Impact of ocean mixed‐layer depth initialization on the simulation of tropical cyclones over the Bay of Bengal using the WRF‐ARW model

Abstract

The sensitivity of the simulated tropical cyclone (TC) intensity and tracks to the different ocean mixed‐layer depth (MLD) initializations is studied using coupled weather research and forecasting (WRF) and ocean mixed‐layer (OML) models. Four sets of numerical experiments are conducted for two TCs formed during the pre‐ and post‐monsoon. In the control run (CONTROL), the WRF model is initialized without coupling. In the second experiment, the WRF‐OML model is initialized by prescribing the MLD as a constant depth of 50 m (MLD‐CONST). In the third experiment, the spatial varying MLD obtained from the formulation of depth of the isothermal layer (MLD‐TEMP) is used. For the fourth experiment (MLD‐DENS), the model is initialized with the density‐based MLD obtained from ARMOR‐3D data. The results indicate that the CONTROL exhibits an early intensification phase with a faster translation movement, leading to early landfall and the production of large track deviations. The coupled OML simulations captured the deepening phase close to the observed estimates, resulting in the reduction of errors in both the vector and along the tracks of the storm. The initialization of the different estimates of the MLD in the WRF‐OML shows that the TC intensity and translation speed are sensitive to the initial representation of the MLD for the post‐monsoon storm. The gradual improvements in the intensity and translation speed of the storm with the realistic representation of the OML are mainly due to the storm‐induced cooling, which in turn alters the simulated enthalpy fluxes supplied to the TC, leading to the better representation of secondary circulation and the rapid intensification of the storm.

Simulations of Bay of Bengal tropical cyclones in a regional convection-permitting atmosphere–ocean coupled model

“…Regional-scale coupled predictions are already operational for some TC cases in the Atlantic and North Pacific (Biswas et al, 2018;Jin et al, 2013;Saito et al, 2006), although the models used in these studies do not explicitly resolve convection. Fewer studies assess air-sea coupled models in the Bay of Bengal; the majority use mesoscale models with parameterised convection (Agrawal et al, 2020;Baisya et al, 75 2020;Greeshma et al, 2019;Yesubabu et al, 2020). They show a high sensitivity of TCs to air-sea interactions (Prakash & Pant, 2017), ocean mixed layer depth (Yesubabu et al, 2020) and ocean eddies (Anandh et al, 2020).…”

“…Fewer studies assess air-sea coupled models in the Bay of Bengal; the majority use mesoscale models with parameterised convection (Agrawal et al, 2020;Baisya et al, 75 2020;Greeshma et al, 2019;Yesubabu et al, 2020). They show a high sensitivity of TCs to air-sea interactions (Prakash & Pant, 2017), ocean mixed layer depth (Yesubabu et al, 2020) and ocean eddies (Anandh et al, 2020). Ocean coupling in mesoscale models generally improves TC track and intensity predictions (Greeshma et al, 2019;Srinivas et al, 2016;Yesubabu et al, 2020).…”

“…They show a high sensitivity of TCs to air-sea interactions (Prakash & Pant, 2017), ocean mixed layer depth (Yesubabu et al, 2020) and ocean eddies (Anandh et al, 2020). Ocean coupling in mesoscale models generally improves TC track and intensity predictions (Greeshma et al, 2019;Srinivas et al, 2016;Yesubabu et al, 2020). Atmosphere-only (uncoupled) NWP models generally assume constant SST over the forecast, derived from ocean analysis systems, which has been the operational 80 practice for TC prediction over the Bay of Bengal (Mohanty et al, 2015;Osuri et al, 2017;Routray et al, 2017;Srinivas et al, 2013;Nadimpalli et al, 2020).…”

“…Therefore, CPL overestimates the cooling response for a given TC intensity. SST cooling caused by wind-driven mixing is strongly modulated by pre-storm oceanic conditions, including mixed layer depth (Dutta et al, 2020;Vincent et al, 2012;Yesubabu et al, 2020). 600 Analysis of a month-long period in summer 2016 has shown that coupled IND1 simulations produce a shallower mean ocean mixed layer depth (depth < ~15 m) across the Bay of Bengal in IND1 compared to FOAM (Forecast Ocean Assimilation Model) analyses and the three RAMA moorings in the Bay of Bengal.…”

Modulation of environmental conditions on the significant difference in the super cyclone formation rate during the pre- and post-monsoon seasons over the Bay of Bengal

“…Some studies have focused on the formation of SCs over the BoB during the Pre-MS (Kotal et al 2009a;Li et al 2016Li et al , 2019a, but few studies have attempted to explain why the SC formation rate is far lower in the Post-MS than the Pre-MS. Balaguru et al (2012) indicated that the ocean barrier layer is able to prevent a sharp reduction of sea surface temperature (SST) by strengthening the stratification to weaken storminduced Ekman pumping when cyclones pass over barrierlayer regions, which is conductive to enhancement of the cyclone intensity. Unlike the Post-MS, the Pre-MS in the BoB exhibits an obvious shoaling and thin barrier layer (Thadathil et al 2007;Vissa et al 2013;Yesubabu et al 2020); therefore, the barrier layer may not explain why the Pre-MS is more prone to SC occurrence than the Post-MS. Neetu et al (2019) postulated that BoB TCs are stronger before compared with after the monsoon due to favorable large-scale background conditions; however, there is a lack of quantitative diagnostic proof to illuminate the inner physical processes that determine the difference in the SC formation rate between the Pre-and Post-MS. Considering the current situation, a more specific analysis is needed to explain what and how environmental conditions and physical processes modulate the significantly different SC formation rate between the Pre-and Post-MS in the BoB.…”