Oozeer, Muhammad Yaasiin
(2018)
On the occurrence and transport of biomass burning haze in south-southeast Asia using observation data and computational methods.
PhD thesis, University of Nottingham Malaysia Campus.
Abstract
Biomass-burning haze (BBH) is an environmental concern which has a tremendous impact on human health and the economy in Southeast Asia (SEA). One of the worst haze events to ever hit Peninsular Malaysia occurred in June 2013 due to smoke from Riau, Central Sumatra. While biomass-burning in the region is common, the early occurrence of a haze episode of this magnitude is uncharacteristic of the seasonality of extreme fire events which usually occur between August and October in the Maritime Continent (MC). Previous studies on the June 2013 event mostly include statistical studies of the impacts of haze on air quality and health. Therefore, this study aims to investigate the phenomenology of this peculiar haze event and its underlying meteorological forcing agents. The aerosol and meteorological environment during the event is examined using the Moderate Resolution Imaging Spectroradiometer (MODIS) active fire hotspot detections and aerosol optical thickness (AOT) retrievals, satellite based precipitation retrievals and meteorological indices. Particular attention is given to El Niño Southern Oscillation (ENSO) and Madden-Julian Oscillation (MJO) conditions since these phenomena influence inter-annual and intra-seasonal fire-activity, respectively, as well as the influence of tropical cyclones (TC) over the South China Sea. The above datasets are then supported by a WRF-Chem nested simulation to provide a comprehensive picture of the event’s meteorology and aerosol transport phenomenology. Indeed, while the use of weather models to study BBH has become more popular, more modelling efforts need to be put into studying the June 2013 haze event to identify the mechanisms of long range transport of haze.
A set of 13 sensitivity simulations are run to determine the physics settings which best represent the meteorology over the model domain during the June 2013 haze episode. The physics options used in the sensitivity simulations are selected based on previous WRF physics sensitivity studies and work which include WRF simulations with domains over Asia and SEA. In particular, the microphysics, cumulus parameterisation and planetary boundary layer (PBL) schemes are looked into to obtain the best agreement to observation data. The output from the sensitivity simulations are evaluated with satellite based precipitation retrievals and ground station data over Malaysia. The simulations run with the Lin microphysics scheme, Betts-Miller-Janjić (BMJ) cumulus parameterisation scheme and Mellor-Yamada-Janjić (MYJ) planetary boundary layer scheme performed best overall. These best settings, based on the sensitivity studies, are then used in the numerical simulations which are evaluated with satellite and ground station data. The evaluation shows that model produces similar patterns and magnitudes of AOT and successfully captures the variations in smoke plume height when compared to MODIS AOD and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) aerosol extinction profile datasets.
The analysis of the time series ENSO conditions and MODIS fire count show that while extreme fire events are more characteristic of El Niño years, the MODIS fire count over the MC in June for the years 2001–2015 was highest in 2013 when neutral conditions prevailed. Although, the mean daily precipitation for June 2013 was below average for June for the years 2003–2015. In addition, the highest ratio of 0.89 of fire count for SPM to MC for any month for the period 2001–2015 was recorded in June 2013. An early and active TC season, which could have been the result of a strong transiting MJO, occurred in June 2013. The results show that the combined induced subsidence and flow enhancement due to TC Bebinca and the dry phases of the strong MJO event contributed to the event. The simulations further show that downward vertical motion of at least 6 cm s-1 prevailed over Sumatra on 22 June when TC Bebinca was most intense, while upward vertical motion reaching at least 9 cm s-1 prevailed over the same region before TC Bebinca on 17 June. Indeed, smoke sources were concentrated under this particular region of subsidence, where surface PM2.5 concentrations reached at least 1000 µg m-3 on 22 June. Vertical cross-sections across the model domain also show that subsidence during phase 6 of the MJO prevailed over Sumatra. Intense and early TC seasons over the Western North Pacific can therefore be an indication of the occurrence of early and extreme haze events over the MC.
The numerical simulations are also used to study the convective mechanisms which are responsible for uplifting biomass-burning haze in the troposphere. These mechanisms over Sumatra and Peninsular Malaysia are under-studied and their physical mechanisms remain unclear. The PM2.5 mass concentration, vertical wind speed plots at different levels and vertical cross-sections of major smoke plumes are analysed and the corresponding convective mechanisms identified. Three main convective mechanisms are identified, namely, orographic motion over the Barisan Mountains of Indonesia, morning convergence over the strait of Malacca and orographic motion over Peninsular Malaysia. Results show that smoke is lifted to heights of at least 10 km in the atmosphere due to orographic lifting over Peninsular Malaysia while the average plume height increased to higher than 2 km as TC Bebinca subsided, due to the resulting decrease in subsidence. The identified mechanisms are able to uplift the biomass-burning emissions to the upper troposphere and this could have significant long-range transport and global climatic effects.
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