Sensitivity of high-resolution tropical cyclone intensity forecasts to surface flux parameterization

Surface flux parameterization schemes used in current dynamic models are primarily based upon measurements at low and moderate wind speeds. Recent studies show that these parameterization schemes may be incorrect at high wind speeds (e.g., tropical cyclone forecasts). Five high-resolution numerical model experiments are designed to assess the sensitivity of tropical cyclone intensity forecasts to changes in the surface flux parameterization. The sensitivity experiments are conducted by running 48 h forecasts of the Coupled Ocean/Atmosphere Mesoscale Prediction System (COAMPS®) for 6 selected tropical cyclones with individual modifications to surface flux calculation that include: (1) limiting the surface stress for wind speeds greater than 33 m s^-1; (2) modifying the stress calculation at low levels for wind speeds greater than 33 m s^-1 to mix the results from surface layer similarity and turbulence mixing parameterization; (3) increasing the roughness lengths for heat and moisture transfer by a factor of ten; (4) setting the roughness lengths for heat and moisture transfer to 1/10 of the momentum roughness length; and (5) cooling the sea surface temperature by a prescribed rate at high winds. Averaged responses for the six storms to these sensitivity tests show that: (a) the limit on surface stress at high winds significantly increases the cyclone intensity in a 48 h forecast; (b) the modified surface layer stress at high winds increases the cyclone intensity but to a much lesser degree than limiting the surface stress; (c) large increases in the roughness lengths for heat and moisture transfer are needed to significantly impact the intensity forecast; and (d) cooling of the SST by -5.8 ^oC in 48 hours reduces the maximum surface wind speed by –32 knots at 48 h forecast. All modifications to surface flux calculation have little influence on 48 h track forecasts, even though they may significantly impact the intensity forecasts.