1. Why Weather Radar Shows Radio Wave Echoes, Not Rain Itself
If you shout "Hey!" toward a mountain, an echo returns after a short time. The longer it takes for the echo to return, the farther away the mountain is. The concept of weather radar is the same. An antenna rotates while sending out radio waves (microwaves) to observe rain and snow within a radius of several hundred kilometers [1].
By measuring the time it takes for the radio waves to return, we can calculate the distance to the rain or snow. From the strength of the returned radio waves (radar echoes), we can determine the intensity of the rain or snow [1]. We are not seeing the rain clouds with our eyes, but reading the radio waves that bounced off the raindrops.
The JMA began operating radar in 1954 and currently has 20 radar sites across the country [1]. There are also radars operated by the Ministry of Land, Infrastructure, Transport and Tourism. Both types are used later to create analyzed rainfall data [5].
2. How a Radar on Mount Fuji Found a Typhoon 800 Kilometers Away
Mount Fuji, the highest mountain in Japan at 3,776 meters (about 12,388 feet), once had a weather radar on its summit. It was completed in 1964 and was, at the time, the highest weather radar in the world. Documents from the Japan Council of the Institute of Electrical and Electronics Engineers (IEEE) state that it could detect a typhoon 800 kilometers (about 500 miles) away [2].
Placing a radar in a high location makes it less likely to be blocked by mountains, allowing radio waves to be sent far. According to encyclopedia descriptions, the plan was initiated to detect approaching typhoons earlier, following the damage caused by the Ise Bay Typhoon in 1959 [3].
Later, weather satellites developed, and other radar sites could cover the area, so the Mount Fuji radar ended its service. After it was removed, it was preserved at the "Mount Fuji Radar Dome Museum" in Fujiyoshida City, Yamanashi Prefecture [2][3]. Sources differ on whether its operation ended in 1999 or 2000.
3. How the Doppler Effect and Dual Polarization Show Rain's Movement and Shape
The sound of an ambulance sounds high-pitched as it approaches and low-pitched as it moves away. This is the Doppler effect. The same thing happens with radio waves. From the shift in the frequency of the returned radio waves, we can understand the movement of rain or snow and the wind in the precipitation area [1].
Since March 2020, the introduction of dual-polarization weather Doppler radars has begun. These radars send and receive radio waves that vibrate horizontally and vertically at the same time. This allows for high-precision estimation of rain intensity. It also makes it easier to distinguish the shape and type of objects hit by the waves, such as rain, hail, and debris lifted by tornadoes [4].
4. Why Wind Turbines Can Appear as Fake Rain on Weather Radar
Radar can display things that are not rain if they reflect radio waves back. A leaflet from the JMA and the Ministry of Land, Infrastructure, Transport and Tourism states that when wind turbine blades scatter radio waves, a "false echo" (a precipitation area that does not actually exist) can appear, leading to incorrect precipitation information. It can also block radio waves, making distant areas invisible [5].
The initial idea that "we are reading radio wave reflections, not seeing rain clouds" appears again here. A radar screen is not a photograph of rain as it appears to the naked eye.
5. Comparing Rain Gauges, Which Are Accurate at One Point, With Radar, Which Is Broad but Rough
Rain gauges measure the collected rain directly, so they provide accurate rainfall amounts. However, they only observe the specific location where they are placed, leaving gaps in the overall coverage. Radar can estimate rainfall over a wide area without gaps, but its accuracy is lower compared to rain gauges [6].
Imagine a map with accurate points scattered around and a roughly painted picture covering the whole area. Neither one alone is sufficient. Combining both is the idea behind the next section.
6. How Analyzed Rainfall Combines Weather Radar and Rain Gauge Data
"Analyzed rainfall" is a distribution of rainfall analyzed at a resolution of 1 square kilometer (about 0.4 square miles). It combines weather radar with rain gauges from AMeDAS (Automated Meteorological Data Acquisition System) and local governments [7]. The wide observations from radar are corrected using the accurate numbers from rain gauges.
These maps are created every 30 minutes, but they show rainfall for a 1-hour period. Preliminary versions of analyzed rainfall are created every 10 minutes [7].
Analyzed rainfall is not used only for monitoring rain. It is also used to calculate soil rainfall indices, surface rainfall indices, and basin rainfall indices, which form the basis for heavy rain warnings. It is also used for short-term forecasts [7]. It is the preparation work for the rain information we usually see.
7. How the High-Resolution Precipitation Nowcast Predicts Rain Up to 30 Minutes Ahead
The "High-Resolution Precipitation Nowcast" tracks the distribution of rain inside clouds in three dimensions to predict rain in the near future. It updates every 5 minutes. For up to 30 minutes ahead, the resolution is 250 meters (about 820 feet) square. For 35 to 60 minutes ahead, the resolution is 1 kilometer (about 0.6 miles) square [8][9].
The 250-meter resolution applies to land and areas near the coast. For other sea areas, the resolution is 1 kilometer square [8].
8. How to Investigate Rain Clouds Yourself With the JMA's Rain Cloud Movement Site
On a rainy day, open the JMA's "Rain Cloud Movement" site (available in Japanese) and compare the color on the screen with the strength of the rain outside your window. If they feel different, think about the difference between rain gauges and radar.
The Mount Fuji radar is preserved at the "Mount Fuji Radar Dome Museum" in Fujiyoshida City, Yamanashi Prefecture. If you visit, look at the size and height of the part that sends out radio waves with your own eyes.
Read the explanation of analyzed rainfall on the JMA website [6] and write in your own words how the weaknesses of rain gauges are covered by radar, and vice versa.