Satellites Suggest Rising Tropical High Cloud Altitude: 2002–2021

Abstract

Warming commonly causes high clouds to rise in cloud-resolving models (e.g., Kuang & Hartmann, 2007) and Global Climate Models (e.g., Soden & Vecchi, 2011). All else being fixed, increased cloud top height amplifies warming by suppressing changes in cloud top longwave radiative cooling to space. Furthermore, clouds respond to the detailed spatial structure of temperature and dynamical changes (Andrews et al., 2015;Su et al., 2017;Zhou et al., 2016), so responses to internal variability, such as the El Nino Southern Oscillation (ENSO), and longterm climate change may differ. It is desirable to distinguish between such changes to comprehensively evaluate observed cloud-height changes and model performance. To that end, here we use the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments on NASA's Terra and Aqua satellites to simultaneously diagnose the linear trend and El Nino Southern Oscillation (ENSO)-correlated response of high-altitude cloud heights.The canonical fixed anvil temperature (FAT, Hartmann & Larson, 2002) hypothesis is that the altitude of maximum radiative cooling by water vapor in clear-sky conditions rises under warming as governed by the Clausius-Clapeyron relation, thereby driving anvil outflow at those levels (Boucher et al., 2013). However, full-physics climate models (Zelinka & Hartmann, 2010) and cloud-resolving models (Seeley et al., 2019) support a Proportionally Higher Anvil Temperature (PHAT) behavior where anvil clouds warm, albeit by less than the surface. Observational evidence links a warmer surface to higher cloud altitudes, although this is often limited to interannual variation rather than trends (e.g., Igel et al., 2014).High-cloud feedback also depends on changes in cloud fraction (CF) and optical depth. Multiple factors can change simultaneously, for example, some models show increased altitude and reduced fraction of tropical high clouds under warming (Bony et al., 2016;Cronin & Wing, 2017). A long-running hypothesis has been a

Observational Quantification of Tropical High Cloud Changes and Feedbacks

“…Using CALIPSO satellite observations, we showed that tropical high clouds are rising. This builds on several recent studies that also found a rise in tropical high clouds using passive and active sensors as well as studies that projected their upward shift associated with warming (Aerenson et al, 2022;Chepfer et al, 2014;Höjgård-Olsen et al, 2022;Norris et al, 2016;Richardson et al, 2022;Saint-Lu et al, 2020;Takahashi et al, 2019). We show that this surface warming-induced rise elicits a vertical dipole in cloud cover, consistent with the FAT mechanism.…”

Has there been a recent shallowing of tropical cyclones?

“…Davies et al (2017) showed that the global cloud height in MISR did not have a significant trend during 2000-2015. For the tropics, Richardson et al (2022) found that the height of the tropical upper-tropospheric clouds (regardless of thickness) as measured by MODIS was increasing from 2002 to 2021. Hence, the declining trend of TC inner-core CTH implied in this study is quite anomalous and remarkable.…”

Has There Been a Recent Shallowing of Tropical Cyclones?

“…For the tropics, Richardson et al. (2022) found that the height of the tropical upper‐tropospheric clouds (regardless of thickness) as measured by MODIS was increasing from 2002 to 2021. Hence, the declining trend of TC inner‐core CTH implied in this study is quite anomalous and remarkable.…”