I am Akshaya Nikumbh, a senior research assistant at AOS, Princeton University. I'm working with Prof. Yi Ming on extreme rainfall events.

My current work focuses on understanding the physical processes that govern heavy rainfall events during the South Asian summer monsoon. Specifically, I'm studying their spatial aggregation characteristics.

Before this, 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.


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 .

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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 .

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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.

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 .

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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

Publications

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

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

Akshaya Nikumbh et. al., "Remote drivers of large-scale extreme rainfall events over central India", in prep.


Education

Indian Institute of Science, Bangalore

PhD

Coursework GPA: 9.2

August 2016 - present

Indian Institute of Space Science and Technology (IIST), Thiruvananthapuram

Earth and Space Science

CGPA: 9.13

August 2014 - May 2016

Cummins college of Engineering, Pune University

Electronics and Telecommunication Engineering

Score: 71.22 %

August 2010 - May 2014



Contact

Ms. Akshaya C. Nikumbh

Senior research assistant

Atmospheric and Oceanic Sciences Program,

Princeton University

Forrestal Rd, Princeton, NJ 08540, United States.

Email: akshaya.nikumbh@princeton.edu / akshaya.nikumbh@noaa.gov