Hurricanes and cloud–radiation interaction

The clouds of tropical cyclones strongly interact with both infrared (or longwave) and solar radiation.  The interaction with longwave radiation — referred to as the “cloud–greenhouse effect” (see figure) – accelerates their formation by heating the storm core.  In our recent paper, we found that removing this effect either delays or completely prevents intensification in both Super Typhoon Haiyan (2013) and major Hurricane Maria (2017).  Our work on this subject was recently highlighted in Science.  Our recent talk on this work from the 2021 AMS Annual Meeting is available here.

Additionally, the interactions of the clouds of TCs with solar radiation leads a dramatic diurnal cycle, which manifests in their clouds, rain bands, and transverse circulation.  Our recent paper on this subject was featured in an Eos Research Spotlight.

Cloud–greenhouse effect accelerates hurricane formation.
From Ruppert et al. (2020, PNAS)

Diurnally phase-locked gravity waves over the Maritime Continent. From Ruppert and Zhang (2019, JAS)

The Maritime Continent

An excellent example of scale interaction is the Maritime Continent – the island archipleago that includes Indonesia.  We have recently examined how diurnal deep convection here excites long-lived gravity waves of a scale ~1500 km, which diurnally phase lock over multiple islands (Borneo and Sumatra), linking the diurnal burst of convection in one island with that of another on the next day. I am currently investigating mechanisms of climatological island rainfall enhancement in this region through such feedbacks.

Gravity waves and diurnal circulation coupling

Through interactions between clouds and radiation, the diurnal cycle drives pronounced mesoscale circulation changes in organized convective systems. This occurs because of how rapidly gravity waves propagate, which results in a rapid circulation adjustment into balance with changes in latent and diabatic heating. Our research on this subject suggests that this effect causes a diurnal cycle of the Hadley Cell and the ITCZ coupled with it.

Time scale of gravity wave adjustment Ruppert and Hohenegger (2018, J Climate)

How the diurnal cycle accelerates the transition to deep convection Ruppert (2016, JAMES)

Diurnal cycle and time-scale feedbacks

I am greatly fascinated by how clouds and moist convection create links across scales, both in space and time. An excellent example of “time-scale feedback” is the effect that the 24-hr diurnal cycle has on the background weather and climate states in which it operates. We have studied this problem using data from the international DYNAMO field campaign in relation to the Madden–Julian oscillation. Through controlled hypothesis testing using cloud-resolving numerical modeling, we managed to identify how this diurnal time-scale feedback works: namely, the covarying diurnal cycles of humidity and static stability lead to a more rapid transition from shallow to deep convection than without the diurnal cycle.