Solving the Atmosphere: Case Study of Tropical Storm Vamei Tieh-Yong Koh School of Physical & Mathematical Sciences, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore. Email: [email protected]

The atmosphere, like any other fluid, is governed by a few fundamental physical principles: the conservation of mass, the First Law of Thermodynamics and Newton’s Second Law. Thus, one can easily write down a set of partial differential equations for the atmosphere and hope to solve for the atmosphere’s behaviour from there. Or can one?

Apart from the general mathematical problems associated with solving the hydrodynamical equations, additional challenges to solving the atmosphere arise from: (1) the discretization of the problem in numerical solutions, hence demanding unresolved processes to be parameterized; and (2) from the representation of sources and sinks for mass, heat and momentum in the atmosphere and on the Earth’s surface. Moreover, because of the chaotic nature of atmospheric dynamics, meteorological observations need to be assimilated into the numerical solutions to keep them from diverging too much from reality. We shall explain the theory and the practice of solving the atmosphere through the realistic simulation of a case of extreme weather in Southeast Asia.

On 27 Dec 2001, Vamei landed on the southern tip of the Malayan Peninsula, bringing heavy rain and widespread flooding in the southern Malaysian state of Johor and nearby Singapore. While tropical cyclones or are no strangers in some countries in Southeast Asia, e.g. the Philippines, the occurrence of Vamei at the very low latitude of 1.5 degrees north is virtually unheard of before (Chang et al. 2003).

The Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) was used to make a high-resolution simulation of the regional weather for the period 17 – 29 Dec 2001. Navy Operational Global Atmosphere Prediction System (NOGAPS) outputs were used to initialize and to provide boundary conditions for the model run. A host of observational data was assimilated every 12 hours. Description of the COAMPS model can be found in Hodur (1997).

A tropical cyclone was successfully spun up in the COAMPS model simulation. To verify the simulated track of the cyclone, we apply an objective criterion to identify the centre of circulation in the lower troposphere, essentially following the same principle as in Power and Davis (2002). The cyclone track so derived is compared to published best-track data of Vamei from the Joint Warning Center in Hawaii. This gives us a quantitative assessment of how good our numerical solution of the partial differential equations for the atmosphere is in this case.

References

[1] C.-P. Chang, C. H. Liu and H. C. Kuo (2003), Geophysical Research Letters, 30(3), 1150. [2] R. H. Hodur (1997), Monthly Weather Review, 125, 1414-1430. [3] J. G. Powers and C. A. Davis (2002), Atmospheric Science Letters, 3, 15-24.

Note: COAMPS® is a registered trademark of US Naval Research Laboratory.

Tropical Storm Vamei was a Pacific tropical cyclone that formed closer to the equator than any other tropical cyclone in the Pacific Ocean. The last storm of the 2001 Pacific typhoon season, Vamei developed on December 26 at 1.4° N in the South China Sea. It strengthened quickly and made landfall along extreme southeastern Peninsular Malaysia. Vamei rapidly dissipated over Sumatra on December 28, and the remnants eventually re-organized in the North Indian Ocean. A New Coupled Ocean-Waves-Atmosphere Model Designed for Tropical Storm Studies: Example of Tropical Cyclone Bejisa (2013–2014) in the South-West Indian Ocean. J. Pianezze1 , C. Barthe1 , S. Bielli1 , P. Tulet1 , S. Jullien2 , G. Cambon2, O. Bousquet1, M. Claeys1, and E. Cordier3. Most studies about sea spray in tropical cyclones have focused on their impact on the ocean-atmosphere uxes (Andreas & Emanuel, 2001; Bao et al., 2000; Fairall et al., 1994; Wang et al., 2001). It is a nonhydrostatic anelastic model, able to simulate the motion of the atmosphere, at scales ranging from syn-optic (hundreds of kilometers) to microscale (tens of meters). 5.2 The storm track of Tropical Storm Vamei retrieved from JTWC best track database. 5.3 (a) Coastal topography and (b) depth isobaths (m) of study domain. 5.4 Atmospheric condition during Tropical Cyclone Vamei generated by MM5; (a) 18 UTC 26 Dec 2001 (b) 00 UTC 27 Dec 2001 (c) 06 UTC 27 Dec 2001 (d) 12 UTC 27 Dec 2001 (e) 18 UTC 27 Dec 2001 and (f) 22 UTC 27 Dec 2001. By incorporating the atmospheric data which has been downscaled by the MM5 earlier, TUNA-SU will simulate these two storm surge cases. We will also present the simulation results such as surge. Tropical Storm Vamei was a Pacific tropical cyclone that formed closer to the equator than any other tropical cyclone in Pacific Ocean. The last storm of the 2001 Pacific typhoon season, Vamei developed on December 26 at 1.4° N in the South China Sea. It strengthened quickly and made landfall along extreme southeastern Malaysia. Vamei rapidly dissipated over Sumatra on December 28, and the remnants eventually re-organized in the North Indian Ocean.