Revisiting the Contributions of Surface Sensible and Latent Heat Fluxes to the Outer Spiral Rainbands of Tropical Cyclones

Abstract

The separate effects of the sensible heat flux (SHF) and latent heat flux (LHF) on the outer spiral rainbands of tropical cyclones (TCs) have not received sufficient attention. This study examines the separate contributions of the SHF and LHF to the outer spiral rainbands of TCs via a series of sensitivity experiments using the three-dimensional cloud-resolving Weather Research and Forecasting model. The results indicate that removing the outer SHF suppresses the activity of the outer spiral rainbands and results in a stronger and smaller-scale TC, whereas decreasing the outer LHF by the same amount has only a slight impact on the outer spiral rainbands. Further investigations indicate that the positive radial gradient of the potential temperature at the cold-pool outer edge is crucial for the strength of outer spiral rainbands. The strong positive radial gradient of the potential temperature gives rise to a negative radial gradient of the horizontal pressure at the cold-pool outer edge, and thus an outward radial pressure gradient force. As a result, strong outflow exists near the cold-pool outer edge, producing horizontal convergence and eventually lifting air to generate new convective cells. Distinct from the LHF, the SHF directly modulates the potential temperature immediately outside the cold pool and thus the radial gradient of the potential temperature at the cold-pool outer edge. Removing the outer SHF leads to a significantly lower potential temperature outside the cold pool, which is averse to the formation of a larger radial gradient of the potential temperature at the cold-pool outer edge.