Paper looking at how model forecasts of what is happening near the ocean surface in tropical cyclones changes as the space between forecast points gets smaller was released online in Monthly Weather Review

The energy that fuels tropical cyclones comes from heat and moisture from the warm ocean below.  This energy is transferred from the ocean to the atmosphere and the momentum is transported from atmosphere to surface by what we call turbulent processes in the atmosphere near the ocean surface (what we call the planetary boundary layer or PBL).  The size of the turbulence is sometimes less than 100 yards across, or comparable to the length of a football field.  However, the computer models we use to predict the weather can only do so on points (called grid points) at least ten times that size. Because we cannot directly predict the turbulent processes in the models, we use a process called parameterization to mimic what is happening on these small scales.

Visible satellite image of Hurricane Earl.

As computers become larger and faster, the spacing between forecast points gets smaller and is now approaching the size of these turbulent processes.  As the difference between these two sizes gets small, we enter what we call the gray zone, where some of the processes, but not all, can be directly forecast by the models.  This can cause problems with the current parameterization schemes, so new scale-aware PBL schemes that understand this problem have recently been developed.  

By performing numerical simulations of Hurricane Earl (2010) with grid spacings less than 1 km, this study investigates the effect of a new scale-aware (Shin-Hong – SH) PBL scheme on TC intensity and structural changes compared to the traditional non-scale-aware version (Yonsei University  – YSU) it is built upon. 

Fig. 1.  (a) maximum wind speed (m s−1), and (b) 10-m radius of maximum wind (RMW) speed (km) from the CTL-YSU (using the YSU PBL scheme), and CTL-SH (using the SH PBL scheme) experiments. Note that the SH scheme makes Earl stronger and have the strongest wind closer to the center than the YSU scheme.

Important Conclusions:

  1. The new scale-aware PBL scheme tends to forecast a stronger TC with a smaller core  (where the strongest wind and heaviest rainfall occur) than the traditional PBL scheme (Fig. 1).  
  2. The new scale-aware scheme comes into effect when the depth of the PBL is bigger than the distance between grid points. When this happens, the strength of the turbulent mixing in the PBL decreases.  
  3. In SH, the reduced vertical mixing of momentum near the surface causes stronger flow from outside the core into it (called radial inflow); meanwhile, the reduced mixing of water vapor help retain more moisture in the PBL. The resulting convergence of moisture in the PBL and convective activity inside the core causes a faster decrease of the RMW before rapid intensification (RI) begins and a faster intensification rate during RI (Fig. 2). 
Fig. 2. A flow chart describing the effect of the scale-aware scheme on intensity and structure.   Black arrows show whether the variable mentioned increases or decreases.  The brown arrows show the direction of how one variable impacts the next.

For more information on this study, contact aoml.communications@noaa.gov.

The full study can be found at https://journals.ametsoc.org/view/journals/mwre/aop/MWR-D-20-0297.1/MWR-D-20-0297.1.xml. This study was supported by the National Key R&D Program of China, the Natural Science Foundation of China. Xiaomin Chen was supported by the NRC Research Associateship Programs. Numerical simulations were performed at the High Performance Computing Center of Nanjing University.

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