A Journey Through the Clouds to Improve Indian Monsoon Forecast

Indian scientist Sandeep Pattnaik and colleagues from the United Kingdom literally took “science to the skies” in the summer of 2016.

Pattnaik, a scientist at IIT-Bhubaneswar, flew over the Arabian Sea aboard an airborne research laboratory, a Facility for Airborne Atmospheric Measurements (FAAM) aircraft – a highly modified BAe-146 four-engined jet that can pack in four tonnes of scientific equipment.

His sortie over the Arabian Sea through the Western Ghats was conducted under the Indo-UK collaborative INCOMPASS project that aims to advance monsoon forecasting capability between the two sovereign states.

INCOMPASS stands for Interaction of Convective Organisation and Monsoon Precipitation, Atmosphere, Surface and Sea and it was the first field campaign to use a foreign-registered aircraft to take measurements over land surfaces in India and the adjacent oceans, scientists said.

From flying in the lower atmosphere to skimming the top of the clouds, the project stacked up 100 hours of sorties over the Indian region, throwing up interesting insights on the land surface, boundary layer, cloud microphysics, and its convective environment, that are now being used to check the accuracy of the weather forecasting models.

Among those sorties, Pattnaik had a chance to fly over the Arabian Sea through the Western Ghats and sample data sets over the coastal and mountainous regions. Pattnaik who works on cloud microphysics and extreme weather events, described his experience onboard the aircraft as a “once-in-a-lifetime” learning experience.

“It is a state-of-art onboard airborne research facility and you be sitting in a spot with a monitor in front of you that consolidates all sorts of data from multiple observing instruments in real-time,” Pattnaik recalled.

He said: “While flying one can come across evolution and growth of different cloud types. Rainfall and key parameters were sampled on its path. Onboard, continuous discussions happen among pilots, air traffic control, instrument engineers and scientists seamlessly on the evolving real-time scenarios to maintain perfect co-ordination.”

Leveraging the data gathered under the project, Pattnaik and colleagues, in a recent study, have spotlighted how accurately cloud microphysics (branch of the atmospheric sciences concerned with particles that make up a cloud) and cloud environment are represented in the Weather Research and Forecasting (WRF) model and worked to better understand the mismatches between what the model forecasts and what actually happens (bias) in predicting key mechanisms shaping rainfall forecast.

The findings, the study said, have direct implications on improving the forecast skills of the model, with special thrust on enhancing the prediction of moderate to heavy rainfall events that have substantially increased in terms of frequency, duration and intensity over the Indian region in a climate change scenario.

Climate scientist Roxy Mathew Koll at the Indian Institute of Tropical Meteorology, Pune, who was not associated with the study, said the accurate simulation of clouds is one of the biggest challenges in forecasting weather and rainfall.

“Models fail to represent the microphysical properties of clouds due to two reasons— our lack of understanding of how clouds behave at finer scales— and also because the model resolution is too coarse to simulate the finer details of the clouds. Most of the weather models hence use a simplified form of the complex cloud process. This is known as cloud parameterization (since the cloud process is defined by simplified equations consisting of the key physical parameters),” Koll told Mongabay-India.

This can sometimes result in inaccurate simulations of clouds and atmospheric processes, resulting in a wrong forecast, Koll pointed out.

An improved forecast is essential because the Indian summer monsoon supplies the majority of water for agriculture and industry in South Asia and is therefore critical to the well-being of a billion people. Active and break periods in the monsoon have a major influence on the success of farming, while year-to-year variations in the rainfall have economic consequences on an international scale.

Filling the gaps in knowledge of Indian monsoon

With the growing population and developing economy of India, understanding and predicting the monsoon is vital. The Indian monsoon also impacts European weather through atmospheric waves, according to INCOMPASS researchers.

But there are still aspects of the Indian monsoon that are not understood even after years of research, said the project’s principal investigator Andrew Turner.

Aspects such as the transition from active to break periods in the monsoon and its influence over European weather still need clarity, added Indian Institute of Science, Bengaluru’s G.S. Bhat, the joint principal investigator of the project.

“This imperfect understanding is carried through to the models that we use for making weather forecasts and climate projections for the monsoon, and many other aspects of world weather and climate,” Turner at the University of Reading, explained.

“While there is some ‘skill’ in making forecasts for the monsoon, the models are dogged by biases that have persisted through many years of model development at institutes around the world, including in the U.K. and India,” Turner told Mongabay-India in email.

This is where the Indo-UK collaborative comes in by gathering new observations over India and combining them with computer modeling in “unprecedented detail” to improve rainfall forecasts.

“By making new observations of the land surface and of the atmosphere above it and how that atmosphere responds to changes in the land surface, we hope to better understand the physics involved in those processes, and therefore work towards the creation of better algorithms in the computer models,” Turner said.

The University of Reading, England and Indian Institute of Science, Bengaluru were the lead partners with joint principal investigators Andy Turner and professor G. S. Bhat. They led a team of more than 10 universities, research institutes and operational forecasting centres to observe the monsoon in India from the air and on the ground. IIT-Bhubaneswar was one of several other partners. The project was funded by the U.K.’s Natural Environment Research Council and India’s Ministry of Earth Science under their “Monsoon Mission” programme.

The aircraft campaign was carried out in May-July 2016, along with the installation of eight ground observation stations such as flux towers, including one at IIT-Bhubaneswar that was facilitated by Pattnaik.

As a flying laboratory, the FAAM aircraft can be equipped with scientific equipment such as Aircraft Integrated Meteorological Measurement System (AIMMS-20), radar altimeter, backscatter lidar among others, to help investigate the atmosphere.

“Rainfall is an outcome of many complex processes. Both cloud physics and dynamics are important in this. INCOMPASS comprised of ground based observations to understand land surface processes, along with aircraft, radar, satellite and disdrometer measurements to understand cloud physics and dynamics,” noted Bhat.

“Since then [2016] to the present, we have been working on analysing the data and publishing our findings, and running model experiments to explore the monsoon in more detail,” said Turner.

“Since clouds cover 70 percent of the Earth’s surface at any given moment, it is quite crucial that we simulate them precisely. This is where field observations like the INCOMPASS can help— they provide important data that provide insights into the behavior of monsoon clouds at finer scales,” added Koll.

For example, the study discussed points out that particular microphysical processes are dominant in the atmospheric column over India during the monsoon. Koll hopes this information can be utilised for correcting the models for better monsoon forecasts.

Airborne research lab shows hows atmosphere behaves as the monsoon develops

The aircraft-based observations are important since they give a unique insight into the behaviour of the atmosphere and how it responds to changes on the surface as the monsoon develops. “The beauty of the flight instrumentation is that it can measure at extremely high resolution in time, measuring temperature, humidity, fluxes of heat and moisture as they pass up from the surface, surface temperature, cloud structure, and constituents etc.,” said Turner.

Two airports-Lucknow in north India and Bengaluru in southern India-became bases for the project.

“While based in Lucknow, we were able to fly both to the east/southeast, going into an increasingly wet environment from the Ganges basin to forests near Bhubaneswar before passing over the coast and over the Bay of Bengal,” recollected Turner.

Similarly, they could cover the west and see how the atmosphere behaves differently over the much drier surfaces in the vicinity of Jaipur and Jodhpur.

“While based in the south (Bengaluru), we could fly west over the Western Ghats and into the Arabian Sea, or instead fly south/east over Tamil Nadu and across the coast of the south Bay of Bengal,” he added.

Interesting things can be discovered when aircraft and flight measurements are combined.

“For example, in northern India, we were able to discover how the behaviour in the lowest levels of the atmosphere changes as the land surface changes from dry to wet soils (either from rainfall a couple of days before, or from irrigation used by farmers),” Turned explained.

This change in wetness on the surface causes friction which then causes the winds to slow down (convergence). The researchers have found this to lead to the creation of shallow clouds which eventually become larger during the day as the land is heated up – eventually producing rainstorms.

“This is initial work that we hope to analyse systematically over the rest of India using satellite and other data, eventually allowing us to design ways to improve our forecasting models,” added Turner.

The ground-based measurements from the towers are enabling the team to unravel what transpires in the boundary layer of the atmosphere which is where the roots of the clouds lie.

“Roots of the clouds are in the boundary layer so we need to understand what happens in the boundary layers, what happens inside the cloud and how the two are linked. Looking at the above data we are able to see how, when and where rain clouds are triggered. Now we have a fair idea of what happens in nature. The observational findings are being compared with model outputs,” noted Bhat.

But there’s still a long way to go before we start seeing improved forecasts.

“What the past observation programs have shown us is that it takes 10 years from carrying out the field experiment and using the data to the incorporation of data into models. That is the time it takes for the observations to be useful for improved forecasts,” Bhat pointed out.

“This is because the forecast models are very complex. If you adjust one (parameter) then something else goes bad somewhere. So we have to look at the total balance because these are interactive systems. So this tuning is a long process,” G.S. Bhat told Mongabay-India.

Success in science diplomacy

Apart from sowing the seeds that can lead to improved models used for weather forecasting and climate projection, the project helped build capacity in terms of better-trained scientists for studying Indian monsoon weather and climate.

“This was the first foreign aircraft campaign in India and the lessons learned and relationships built during that process can be used again in future. At the level of the scientists involved, there are new friends made and research collaborations in operation, which can lead to better science,” said Turner.

“A research flight campaign in India such as INCOMPASS is definitely something that we want to do again,” emphasised Turner, adding that there is nothing to suggest that Brexit would have any impact on similar work in the future.

CITATION

Hazra, V., Pattnaik, S., Sisodiya, A., Baisya, H., Turner, A. G., & Bhat, G. S. (2020). Assessing the performance of cloud microphysical parameterization over the Indian region: Simulation of monsoon depressions and validation with INCOMPASS observations. Atmospheric Research, 104925.

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Source: Mongabay

Image Courtesy: FAAM

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