Asst Prof KOH Tieh Yong
1. Numerical weather prediction in Southeast Asia
In mid-latitude regions, numerical weather prediction (NWP) has reached unprecedented skill in that state-of-the-art predictions provide reliable 3-day local forecasts and reasonable 1-week regional weather outlooks. However, forecasts of tropical weather are hardly reliable even within the same day. The reasons are many-fold: e.g. (1) the dearth of atmospheric soundings (observations) in tropical regions; (2) the inherent shorter spatio-temporal scales of convective weather; and (3) insufficient understanding of tropical weather dynamics. In collaboration with Temasek Laboratories @ NTU, we work as a team on improving NWP in Southeast Asia, using the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®), developed by the US Naval Research Laboratory (Monterey).
Figure 1 shows COAMPS simulation of typhoon Vamei hitting the southern tip of the Malay Peninsula and Singapore on 27 Dec 2001 at 0600 UTC (1400h Singapore Time). Arrows denote 850mb-winds and shading (with the zero contour in magenta) represent the absolute vorticity (a measure of local rotation) in the flow. This is one example of an extreme-weather event that can bring disaster to the region and so there is a need for reliable forecasts.
2. Pollutant transport in highly convective equatorial atmospheres
The motion of a particle in the atmosphere is inherently chaotic, meaning that the particle trajectory is sensitively dependent on initial conditions, even in seemingly regular flows. Thus, it is hard to understand atmospheric transport patterns without the aid of Lagrangian diagnostic tools. In the tropics, the weather is dominated by deep cumulus convection, which chaotically mixes and transports substances vertically between the surface boundary layer and up to 16km high. How chemical tracers, e.g. pollutants, are dispersed vertically in equatorial atmospheres constitute a scientifically interesting and environmentally important subject of investigation.
Figure 2 shows a cloud of tracers representing haze smoke emitted from central Sumatra island being transported eastwards by lower tropospheric winds (magenta streamlines) to Borneo island. The haze is mixed into the surface boundary layer by convective turbulence. The white contours on the surface denote regions where boundary layer heights greater than 1000m.
3. Fundamental geophysical fluid dynamics
Geophysical fluid dynamics (GFD) is the study of the behaviour of the atmosphere and the ocean as fluid bodies, governed by the basic laws of the conservation of momentum, energy and mass. As an applied mathematics discipline, the field has seen tremendous advances over the last few decades. Rigorous theories of slow and fast manifold dynamics in the phase space of these fluids have been discussed in international conferences in Europe and the USA in recent years. This recent advancement in GFD theory is based on Lagrangian and Hamiltonian formulations of fluid mechanics.
deformation of the 286K-isentropic surface in an order-6 symmetric baroclinic wave assuming dry dynamics. The horizontal plane represents a stereographic projection of the Earth’s surface from the equator to the pole. . As atmospheric motion is largely adiabatic (especially when water vapour and clouds are considered within the atmosphere’s entropy budget), isentropic coordinates is the most appropriate framework to analyze and understand the fluid dynamics of the atmosphere.