locations of precipitation but it can also tell the amount of rain the clouds bring. And compared to rain gauges, weather radars have a higher resolution. Each data point of the radar can be thought of as an individual rain gauge – it’s like having lots of rain gauges side-by-side in a dense network.
Radars can tell the location of rain clouds and how much rain it bring.
The extent of Philippine RADARs is 200 km from the station covering 360 degrees, with a spatial 1 km2. It’s like having a rain gauge every 1 km in all directions within the radar coverage.
With high resolution radar data, it’s like having a very dense and equally spaced rain gauge network.
Currently, we have six operational radars: Baguio, Subic, Tagaytay, Mactan, Hinatuan, and Tampakan.
The six operational Doppler Radars from PAGASA.
For a closer look, here are the coverage of each Doppler Radar:
Tagaytay Radar Coverage
Cebu Radar Coverage
Hinatuan Radar Coverage
Tampakan Radar Coverage
With the Radar Network, we can monitor weather systems even before it arrives in our area.
Doppler images of the small-scale cyclone that hit Metro Manila on the night of July 29, 2013.