• GOVINDAN KUTTY

Articles written in Journal of Earth System Science

• The role of mid-level vortex in the intensification and weakening of tropical cyclones

The present study examines the dynamics of mid-tropospheric vortex during cyclogenesis and quantifies the importance of such vortex developments in the intensification of tropical cyclone. The genesis of tropical cyclones are investigated based on two most widely accepted theories that explain the mechanism of cyclone formation namely ‘top-down’ and ‘bottom-up’ dynamics. The Weather Research and Forecast model is employed to generate high resolution dataset required for analysis. The development of the mid-level vortex was analyzed with regard to the evolution of potential vorticity (PV), relative vorticity (RV) and vertical wind shear. Two tropical cyclones which include the developing cyclone, Hudhud and the non-developing cyclone, Helen are considered. Further, Hudhud and Helen, is compared to a deep depression formed over Bay of Bengal to highlight the significance of the mid-level vortex in the genesis of a tropical cyclone. Major results obtained are as follows: stronger positive PV anomalies are noticed over upper and lower levels of troposphere near the storm center for Hudhud as compared to Helen and the depression; Constructive interference in upper and lower level positive PV anomaly maxima resulted in the intensification of Hudhud. For Hudhud, the evolution of RV follows ‘top-down’ dynamics, in which the growth starts from the middle troposphere and then progresses downwards. As for Helen, RV growth seems to follow ‘bottom-up’ mechanism initiating growth from the lower troposphere. Though, the growth of RV for the depression initiates from mid-troposphere, rapid dissipation of mid-level vortex destabilizes the system. It is found that the formation mid-level vortex in the genesis phase is significantly important for the intensification of the storm.

• Comparison of the performance of HYBRID ETKF-3DVAR and 3DVAR data assimilation scheme on the forecast of tropical cyclones formed over the Bay of Bengal

This study compares the performance of hybrid ensemble transform Kalman filter – three dimensional variational data assimilation (HYBRID) system and three dimensional variational (3DVAR) data assimilation system in Weather Research and Forecasting Model (WRF) in simulating tropical cyclones (TC) formed over the Bay of Bengal. An Ensemble Transform Kalman Filter (ETKF) system updates the ensemble system that provides flow-evolving background error covariance for HYBRID data assimilation system. Results indicate that use of flow-evolving ensemble error covariance in 3DVAR system has systematically reduced the TC position and intensity errors in the analysis; however, adding more weights to the ensemble error covariance term in 3DVAR cost function has not made any significant impact. The 3DVAR analysis depicts a stronger TC vortex with a well pronounced warm core structure as compared to that in HYBRID analysis. The forecasts from HYBRID analysis outperform that from 3DVAR in reducing TC track forecast error. The relative improvement in TC landfall position is 43% and 49% for variously configured HYBRID experiments. The forecasts initiated from HYBRID analysis has higher skill in quantitative precipitation forecasts during TC landfall compared to 3DVAR, which may be attributed to improved track prediction in the HYBRID experiments.

$\bf{Highlights}$

$\bullet$ Compared the performance of HYBRID and 3DVAR data assimilation system for Tropical cyclone forecasts.

$\bullet$ HYBRID has systematically reduced the Tropical cyclone position and intensity errors in the analysis.

$\bullet$ The forecasts from HYBRID analysis outperform that from 3DVAR in reducing TC track forecast error.

$\bullet$ The forecasts initiated from HYBRID analysis has higher skill in quantitative precipitation forecasts during Tropical cyclone landfall compared to 3DVAR.

• Simulated dynamics and thermodynamics processes leading to the rapid intensification of rare tropical cyclones over the North Indian Oceans

The life cycle dynamics and intensification processes of three long-duration tropical cyclones (TCs), viz., Fani (2019), Luban (2018), and Ockhi (2017) formed over the North Indian Ocean (NIO) have been investigated by developing a high-resolution (6 km ${\times}$ 6 km) mesoscale analysis using WRF and En3DVAR data assimilation system. The release of CAPE in nearly saturated middle-level relative humidity caused intense diabatic heating, leading to an increase in low-level convergence triggering rapid intensification (RI). The strengthening of the relative vorticity tendency terms was due to vertical stretching (TC Fani) and middle tropospheric advection (TCs Luban and Ockhi). The increase or decrease in uppertropospheric divergence led to RI through two different mechanisms. The increase in upper divergence strengthens the vortical convection (in TC Luban and Fani) by enhancing the moisture and heat transport, whereas its decrease caused a reduction in the upper-level ventilation flow at 200 hPa followed by moisture accumulation, enhanced diabatic heating, and strengthened the warm core (TC Ockhi). The RI caused the vortex of three cyclones to extend up to the upper troposphere. The well organised wind during RI led the unorganised, weak, discontinuous vertical vortex columns to become organised with intense vertical velocity throughout the column. Spatial distributions of Okubo–Wiess (OW) parameter showed TC core dominated by vorticity than strain, since deep depression (DD) stages.

$\bf{Highlights}$

$\bullet$ The saturated middle-level relative humidity caused intense diabatic heating, and then release of CAPE led to a rise in low-level spin-up triggering the RI.

$\bullet$ The strengthening of the relative vorticity tendency terms was due to stretching (TC Fani) and middle tropospheric advection (TCs Luban and Ockhi).

$\bullet$ The increase or decrease in upper-tropospheric divergence led to RI through two different mechanisms.

$\bullet$ The RI caused the vortex of three cyclones to extend up to the upper troposphere.

$\bullet$ RI led unorganised, weak, discontinuous vertical vortex columns to become organised with intense vertical velocity throughout the column.

• Ensemble-based forecast sensitivity approach to estimate the impact of satellite-derived atmospheric motion vectors in a limited area model

Data impact studies have been conducted regularly to assess the value added by each observation type to the data assimilation (DA) systems in all the major operational weather forecast centers. In this study, we implement a novel ensemble-based forecast sensitivity to observation (EFSO) approach in the weather research forecasting (WRF) modelling system to estimate the impact of satellite atmospheric motion vectors (AMV). The EFSO method utilizes ensemble perturbations as a means to estimate the impact of observations in an ensemble Kalman filter DA system. The impact of AMV observation using the EFSO approach is performed during the intensification period of two tropical cyclones (TC) – Hudhud (2014) and Phailin (2013) formed over the Bay of Bengal. Results indicate that the AMV observations substantially reduce the forecast error, in general. Further, it is found that the percentage of AMV observations with a positive impact for both the TCs is a little more than 50% which contributes to improving the forecasts while the rest of the observations degrades the forecasts. It is found that the fractional impact value of AMV observations estimated by EFSO and OSE varies only by 3%, which indicates that the estimated data impact from EFSO agrees well with the corresponding OSE. A sensitivity experiment conducted by inflating/deflating the observational error covariance matrix indicates that higher (lower) observational error corresponds to reduced (enhanced) positive impact of observations. Though AMV observations have a positive impact, in general, the impact of radiosonde observations is significantly higher than AMV in the data assimilation system. The experiments performed with the targeted domain indicate that observations over the upstream region show a larger positive impact when compared to that over the downstream region of the TC.

• # Journal of Earth System Science

Volume 132, 2023
All articles
Continuous Article Publishing mode

• # Editorial Note on Continuous Article Publication

Posted on July 25, 2019