SATELLITES AND WEATHER

North India, plagued by heavy fog since December 2023, witnessed heightened warnings on January 16th, 2024. The India Meteorological Department (IMD) issued alerts for "very dense fog" in Haryana, Chandigarh, and Delhi, with visibility dropping below 50 meters at night and morning. Similar warnings were issued for Uttar Pradesh, Punjab, and Madhya Pradesh, where visibility could range between 50-200 meters. To enhance these alerts, the IMD has leveraged satellite imagery from INSAT 3D and 3DR. These maps clearly depict the fog cover, with annotations highlighting the affected areas.

2. Understanding INSAT 3D Satellite Maps

• To interpret the maps, understanding the color representation is crucial. The clue provided at the bottom right of a 2021 map (Map 1) is 'Night Microphysics.'
• According to a February 2019 paper by IMD scientists, the INSAT 3D satellite utilizes a red-green-blue (RGB) imager, where colors in the images are determined by two factors: solar reflectance and brightness temperature.
• Solar reflectance is the ratio of solar energy reflected by a surface to the incident solar energy. Brightness temperature, on the other hand, involves the relationship between an object's temperature and the brightness of its surface. It differs from our usual understanding of temperature, as it considers how the object emits thermal radiation at different frequencies in various directions.
• The 'day microphysics' data component of INSAT 3D examines solar reflectance at three wavelengths: 0.5 micrometers (visible radiation), 1.6 micrometers (shortwave infrared radiation), and 10.8 micrometers (thermal infrared radiation).
• Detectors onboard the satellite track radiation in these wavelengths from over India.
• The strength of the 0.5 micrometer visible signal determines the amount of green color, the strength of the 1.6 micrometer shortwave infrared signal dictates the amount of red color, and the strength of the 10.8 micrometer thermal infrared signal influences the amount of blue color.
• Through this process, the INSAT 3D computer assigns colors to each point on the image.

3. How Satellites Track Snow with Color Clues?

• The satellite tracks snow by exploiting the distinctive characteristics of its interaction with solar radiation.
• In the paper, it is noted that the color scheme is particularly useful for various applications, including the analysis of different cloud types, initial stages of convection, maturing stages of a thunderstorm, identification of snow areas, and the detection of fires.
• In the visible part of the spectrum, the solar reflectance of both snow and clouds is similar.
• However, snow significantly absorbs radiation at a wavelength of 1.6 micrometers, which corresponds to the shortwave infrared.
• Consequently, when the satellite is monitoring snow, the red component of the color scheme becomes notably weak.
• This phenomenon allows the satellite to distinguish and identify areas covered by snow through the analysis of the weakened red signal in the imagery.

4. INSAT 3D's Night Microphysics Colors

• The determination of colors in the satellite's 'night microphysics' component involves a more complex process.
• In this context, two colors are not determined by a single signal but by evaluating the strength of the difference between two signals.
• The computer calculates the amount of red color based on the disparity between two thermal infrared signals 12 micrometers and 10 micrometers.
• Similarly, the amount of green color varies according to the difference between a thermal infrared signal and a middle infrared signal 10.8 micrometers and 3.9 micrometers, respectively.
• In contrast, the amount of blue color is not derived from a difference but is determined by the strength of a thermal infrared signal with a wavelength of 10.8 micrometers.
• For instance, in Map 3, the data reveals three types of clouds, denoted by temperature differences in kelvin.
• A mature cumulonimbus ("CB") cell, possibly part of a tropical storm, appears over West Bengal, predominantly in red, with its blue component indicating a very cold temperature. North of Delhi, green flecks dominate, suggesting a prevalence of lower clouds.
• Further north, a high and substantial cloud system encompasses both lower and upper clouds.
• By combining day and night microphysics data, atmospheric scientists can discern the presence of moisture droplets with different shapes and temperature differences over time.
• This approach enables tracking the formation, evolution, and dissipation of cyclones and other weather events.
• As an example of the satellite's capabilities, leveraging the ability of INSAT 3D to generate images based on signals of multiple wavelengths, the authors of the 2019 paper propose day and night microphysics data that they believe can indicate an impending thunderstorm within one to three hours.

• Satellites such as INSAT 3D and INSAT 3DR collect weather data using radiometers and atmospheric sounders.
• Radiometers are devices designed to measure different properties of radiation by leveraging its interaction with matter, often in the form of temperature or electrical activity.
• Additionally, both satellites are equipped with atmospheric sounders. These instruments are designed to measure temperature and humidity while studying water vapor at different heights above the ground.
• By combining the measurements from radiometers and sounders, scientists gain insights into various atmospheric characteristics.
• This integrated approach allows for a comprehensive understanding of the complex dynamics and conditions within the Earth's atmosphere.

• India currently operates several weather satellites, including Kalpana 1, INSAT 3A, INSAT 3D, and INSAT 3DR. The radiometer on INSAT 3DR is an upgraded version of the Very High-Resolution Radiometer (VHRR) used by Kalpana 1 and INSAT 3A, launched in 2002 and 2003, respectively.
• The Space Application Centre's information on INSAT 3A highlights its three-channel VHRR with 2 km resolution in the visible band and 8 km resolution in the thermal infrared and water vapor bands for meteorological observation. The radiometers on INSAT 3D and INSAT 3DR feature significant improvements in spatial resolution, spectral channels, and functionality compared to their predecessors.
• Over the years, India has continually enhanced its weather monitoring capabilities with each new satellite being an improved version of the previous one. For instance, INSAT 3DR, launched in 2016, had a launch mass of 2,211 kg and carried an upgraded VHRR, a sounder, a data-relay transponder, and a search-and-rescue transponder.
• While Kalpana 1 served India's weather monitoring and warning services for 15 years before being deactivated in September 2017, INSAT 3D and INSAT 3DR are currently active in geostationary orbits at 82 degrees and 74 degrees east longitudes, respectively.
• In February 2024, the Indian Space Research Organisation plans to launch the INSAT 3DS meteorological satellite on its GSLV Mk II launch vehicle. The "3DS" designation stands for "3D second repeat," indicating a continued commitment to advancing and repeating the success of the 3D series.