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Featured:
Li RKK, Tam CY, Lau NC. (2021) Effects of ENSO Diversity and Cold Tongue Bias on Seasonal Prediction of South China Late Spring Rainfall. Climate Dynamics. https://doi.org/10.1007/s00382-021-05732-w

Summary:
The featured paper investigated the prediction of South China rainfall. The late spring period of April and May usually marks the end of the dry seasons and the beginning of the wet season over South China. However, the rainfall exhibits large year-to-year variability. Enhanced rainfall can advance the wet season and reduced rainfall can prolong the dry seasons, where both can cause agricultural and socio-economical losses. Therefore, accurate predictions of South China late spring rainfall a few months in advance can aid water management and provide early warnings.
The prediction skill of a state-of-the-art climate model, which is used operationally in many weather agencies worldwide, was investigated. It was discovered that during the time between model initialisation in March and the forecasts being made for April and May, the model develops a cold sea surface temperature bias in the tropical Pacific. Results showed that the cold bias is clearly seen in the tropical Pacific. It was proposed a dynamical mechanism on how this oceanic bias affects the atmosphere over the tropical Pacific, and subsequently reduces the skill of this model in predicting the rainfall over South China. For better prediction of South China rainfall, this project suggests that one solution is to first model the tropical Pacific sea surface temperature better.

Featured:
Xu, K., Tam, C. Y., Zhu, C., Liu, B., & Wang, W. (2017). CMIP5 Projections of Two Types of El Niño and Their Related Tropical Precipitation in the Twenty-First Century. Journal of Climate, 30(3), 849–864. https://doi.org/10.1175/jcli-d-16-0413.1

Summary:
The featured paper investigated two types of El Niño in the twenty-first century, the eastern-Pacific (EP) and central-Pacific (CP) types, including their associated tropical circulation and precipitation vavriablility. The team used the historical runs and representative concentration pathway 8.5 (RCP8.5) simulations from 31 coupled models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the projection.
The model results projected the El Niño sea surface temperature (SST) variability, changes of relative frequency from CP to EP El Niño, tropical overturning circulation, and the El Niño associating precipitation. These suggest that elements, apart from the tropical enhanced basic-state hydrological cycle, like the anomalous overturning circulation might also be involved in changing the ENSO precipitation under the warmer background climate.

Featured:
Wang, Z., Xiao, Z., Tam, C. Y., Pan, W., Chen, J., Hu, C., Ren, C., Wei, W., & Yang, S. (2021). The projected effects of urbanization and climate change on summer thermal environment in Guangdong-Hong Kong-Macao Greater Bay Area of China. Urban Climate, 37. DOI: https://doi.org/10.1016/j.uclim.2021.100866

Summary:
The team projected the near-future urbanization and climate change impacts on thermal environment in July over the Guangdone-Hong Kong-Macao Greater Bay Area (GBA). Individual and combined impacts are investigated.
It is found that both the urbanization and climate change cause the near-future tmeperature to increase. The probability of extreme heat events is enhanced as well. For the human thermal comfort, the results suggest that urbanization creates more comfortable daytime but uncomfortable nighttime. On the other hand, the warming climate leads to a significant and uniform increase of Humidex during whole day, so creating a thermally uncomfortable condition throughout whole day. The warming climate relatively contributes a much higher percentage of the total increased Humidex. As a result, it is suggested that there is a necessity of mitigating climate change in future magacities.

Featured:
Xu, Z., Y. Han, C.-Y. Tam, Z.-L. Yang and C. Fu, 2021: A bias-corrected CMIP6 global dataset for dynamical downscaling of future climate (1979–2100), Scientific Data, DOI: 10.1038/s41597-021-01079-3.

Summary:
A set of bias-corrected global dataset based on 18 models from the Coupled Model Intercomparison Project Phase 6 (CMIP6) and European Centre for Medium-Range Weather Forecasts Reanalysis (ERA5) is constructed for the dynmamical downscaling projections of future climate, atmospheric environment and other related fields. Comparing with the individual CMIP6 models, this dataset has better quality in terms of climatological mean, interannual variance and extreme events.

Personal profile

Featured:
Wang, L., Tai, A. P. K., Tam, C.-Y., Sadiq, M., Wang, P., and Cheung, K. K. W.: Impacts of future land use and land cover change on mid-21st-century surface ozone air quality: distinguishing between the biogeophysical and biogeochemical effects, Atmos. Chem. Phys., 20, 11349–11369, https://doi.org/10.5194/acp-20-11349-2020, 2020.

Summary:
The individual and combined biogeophysical and biogeochemical effects of land use and land cover change (LULCC) on ozone is investigated and the critical pathway that LULCC affects ozone pollution is examined. LULCC in the boreal summer from 2000 to 2050 under RC4.5 and RCP8.5 scenarios is projected using a global coupled atmosphere-chemistry-land model. The results suggest that the biogeophysical effects of LULCC are significant pathways to influence ozone air quality locally and in remote places without LULCC. Several mechanisms are proposed to explain the effects.

Featured:
Li, J., Tam, C. Y., Tai, A. P., & Lau, N. C. (2021). Vegetation-heatwave correlations and contrasting energy exchange responses of different vegetation types to summer heatwaves in the Northern Hemisphere during the 1982–2011 period. Agricultural and Forest Meteorology, 296. https://doi.org/10.1016/j.agrformet.2020.108208

Summary:
The study investigates the vegetation and heatwaves relationships in the boreal summer during 1982-2011. It uses the Global Heatwave and Warm-spell Record (GHWR) and leaf area index (LAI) products from satellites during the period. The results reveal that there is a strong interannual correlations between the summer heatwaves and plant function types. It is found that for places that are domainated by grassland, temperate deciduous forests or temperate needleleaf forests, tends to experience a higher frequency of heatwaves in the years with lower LAI. On the other hand, places where boreal evergreen forests dominate, the frequency of heatwaves increases when there is a higher-than-norma LAI. Energy exchange processes for different type of vegetation under backgound climate conditions are suggested to explain the mentioned phenomena. The study also wants to emphasize the management of forest resources in view of worsening heatwave severity under the warming climate.

Featured:
Zhang, T., Tam, CY., Jiang, X., et al. Roles of land-surface properties and terrains on Maritime Continent rainfall and its seasonal evolution. Clim Dyn 53, 6681–6697 (2019). https://doi.org/10.1007/s00382-019-04951-6

Summary:
The paper investigates the roles of surface properties and terrains in controlling the climatological rainfall of Maritime Continent (MC), using the general circulation model.
Model results show that the presence of terrain can increase the MC land rainfall but only moderately affect the rainfall patterns over the MC ocean. In addition, there is a seasonal dependence to the impacts of MC land-sea distribution on the regional rainfall. The results suggest a larger sensitivity of model bias in representing land-sea distribtuion over the MC region during the boreal summer than in the boreal winter.

Featured:
Jiang, X., Zhang, T., Tam, C., Chen, J., Lau, N., Yang, S., & Wang, Z. (2019). Impacts of ENSO and IOD on Snow Depth Over the Tibetan Plateau: Roles of Convections Over the Western North Pacific and Indian Ocean. Journal of Geophysical Research: Atmospheres, 124(22), DOI: https://doi.org/10.1029/2019jd031384

Summary:
The study investigates the impacts of El Niño-Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) on the snow depth over the Tibetan Plateau (TP). In situ observational data are taken and analyzed. Model simulations are performed. The results show that the snow depth over the TP are different during early winter under the impacts of ENSO and IOD. Spatially, ENSO has a higher contribution to the change in snow depth over the eastern TP, while IOD effects are more dominant over the central western TP. A positive anomalous snowfall and cold temperature anomaly result in a positive anomalous snow depth. It is suggested that the positive anomalous snowfall in the eastern TP is the results from the positive phase of ENSO while that in the central western TP is mainly contributed by the positive IOD. If the phases of ENSO and IOD become negative, the associated anomalous snowfall will tend to be negative.

Featured:
Li, Z., C.-Y. Tam, Y. Li, N.-C. Lau, J. Chen, S.T. Chan and D.-S. D. Lau, 2021: How does air-sea wave interaction affect tropical cyclone intensity? An atmosphere–wave–ocean coupled model study based on super typhoon Mangkhut (2018), Earth and Space Science, 9, e2021EA002136, DOI: https://doi.org/10.1029/2021EA002136

Summary:
The contribution of the atmosphere-ocean and atmosphere-sea wave coupling to the intensity of super typhoon Mangkhut (2018) is studied by taking numerical simulations based on hierarchical Atmosphere-Wave-Ocean (AWO) coupling configurations. TC-related strong winds, oceanic cold wake, and wind waves with high fidlity are simulated. The experimental results show that there are two mechanisms, which are related to the surface drag coefficient and the wind speed distribution of the TC, contributing to the improvement of TC intensity. The paper also discusses the related implications of the results on the meteorological and sesa-state forecasts over marine and coastal areas during TC passages.

Featured:
Li, Y., N.-C. Lau, C.-Y. Tam, H.-M. Cheung, Y. Deng and H. Zhang, 2021: Projected Changes in the Characteristics of the East Asian Summer Monsoonal Front and their Impacts on the Regional Precipitation, Climate Dynamics, https://doi.org/10.1007/s00382-021-05687-y

Summary:
The interannual variability of the East Asian summer monsoonal front (EASMF) and its projected trends under the RCP8.5 scenario are investigated by using the Community Atmospheric Model (CAM5.1) with a 50km horizontal resolution.The model can reasonably reproduce the seasonal march of the EASMF. According to the results, it is found that in the late twenty-first century, the intensity of the EASMF will increase and the location of it will be displaced eastward and southward from the present-day mean location. The EASMF events will also expand wider meridionally and the life span will be longer.
Since the monsoonal precipitation over East Asia is sensitive to the meridional displacement of EASMF, an enhanced rain band is observed extending northeastward from southern China to the northwestern Pacific south of Japan. This feature is associated with the jet streams that are linked to tropical warming in the upper troposphere and warming over the South China Sea in the lower troposphere during the twenty-first century. It is suggested that the long-term trends in the temperature and circulation patterns associated with climate change will alter the location at which the "jet stream-precipitation" relationship prevails.

Personal profile

Featured:
Au-Yueng, A. Y. M., and C.-Y. Tam, 2017: Dispersion characteristics and circulation associated with boreal summer westward travelling mixed-Rossby gravity wave-like disturbances, J. Atmos. Sci., https://doi.org/10.1175/JAS-D-16-0245.1

Summary:
The study develops an algorithm tracking the mixed Rossby-gravity (MRG) wave-like disturbances with a cross-equatorial component. The locations with the Gaussian-shaped wind structures stated in the solutions of shallow-water equations are found from the space-time-filtered meridional wind data. The low-level MRG wave-like disturbances can be classified into two groups, longer wavelengths and shorter wavelengths. Most waves found in the eastern Pacific belong to the long-wavelength group while those in the western Pacific tend to have a wider range of wavenumbers. Further analysis on the energetics of both east and west of ~140°E low-level disturbances, it is found that there is a downward energy dispersion in the mid- to upper levels, suggesting that the upper-level wave activity might play a significant role in triggering these low-level waves in the Pacific basin.

Featured:
Kwok, G. L. K., Y. F. Lam and C.-Y. Tam, 2016: Developing a Statistical Based Approach for Predicting Local Air Quality in Complex Terrain Area, Atmospheric Pollution Research, http://dx.doi.org/10.1016/j.apr.2016.08.001

Summary:
The study develops a statistical framework for predicting the next-day air quality for Hong Kong. Generalized Additive Models (GAMs) connecting the air pollutant concentration with meteorological data were built. To forecast the air quality, GAMs used the weather predictions from GFS model, dynamical downscaling of GFS using WRF (GFS-WRF) and bias corrected GFS-WRF (Improved GFS-WRF). In one-year period from December 2009 to November 2010, verification of the system was performed by conducting retrospective daily air quality predictions.
It was found that GAMs combined with Improved GFS-WRF could provide better results than those just based on GFS and GFS-WRF. The model inputs performed well in forecasting urban and sub-urban pollutant concentrations. With the Improved GFS-WRF, the hit rate for categorical forecasts of events with daily air pollution index over 100 is higher. The implementation of Improved GFS-WRF to detect the onset of O₃ episodes before the presence of the tropical cyclones is described. This statistical model can be indicated as a useful tool for air quality prediction for sites like Hong Kong.

Personal profile

Featured:
Zhao R., C.-Y. Tam, S.-M. Lee, J. Chen and P. Gao, 2022: Attributing extreme precipitation characteristics in South China Pearl River Delta region to anthropogenic influences based on pseudo global warming, Climate Dynamics, submitted. (Preprint DOI: 10.21203/rs.3.rs-2065981/v1)

Summary:
This study elaborates on the human-contributed variations in daily-mean and extreme precipitation characteristics over the Pearl River Delta (PRD) region. By applying the Weather Research and Forecasting (WRF) model and the pseudo global warming (PGW) method, attribution analyses on 40 extreme rainfall events that occurred in different seasons during 1998-2018 over the PRD are conducted. The results show that PRD near-surface temperature has been raised respectively by 0.9-1.1 K in the May-to-September (MJJAS) and 0.6-0.8 K in non-MJJAS seasons. Accordingly, extreme daily rainfall increased by 8%-9.5% in MJJAS and 12.4% at most in non-MJJAS season, and the probability of occurrence increased by 10%-30% during MJJAS (20%-40% in non-MJJAS season). The paper suggests that changes in MJJAS extremes are related to stronger low-level southerly winds, while non-MJJAS rainfall is worsened by strengthened low-level wind convergence and updrafts. Moreover, it is suggested by moisture budget analysis that thermodynamic effects associated with the increased moisture amount account for the mean rainfall increase, whereas dynamic effects related to wind circulation changes are responsible for extreme precipitation, regardless of seasons.

Featured:
Chen, J., Wang, Z., Tam, CY. et al. Impacts of climate change on tropical cyclones and induced storm surges in the Pearl River Delta region using pseudo-global-warming method. Sci Rep 10, 1965 (2020). https://doi.org/10.1038/s41598-020-58824-8.

Summary:
Applying the pseudo-global-warming technique, the changes of western North Pacific land-falling tropical cyclone (TC) characteristics due to global warming is investigated. WRF model is used to simulation three historical intense TCs that made landfall in the Pearl River Delta (PRD). The near- and far- future temperature and humdity changes, which are obtained from the CMIP5 projections based on RCP8.5 scenario, are applied to the background climate. The results suggest that the peak intensities of TCs will increase. Analysis is conducted and shows that the surface warming alone can intensify TCs, while the warmer atmosphere provides a higher stability. SLOSH model is used and found that the corresponding storm surges will increase. Under the impacts of both sea level rise and land vertical displacement, the surges affecting PRD will increase by about 1m in the far-future.

Featured:
Wang, P., C.-Y. Tam, N.-C. Lau and X. Kang, 2020, Future Impacts of Two Types of El Niño on East Asian Rainfall Based on CMIP5 Model Projections, Climate Dynamics, http://link.springer.com/article/10.1007/s00382-020-05510-0

Summary:
Based on RCP8.5 scenario from CMIP5, the future change of El Niño-related East Asian (EA) rainfall and the diversity of the relationship are investigated. The East Asian Summer Monsoon (EASM) climatology and interannual variations are evaluated. The results suggest that the general patterns of the anomalous circulation and rainfall persist under a warming climate, but the rainfall intensity amplifies during mature boreal winter and decaying summer for both Eastern-Pacific (EP) and Central-Pacific (CP) El Niño. The CP type-related rainfall seems to show a stronger amplification than that for the EP type. Moreover, it is suggested that a moister atmosphere favours the strengthening of the rainfall varations for types. The paper suggests that the changes in circulations might be associated with the modified sea surface temperature anomalies of El Niño and its diversity under global warming. These results indicate the significance of better preparedness and higher resilience in the EA region to enhanced El Niño-induced hydrological variaitons under global warming.

Featured:
Fung, K.Y., C.-Y. Tam, T.C. Lee and Z. Wang, 2021: Comparing the Anthropogenic Heat and Global Warming Impacts on Extreme Precipitation in urbanized Pearl River Delta area based on Dynamical Downscaling, J. Geophys. Res, DOI: 10.1029/2021JD035047

Summary:
The comparison between the impacts of global warming and intense anthropogenic heat (AH) on extreme hourly precipitation over the Pearl River Delta megacity is carried out. The study uses the cloud resolving Weather Research and Forecasting (WRF) model coupled with the single-layer urban canopy model (SLUCM) to perform three downscaling experiments under different AH values and climate projections. The results show that about 20 to more than 100% increase in the probability of hourly precipitation with the magnitude of 20-100mm/hr over urban locations was resulted by the global warming forcing. The intense AH forcing also creates similar results. In terms of modulating the thermodynamic environment, intense AH results in enhanced convective available potential energy (CAPE) and reduced convective inhibition (CIN) within the megacity, and its warming is limited to the lowest 1km above ground. On the other hand, global warming enhances both CAPE and CIN over urban and rural areas, and the warming is observed over the whole troposphere. This study highlights the different physical mechanisms of AH and global warming in exacerbating extreme urban rainfall.

Featured:
Lui, Y.S., K.-S. Tse, C.-Y. Tam, K.H. Lau and J. Chen, 2020: Performance of western north Pacific tropical cyclone track and intensity evolution in MPAS-A and WRF simulations, Theor. Appl. Climatol.,https://doi.org/10.1007/s00704-020-03444-5

Summary:
Comparison of the performance of MPAS-A and WRF model in simulating western North Pacific (WNP) tropical cyclone (TC) tracks and intensities is conducted. Eight historical storms making landfall over southern China are simulated parallelly with initial conditions taken from ERA-interim. The results show that the storm intensities are underestimated in general while the TC tracks are reasonably captured by both models. It is suggested that the performance of MPAS-A is comparable as WRF, given MPAS-A runs are initial value predictions while WRF runs are dynamically downscaled from ERA-interim. Further investigation is taken and it suggests that the underestimation of the TC intensity by MPAS-A is probably related to the inferior representation of storms in the ERA-interim initial fields. The paper also points out that the track forecast accuracy of MPAS-A in TC is sensitive to the grid resolution in the coastal part of the variable-resolution mesh used.

Featured:
Liu, Y.L., C.-Y. Tam and A.Y.M. Au-Yueng, 2021: Sensitivity of western north Pacific summertime tropical synoptic-scale disturbances to extratropical forcing – A regional climate model study, J. Meteor. Soc. Japan, 100, DOI: https://doi.org/10.2151/jmsj.2022-008

Summary:
Parallel integrations of the RegCM are performed to study the role of extratropical forcing on the summertime tropical synoptic-scale disturbances (TSDs) in the western north Pacific. A control experiment (CTRL) with ECMWF Reanalysis data as boundary conditions, and an experimental run (EXPT) with the same setting, except that signals with zonal wavenumber > 6 are suppressed at the northern boundary of the model domain, are performed. The results comparison shows that the TSD activity in JJA is weakened with reduced precipitation over its pathway with the absence of extratropical forcing. Hence, less active MRG waves in the equatorial western Pacific area shall be observed. Further analysis is carried out and finds that the TSD wavetrain and the coherence structure can be modulated by the extratropical forcing and associated circulation changes. From the energetics perspective, the changes in low-level background circulation can influence TSD characterisitics. Theses results quantify how extratropical forcing and related general circulation features influence western north Pacific summertime TSD activities. Discussion on the implications on understanding the initiation of TSD and their variability on longer time scales is carried out in this paper.

Featured:
So, K. W., C.-Y. Tam and N.-C. Lau, 2021: Impacts of global warming on Meiyu-Baiu extreme rainfall and associated mid-latitude synoptic-scale systems as inferred from 20km AGCM simulations, Climate Dynamics 59, 1849-1861, DOI: https://doi.org/10.1007/s00382-021-06072-5

Summary:
The impacts of global warming on Meiyu-Baiu extreme rainfall and the associated mid-latitude synoptic-scale weather systems over the Eastern China (EC) and the Baiu rainband (Bu) regions in the East Asia are studied with the aid of MRI-AGCM3.2S. To identify the upper-level propagating wave signals together with the presence of extreme precipitation in either EC or Bu, a wave-selection algorithm is used based on the 300hPa wind. The climate simulations for the present and future is performed using the AGCM to infer the impacts of global warming. It is found that there is obvious decrease of intensity of systems influencing both Bu and EC in the future climate. It is suggested to be related to the changes in the background circulation in future over the East Asian mid-latitude zone. Nonetheless, the wave-associated precipitation over the regions is enhanced which can be attributed to more strong intensity rainfall. These results reveal that the thermodynamic effect plays the major role in changing the wave-related extreme precipitation in EC and Bu.

Oscar Tam is a graduated MPhil student co-supervised by Prof Amos TAI and Prof Francis TAM.

Before joining EASC, he completed his BSc study in the Department of Physics, The Chinese University of Hong Kong, where he polished computational skills and genuine interest in meteorology.

During his MPhil study, he studied how regional climate and air quality would respond to irrigation using WRF coupled with GEOS-Chem (WRF-GC) over semiarid Northwest China.

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