In recent years, the importance of cloud–radiation interactions in controlling the intensity and structural evolution of tropical cyclones (TCs) have been increasingly recognized. Clouds influence the atmospheric thermal structure by reflecting and absorbing incoming shortwave solar radiation and by absorbing and re-emitting longwave (infrared) radiation from the surface and the lower atmosphere. In particular,upper stratiform clouds, that extensively cover the upper part of a TC and are primarily composed of ice particles (cloud ice), are considered to have a substantial impact on the atmospheric temperature structure. However, the physical mechanisms by which radiative processes associated with cloud ice in these upper stratiform clouds influence TC development remain poorly understood.

The purpose of this study is to clarify the impacts of radiative processes associated with cloud ice in upper stratiform clouds on TC intensity and structural evolution. Idealized experiments were conducted using the Cloud Resolving Storm Simulator (CReSS), comparing experiments that considered the longwave radiative processes of cloud ice with those that neglect radiative processes. The results indicate that radiative processes of cloud ice play distinct roles depending on the TC developmental stage.

During the early developmental stage, longwave radiative processes associated with ice clouds promoted TC intensification and expanded the upper stratiform clouds region. Radiative heating near the cloud base destabilizes the stratiform cloud layer, inducing upward motion near the cloud base and strengthening upper-level outflow through mass continuity. The compensating midlevel inflow associated with the enhanced outflow transports large absolute angular momentum from the outer region toward the storm center, leading to an increase in tangential wind speed in the midlevel of the eyewall and thereby accelerating TC intensification. In addition, the enhanced updrafts near the cloud base and strengthened upper-level outflow facilitate efficient cloud ice production and outward radial advection, resulting in a broader radial distribution of cloud ice.

In contrast, during the later developmental stage, longwave radiative processes associated with ice clouds suppressed the maximum TC intensity while increasing TC size, defined by the extent of the strong wind region. Radiative heating by cloud ice near the cloud base induces upward motion and promotes the horizontal expansion of the upper-level outflow region. Beneath the upper stratiform clouds, subsidence is suppressed and convective activity is enhanced. As a result, extensive rainbands develop outside the eyewall, consuming low-level water vapor and reducing the moisture supply to the eyewall, thereby inhibiting further intensification. Moreover, the inward transport of absolute angular momentum by the midlevel inflow is blocked by the outer spiral rainbands, and conservation of angular momentum leads to an enhancement of tangential wind speeds in the outer region, contributing to an increase in storm size.

In conclusion, this study reveals that cloud-base heating associated with the longwave radiative processes of cloud ice in upper stratiform clouds exhibits a dual role depending on the developmental stage: promoting intensification through the enhanced midlevel inflow and increased absolute angular momentum supply to the eyewall in the early stage, and suppressing intensification by reducing the moisture supply to the eyewall through the suppression of subsidence outside the eyewall in the later stage. These findings highlight the importance of accurately representing cloud ice radiative processes in understanding and predicting changes in TC intensity and structure.