April 16, 2021

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Paper on improving model forecasts through understanding of the region near the ocean surface released online in Earth and Space Science – Hurricane Research Division

3 min read

An important tool that forecasters use to predict the weather is a set of computer models that forecast things such as the wind, temperature, moisture, and pressure on a set of places called grid points.  In NOAA’s models to predict hurricanes, the distance between these points is about 1-3 km. Although hurricanes are hundreds of kilometers across and can be bigger than the state of Texas, the strongest wind occurs in gusts that are only a few meters across. So, the computer models cannot see these gusts.

These gusts are associated with swirling flow and mix the heat, moisture, and momentum (mass and velocity) between the ocean and the atmosphere nearby, and in the part of the atmosphere closest to the surface (what we call the boundary layer).  To forecast them, their average effects are included in the computer models using what we call parameterization schemes, especially near the ocean and land surface where hurricanes get heat and moisture that fuels their engine. However, different boundary parameterization schemes produce different forecasts, some of which could be far away from reality.  

The sensitivity of the forecasts of Hurricane Michael in October 2018 to the eddy diffusivity parameters for (a) track and (b) minimum sea level pressure (intensity) and how when the eddy diffusivity is calibrated using flight-level observations, the forecasts improve. The original scheme, EDMF-GFS (green) gets the track right, but not the intensity.  A new scheme, MEDMF-GFS (red) is the opposite.  The study concluded that a new scheme, called MEDMF-TKE (purple) provided the best estimates for forecasting tracks, intensity, and other things like where the strongest winds occur.

We used the new Hurricane Analysis and Forecast System (HAFS), part of NOAA’s Unified Forecast system, and observations from NOAA Hurricane Hunter aircraft to understand why two different boundary layer parameterization schemes make different forecasts, and how well the schemes forecast what is actually happening in the boundary layer.

Changes in eddy diffusivity with wind speed 500 m above the surface from the HAFS model for different schemes, compared with flight-level data from NOAA Hurricane Hunter aircraft (shown in black). EDMF-GFS (green) is the existing scheme in NOAA’s Global Forecast System (GFS), EDMF-TKE (blue) is a more advanced scheme proposed to be used in the future.  Both MEDMF-GFS (Red) and MEDMF-TKE (purple) show both these schemes updated to match the data from Hurricane Hunter aircraft.  Note that the red and purple points correspond to the black data points better than the green and blue ones do.

Important Conclusions:

●  The amount of mixing that the parameterization schemes produce is controlled by a variable known as the eddy diffusivity.  Small changes to the eddy diffusivity sometimes lead to large changes in forecasts which could deviate from reality.

●  The amount of mixing, and thus the eddy diffusivity, can be calculated based on observations from NOAA Hurricane Hunter aircraft.  Modified schemes using these values can improve forecasts. 

For more information, contact aoml.communications@noaa.gov. You can read the study at https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2020EA001422.

noaahrd

2021-03-09 17:44:12

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