Monday, March 28, 2011

How to forecast weather from Radar Images.

Akshay Deoras
Severe Weather Forecaster

Radar Images-

Radar is an acronym for Radio Detection and Ranging.
Radar works on Doppler Shift principle or Doppler Effect and so the weather radars are named as Doppler Radar.
Doppler Shift -( In simplified words)
When Observer and Source are in relative motion,there is a change in the frequency
( and likewise wavelength ) of a wave when detected by an observer when the source
of wave is moving with respect to the observer.

That is on a road, if an approaching car is blowing horn, the sound waves reaching the observer have higher frequency and smaller wavelength and so we hear a loud sound and the sound dims as the car passes by ( SEE IMAGE. APPROACHING CAR'S SOUND WAVE HAS HIGHER FREQUENCY AND LOWER WAVELENGTH)

The effect caused by approaching source is the Blue shift while that caused by sources moving away from observers is Red Shift and is widely used in ASTRONOMY

How Doppler Weather Radar (DWR) works?
Doppler Weather Radar works using the Doppler Effect by beaming a microwave signal from the Radar dish towards the clouds from the microwave transmitter.
The reflected beam is received by the receiver. The change in the frequencies of transmitted and received signal is calculated

Frequency is directly proportional to the velocity
Thus an increase in the frequency indicates velocity of the target( say clouds is increasing) while a decrease in frequency indicate decreasing velocity.

Thus the radars can detect Storm Motion Vectors

Radar in Mumbai and Nagpur
The recently installed RADAR in Mumbai and Nagpur are S band Radar
They have a wavelength of 8-15cm and frequency of 2-4GHz
Since the wavelength of this band is less, this wave is not easily attenuated and hence the RADAR have a range upto 500km!

The Indian Meteorological Dept has avail following datas from the radar product online

Foll are the included products
1] Maximum Reflectivity ( Z)
2] Plan Position Indicator
3] Volume Velocity Processing
4] Plan Position Indicator
5] Surface Rainfall Intensity
6] Precipitation Accumulation

1] Maximum Reflectivity-
This product determines the strength or intensity of the reflected beam ( Echo)
The stronger the reflectivity, the more intense cloud is.

By Analyzing the color codes of the Reflectivity image, we can determine the intensity of thunderstorm, expected precipitation from it, position of Hails etc.

Following color codes are used-

magenta: 65 dBZ (extremely heavy precipitation, possible hail)
red: 52 dBZ
yellow: 36 dBZ
green: 20 dBZ (light precipitation)
Above img-
The above image is a screen shot of Radar reflectivity around Atlanta,Georgia of 27th March 2011
The Intense white core represents stronger reflectivity of 74 dBz and the thunderstorm dropped hail of diameter 1.85" in the region
Once can determine the precipitation from the posted color code

For Weather Forecasters in India
Weather forecasters including me in India use the Doppler Image from IMD radars.
The above is the Radar image from Agartala,India
The main map shows concentric circle which is the field of radar. The blue, orange shades are the radar reflectivities from the region
Shown at the top of image is the Elevation plot or the structure of Thunderstorm from the ground.
Once can see the Radar view of Storm from 0-14km due to the radar. On the extreme right is the Reflectivity scale and left of it is the zoomed max reflectivity regions

2] Plan Position Indicator PPI
The plan position indicator (Z) has a range of 500km and determines a complete view of the area.
Zoomed reflectivity as seen in Max reflectivity plot is absent here.

2] Plan Position Indicator is a large scale map showing a field view of radius 500km
At Nagpur as a center
North extent- Sagar,MP
SE of Kota,Rajasthan
Eastern Extent- East of Bilaspur
Southern Extent- Somewhere around Hanamkonda,AP
Western Extent - West of Jalgaon

3] Volume Velocity processing
A very important component of the Radar product.
This plot shows the wind direction and magnitude upto 8kms from the surface in vertical direction i.e Vertically above or Vertical Wind profiles

It has a maximum capacity to detect winds upto 30km!

4] Surface Rainfall Intensity-
This plot determine the Rainfall rate.
Note- The Max Z reflectivity shows how much precipitation can occur from the system while the Surface Intensity Rainfall shows how much rain is falling ( Also available for other forms of precipitation)

5] Precipitation Accumulation
It shows the accumulated precipitation in different region

Additional Helps-

To see Tornadic supercells and hook echos from Yazoo City,MS US in April 2010 when a tornado outbreak had happened.
See here

2] IMD Radar Frequently Asked Questions

For general Radar facts.

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