Simulations of Tropical Storms over the Pacific and the Atlantic Using a Coupled Regional Climate Model

J. Hsieh, R. Saravanan, P. Chang, H. Seidel

To better simulate the air-sea interaction phenomena, e.g., the formation of tropical storms in the Atlantic, a coupled regional climate model (CRCM) is developed by coupling the Advanced Weather Research & Forecasting Model (WRF-ARW 2.2) to the Regional Ocean Modeling System (ROMS 3.0). The coupling between WRF and ROMS is conducted by exchanging the surface heat & momentum fluxes with sea surface temperature (SST) between these two models every 6 hours of time integration.

The domain for WRF simulation covering the Atlantic and part of the eastern Pacific consists of 466x460 grid points with 30-km horizontal resolution and 28 vertical levels. The similar domain for ROMS over the Atlantic is composed of 482x342 grid points with 1/4° horizontal resolution and 30 vertical layers. In contrast to the SST provided by ROMS over the Atlantic, the observed SST climatology is used as the SST input for WRF over the Pacific ocean. In this simulation, the monthly SST climatology was linearly interpolated into 6-hourly time series to include the monthly variation of SST in the eastern Pacific. The time integration of the coupled model was initiated on January 1st and ran through September using 128 processors on the IBM P575 supercomputer (Hydra) at the Texas A&M Supercomputing Facility.

Preliminary results have demonstrated the ability of CRCM to realistically capture the major climate and weather features over the Atlantic ocean, such as the inter-tropical convergent zone (ITCZ), SST distribution and the formation of tropical storms (hurricanes) from African easterly waves. The figures below display the simulated mean ITCZ, as manifested by rainfall in the left figure, and SST averaged from February to August in the right figure. A robust ITCZ strip north of the equator is well simulated with respect to its location and intensity except for some cold biases of SST near the Caribbean sea. In addition, winter storm tracks in the northern Hemisphere also appear in the coupled simulation, as indicated by the relatively weak precipitation between 30 ° and 40 ° N in the northern Hemisphere.

Simulated Feb-Aug mean rainfall and surface winds Simulated Feb-Aug mean surface temperature
Figure 1: (left) Simulated Feb-August mean rainfall (mm/day) and surface winds at 10 m. (right) Simulated Feb-August mean surface temperature (°C)

Figure 2 shows the formations of tropical storms over the Atlantic and the eastern Pacific. The one in the Atlantic shows the transition process from a intensified trough of African easterly waves to a tropical storm over the Caribbean, which can be demonstrated by heavy rainfall east of the trough on 19 July and rainfall surrounding the vortex on 21 July. On the other hand, the tropical storm over the eastern Pacific appears to form near the ITCZ strip. Strengthening rotational winds cause the undulation of the ITCZ strip on 19 July, as suggested by the simulated precipitation.

Figure 2: (Left) Simulated rainfall (color shading interval with unit of mm/day) and surface winds (mm/day) at 10 m on 19 July. (Middle) the same as above but for 20 July. (Right) the same for 21 July.

Figure 3 displays the simulated cloud distribution and the streamlines of surface winds at 10m. The spiral cloud bands, as shown by the vertically averaged cloud water mixing ratio in the model, demonstrate the significant swirling characteristics of tropical storms.

Figure 3: (Left) Simulated vertically averaged cloud water mixing ratio (shading interval with the scale of 10-5 kg/kg) and surface winds streamlines at 10 m on 19 July. (Middle) the same as above but for 20 July. (Right) the same for 21 July.

In addition to these atmospheric responses, the well-known oceanic loop current eddies through the Yucatan Channel and the Gulf Stream along the eastern coast of the US are both captured by CRCM, as shown below.

Figure 4: (Left) Simulated Sea Surface Temperature (SST) (color shading interval of 1 °C) and ocean current (m/s) on 20 July. (Right) Simulated sea surface elevation, as indicated by shading interval of 0.1 m. The magnitude of flow vector is indicated by the arrow below the figure.