This study investigates anomalous low cloud fractions (LCFs) in the Mascarene High(MH) environment of subtropical Indian Ocean (SIO) during June-September, and their sub-seasonal (10-90 day) circulation changes in the SIO and associated variations of the Indian summer monsoon (ISM) using observations and ERA5 circulation products based on 1999-2014 period. Periods of anomalous excess and deficits in LCFs in the SIO clearly reveal different sub-seasonal circulation attributes across the equator with precursor signals to the strength of ISM. Anomalous circulation composites from the excess LCF periods shows mean sea level pressure (MSLP) enhancements of about 2 hPa in the MH region in correspondence with increasing areal extent and intensifications in LCFs, and a net increase in low-level southerly momentum between MH and monsoon trough (MT) environments. The MSLP reinforcements in the MH are clearly demonstrated to emerge from the strength of cloud-top radiative cooling and associated winds and mass adjustments. The 10-20 [30 -50] day modes of the circulation in the SIO further elucidates zonally propagating [quasi-stationary] manifestations on MH reinforcements. There is an increase in meridional transport of moisture fluxes, by about 7 times relative to deficit LCF periods, channelled through aconduit region (15-30°S, 60-90°E) juxtaposing the cross-equatorial circulation (CEC) from both western and eastern sides of the Indian Ocean. This occurs in tandem with a zone of moisture flux convergence in the ISM region advancing poleward towards the climatological MT region - implying that excess LCF periods portend the likelihood of stronger ISM. Deficit LCF periods, on the contrary, show a mirrored scenario of the above with a net northerly low-level wind anomalies between MH and MT, pressure deficits in the MH region, and also portend the likelihood of weaker ISM. Low cloudsin the SIO are not only instrumental for MH stability, but also essential for circulation and moisture support across the equator and the signals for the strength of ISM on sub-seasonal scale.

In this study, a suite of ten Weather Research and Forecasting (WRF) model simulations is conducted using various choices of local and non-local representations of vertical mixing adopted in the planetary boundary layer (PBL) parameterizations to assess the boreal summertime marine layer cloud environment in the southern Indian Ocean (SIO). The local, non-local, and hybrid type PBL schemes used in the simulations are generally able to produce the pertinent features of the marine layer embedded with stratocumulus clouds in the SIO region, viz., nocturnal coverage of low clouds with warm cloud tops, well-mixed marine layer, inversion capping, strong temperature and moisture jumps across the inversion layer, cloud water generation and its coupling with cloud-top radiative cooling, as conceptually envisaged in earlier investigations. However, there is a large spatial variability noticed in the simulated low clouds over the SIO region. While the satellite diagnosed cloud liquid water paths (LWPs) are noted as high as 0.4 mm in the region 60° E–90° E, 15° S–25° S, the simulated cloud LWPs show a large mesoscale variability exhibiting a range of values between 0.04 and 2.8 mm, corresponding to cloud depths as small [high] as 200 [1700] m. It is further noted that LWPs reproduced from hybrid PBL schemes employed with eddy diffusivity mass flux (EDMF) formulation are closer to the observed estimates in the SIO region. While the non-local PBL mixing schemes tend to produce more organized sheeted low-cloud layers, the local closure schemes tend to produce more discreteness in the low-cloud decks and also exhibit substantial cloud depth variability. In addition, the coupling of in-cloud turbulence in association with cloud-top radiative cooling and sub-cloud layer turbulence are not uniquely reproduced by these schemes. The inferences from this study suggest that non-local/hybrid type of PBL parameterizations accounting for stratocumulus cloud-top driven mixing processes shows greater promise in the reproduction of low-clouds in the SIO region.

This study investigates the influence of climate change on widespread fog conditions over the Indo-Gangetic Plains (IGP) of north India using observations, reanalysis data of atmospheric parameters, coupled model inter-comparison project 6 (CMIP6) projections following four future scenarios based on the shared socio-economic pathways (SSP126, SSP245, SSP370, SSP585), and advanced analysis techniques including machine learning. Two parameters fog fraction and widespread fog days (WFDs) are estimated in this study by functional mapping of fog observations with 8 atmospheric parameters for the period 1981–2018 using three empirical/machine learning approaches. Of these, we note that the deep learning convolutional neural network (CNN) exhibits superiority in performance by showing the mapping closer to the observed, and also offers promising potential for operational purposes to provide fog outlooks for the IGP region. Temporal evolution of fog fractions and WFDs is analyzed from the CMIP6 projections following the aforementioned four future scenarios using CNN for the future periods of the twenty-first century. It is noted that there is a substantial enhancement in the CMIP6 projected fog fractions as high as 57% during the period (2015–2045) relative to the historical (1981–2014) period, while the largest increase of 154% is seen in projected WFDs. It is also seen that the near-future period (2015–2045) witnesses a larger prevalence of WFDs, for all scenarios except SSP126, due to the combined effects of air pollution and greenhouse warming. The post-2046 periods, however, generally indicate signatures of decline in foggy days with widespread conditions relative to historical period in most of the scenarios except SSP370. The severity in fog conditions following the high-emission scenarios SSP370 and SSP585 during this period comes from the relative impact of mitigation strategies of pollutants. The findings provide insights into the possible future changes in widespread fog conditions suitable for the IGP region.