Akshaya Nikumbh

Welcome to my website! I am Akshaya Nikumbh, a postdoctoral research associate at AOS, Princeton University and GFDL. I'm working with Prof. Yi Ming and Dr. David Paynter , supported by the CIMES postdoctoral fellowship program .

My current work focuses on how well tropical convection and rainfall are simulated in GFDL's AM4 model and its sensitivity to the horizontal resolution. I am also examining tropical-extratropical interactions during the lifetime of tropical synoptic systems and extreme precipitation events. I am generally interested in understanding different atmospheric phenomena that can be explained using atmospheric dynamics and thermodynamics. I enjoy utilizing various image processing techniques to study meteorological data and develop algorithms for the objective detection of atmospheric processes.

I've worked with Prof. Arindam Chakraborty and Prof. G. S. Bhat as a graduate student (Grantham research Fellow) at CAOS, IISc, and with Prof. Dargan Frierson at UW as a visiting Fulbright Scholar. During PhD, I examined physical processes that govern the spatial aggregation characteristics of heavy rainfall events during the South Asian summer monsoon. I showed that the changing size could explain the recent increase in heavy rainfall events over central India instead of their frequency of occurrence. I proposed mechanisms for the formation of events of different sizes. Please refer Research section to know more.

I am leading an initiative on mental health awareness as part of Science for rural India. We aim to create a miniature course (Mental Health First Aid) that provides basic knowledge about handling emotions for age 14 to 16, with the help of experts.

Let me know if you would like to discuss further! You can reach to me via contact .

Research

Recent spatial aggregation tendency of rainfall extremes over India

Significant increase in the frequency of occurrences of rainfall extremes has been reported over several parts of the world. These extreme events are usually defined at individual grids without considering their spatial extent. In this work, we show that the consideration of spatial collocation of extreme rainfall events while defining them gives a better understanding of the observed trends and underlying physical processes than using the traditional grid-based definition. The observed changes in the extreme rainfall events over central India after 1980 are mainly due to the changes in the average size of spatially collocated rainfall extremes and not due to the frequency of their occurrences (Fig. 1a). Based on the size, we classify extreme rainfall events into 3 different types viz., small, medium and large. Events of all sizes show a strong association with monsoon low-pressure systems (LPS) and occur mainly in the western sector of LPSs (Fig. 1 b). The large-scale background conditions, however, are different for different sizes of events (Fig. 1 c). For more details, check this work here .

abc — Fig.1 Smoothed time series of: (a) total number of 1° × 1° grids with extreme rainfall in a summer monsoon season (NT, black line) and the number of extreme rainfall events (NE, blue line), average size (S) of extreme rainfall events(brown line).(b) Relative location of extreme rainfall events of different size viz., small, medium and large with respect to the centre of monsoon low-pressure system. (c) Daily surface pressure anomalies.

Large‐Scale Extreme Rainfall‐Producing Synoptic Systems of the Indian Summer Monsoon

In this work, we propose a mechanism for large-scale extreme rainfall events (area > 70,000 km²) of central India. The large-scale extreme rainfall events, though rare, have become more frequent recently. We show that these events occur when multiple monsoon low‐pressure systems are present at the same time. While the individual systems themselves need not be very strong, their simultaneous presence makes the environment conducive for sustained and organized deep convection, leading to large‐scale heavy rainfall events over central India. For more details, check this work here .

abc — Fig 2. Schematic explaining the mechanism of LEREs. The interaction of an LPS and a secondary cyclonic vortex forms conditions favorable for long-lived, organized, and slow-moving convective systems on the western flank of the LPS, which result in large-scale extreme rainfall events. The numbers from 1 to 6 in the figure are representative of terms, namely, midtropospheric vortex, dynamic lifting, static instability, moisture supply, an organization at the low level, and LPS translation speed, respectively.

How extreme rainfall events of different spatial extents are triggered over central India?

In this work, we examine under what conditions monsoon low-pressure systems give extreme rainfall events of different spatial extents viz., small, medium, and large over central India. The bigger size EREs (medium and large) are triggered by LPSs during the positive phase of the MISO. On the other hand, LPSs producing small events occur manily during the negative/neutral phase of the MISO. For more details, check this work here. You might a short video that covers this work useful.

abc — Fig. (A) (a) The probability of occurrence (bars) and a presence of LPSs (violet markers) for Extreme Rainfall Events (EREs) (b) Rainfall intensity (mm day−1) distribution with respect to a geometric centre of the EREs. (B) Processes controlling the spatial extent of EREs

Cloud fraction retrieval and its classification

I developed an alogorithm for the retrieval of cloud fraction and its classification for a ground-based sky imager as a master's project. It is based on RBR thresholding technique. This algorithm is currently used to process the images captured by a Total sky imager (TSI) at the Indian Institute of Tropical Meteorology, Pune . Details of this work can be found here .

abc — Captured images by total sky imager (first panel), the corresponding RBR images (second panel) and the processed images (third panel) obtained using the cloud detection algorithm.

Publications

Akshaya Nikumbh, A. B. S. Thakur, A. Chakraborty, G. S. Bhat, J. Sukhatme "The role of North Atlantic blocking high during large-scale heavy rainfall events over central India", Journal of Atmospheric Sciences, March 2023. Link

Akshaya Nikumbh, A. Chakraborty, G. S. Bhat, D. M. W. Frierson "Multiscale interactions between monsoon intra-seasonal oscillations and low-pressure systems that produce heavy rainfall events of different spatial extents", Journal of Climate, September 2021. Link.

Akshaya Nikumbh, A. Chakraborty, G. S. Bhat, D. M. W. Frierson "Large-Scale Extreme Rainfall- Producing Synoptic Systems of the Indian Summer Monsoon." GRL, May 2020. Link

Akshaya Nikumbh, A. Chakraborty, G. S. Bhat, "Recent spatial aggregation tendency of rainfall extremes over India." Scientific Reports, June 2019. Link

Akshaya Nikumbh, B. PadmaKumari, Sneha Sunil, "Cloud Fraction Retrieval and its Variability during daytime from Ground-based Sky Imagery over a Tropical Station in India." Journal of Atmospheric and Solar-Terrestrial Physics, May 2019. Link

In preparation/ review

D. M. W. Frierson et. al., " Atmosphere and Ocean Energy Transport in Extreme Warming Scenarios", in prep.

Akshaya Nikumbh et. al., "Sensitivity of tropical convection to the horizontal resolution of GFDL’s AM4 model", In prep.