Intercomparison between IMD ground radar and TRMM PR observations using alignment methodology and artificial neural network

Echolocation refers to locating the size and distance of objects in the surroundings using echo. Thousands of species use echolocation to navigate the world. Drawing inspiration from this nature-driven technology, humans have designed and built radar systems to detect and track objects remotely.

During the Second World War (when applied sciences flourished), radar technology was used to detect and target enemy's warplanes. A series of false-alarm whenever it rained offered an opportunity for developing radar to observe weather systems.

There has been no turning back since then. Radar is considered an irreplaceable modern technology that can provide accurate rainfall measurement over a large area. Meteorological organizations from around the world soon integrated weather radar into their observation systems. The polar plot (aka radar chart) helped us detect and track organized precipitation systems and provided more insight into the microphysics of rainfall.

Today, more than 4000 radar stations are built worldwide that continuously monitor the weather on a large scale. When ground stations are not sufficient due to their restricted mobility, radars are also carried on specially designed aircraft for conducting field observations such as the Indian summer monsoon.

In 1997, NASA of the U.S and JAXA of Japan jointly launched the Tropical Rainfall Measuring Mission (TRMM) satellite. The satellite was the first to carry a radar onboard to monitor precipitation systems on a global scale within the tropics. This allowed researchers worldwide to tune in to their surface observations whenever the satellite made an overpass.

However, cross-comparison of radar observations from satellite and ground-based poses several challenges. Even though technically, both the instruments work on the same principle, there could be a mismatch between them due to the difference in their viewing geometry, radar frequency, and other issues such as clutter.

Concept diagram to illustrate intercomparison of ground and space radar

In a recent paper co-authored by my research scholar [1], we had compared the ground radar observations maintained by the Indian Meteorological Department (IMD) with TRMM's Precipitation Radar using alignment methodology. The comparison study showed that the ground radar overestimates rainfall during the Indian summer monsoon period of 2013. We demonstrated that the positive bias of the ground radar measurement could be "corrected" to match with TRMM PR observations using an artificial neural network.

[1] Alok Sharma and Srinivasa Ramanujam Kannan, 2021, Intercomparison between IMD ground radar and TRMM PR observations using alignment methodology and artificial neural network, Journal of Earth System Science, Vol. 130, Article ID 0020.

Effect of humidity on the performance of rooftop solar chimney

When the air gets heated up, it expands and hence, becomes lighter. The less-dense warm air rises up against the gravity, allowing cold, heavy air to sink. This is the fundamental mechanism by which heat gets redistributed with the fluid medium following the natural convection mode.

Natural convection, well, happens naturally. It means no external source of power is used to push the air around. Take a flat black plate on a rooftop and expose it to sunlight. You will notice that after a while, the plate gets hotter. Natural convection heat transfer will then take place between the hot plate and the surrounding air. If we expose the surface to solar radiation for a longer duration, its temperature will further increase. But air can only gain a certain amount of heat. So, in long exposures, the plate loses heat by the radiation mode of heat transfer as well.

Concept diagram of a solar chimney

In solar thermal applications, heat transfer by natural convection is almost always coupled with the radiation mode. Take a solar chimney, which is a device that consists of an absorber plate (to absorb sunlight) and a glass cover. The daylight first passes through the glass cover before it strikes the absorber plate. The glass cover is added to trap the long-wavelength radiation emitted by the absorber plate as it gets heated up. The chimney, with its bottom end open, is usually mounted on the rooftop of a room. 

The air present in the space between the absorber plate and glass cover will get lighter and rise upward. This creates a vacuum inside the room. Fresh cool air through windows or open ventilation inside the room will move towards the chimney, inducing air circulation.

While the air inside the chimney predominantly picks up the heat by convective mode of heat transfer, the fundamental mechanism changes when air carries some moisture with it. The thermal properties of water vapour are different from dry air and can potentially affect the natural convection inside the chimney. Also, water vapour can absorb long-wavelength emitted by the absorber plate.

Graduate students under my guidance had investigated the effect of humidity (amount of water vapour present in air) on the performance of a rooftop solar chimney. Our study [1] shows that water vapour present in the air can improve the overall performance of the solar chimney (measured in terms of air change per hour inside the room) by 10%.

[1] Himanshu Dahire, Srinivasa Ramanujam Kannan, and Sunil Kumar Saw, 2021, Effect of humidity on the performance of rooftop solar chimney, Thermal Science and Engineering Progress, (available online).