Satellite Observed Sensitivity of Tropical Clouds and Moisture to Sea Surface Temperature on Various Time and Space Scales: 1. Focus on High Level Cloud Situations Over Ocean

Abstract

This study examines variations in high cloud properties and relative humidity (RH) with sea surface temperature (SST) over tropical oceans (30°N–30°S) using spaceborne lidar and microwave radiometer observations. The mean values over the tropics indicate that middle‐tropospheric RH increases, high cloud covers decrease and cloud altitudes rise with SST. These signatures are consistent with the hypotheses proposed in the literature. The analysis of this same data set but at the scale of local processes shows different behaviors for SSTs <302 K and SSTs >302 K. Between 299 and 302 K, middle‐tropospheric RH, opaque cloud cover, and cloud top altitude increase together with SST, while optically thin cloud cover decreases. Over SSTs >302 K, middle‐tropospheric RH, opaque cloud cover, and opaque cloud top altitude decrease with SST, while the cover of optically thin clouds increases. Interestingly, the altitude of high clouds (not the cloud top) increases monotonically with SST from 299 to 305 K on a range of space and time scales, and the altitude of optically thin clouds remains higher than that of opaque clouds. The observed relationships on different time and space scales are compared to simulations of a global atmospheric model. Despite systematic biases, the model reproduces the sensitivity of the middle‐tropospheric RH and cloud altitude to SST rather well but fails to reproduce the variations of the balance between high opaque cloud cover and high optically thin cloud cover.

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.…”

“…Most studies, supported by modeling and theory, suggest a decrease with warming albeit without consensus on magnitude (e.g., Beydoun et al., 2021; Bony et al., 2016; Wing et al., 2020; Zelinka & Hartmann, 2010). Only a few studies have brought observations to bear on this particular problem (Chepfer et al., 2014; Del Genio et al., 2005; Höjgård‐Olsen et al., 2022; Igel et al., 2014; Saint‐Lu et al., 2020).…”

Anvil cloud thinning implies greater climate sensitivity

“…7), demonstrating that this thinning can occur regardless of whether f ice increases, decreases, or stays the same. The thinning is qualitatively consistent with recent observational and model-based analyses [8,16,21,39,40].…”

Basic physics predicts stronger high cloud radiative heating with warming

“…Observational studies have confirmed altitude shifts of clouds in response to warmer temperatures based on interannual climate variability [39][40][41][42], while first evidence for long-term shift trends is also emerging from the climatic variability [43]. These findings suggest that the altitude-driven CRH change may already influence the role of high clouds in the climate system beyond their role in interannual variability.…”