Comment on acp-2021-1038

doi:10.5194/acp-2021-1038-rc1

Abstract

<strong class="journal-contentHeaderColor">Abstract.</strong> Ice clouds play an important role in regulating water vapor and influencing the radiative budget in the atmosphere. This study investigates stratospheric ice clouds (SICs) in the latitude range between <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M1" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>Â±</mo><mn mathvariant="normal">60</mn><msup><mi/><mo>â</mo></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="26pt" height="11pt" class="svg-formula" dspmath="mathimg" md5hash="3f386b3c4c65d74f8ea84f8ccc9562ae"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="acp-22-6677-2022-ie00001.svg" width="26pt" height="11pt" src="acp-22-6677-2022-ie00001.png"/></svg:svg></span></span> based on the CloudâAerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). As polar stratospheric clouds include other particles, they are not discussed in this work. Tropopause temperature, double tropopauses, clouds in the upper troposphere and lower stratosphere (UTLS), gravity waves, and stratospheric aerosols are analyzed to investigate their relationships with the occurrence of and variability in SICs in the tropics and at midlatitudes. We found that SICs with cloud-top heights of 250âm above the first lapse rate tropopause are mainly detected in the tropics. Monthly time series of SICs from 2007 to 2019 show that high occurrence frequencies of SICs follow the Intertropical Convergence Zone (ITCZ) over time in the tropics and that SICs vary interannually at different latitudes. Results show that SICs associated with double tropopauses, which are related to poleward isentropic transport, are mostly found at midlatitudes. More than 80â% of the SICs around 30<span class="inline-formula"><sup>â</sup></span>âN/S are associated with double tropopauses. Correlation coefficients of SICs and all the other abovementioned processes confirm that the occurrence of and variability in SICs are mainly associated with the tropopause temperature in the tropics and at midlatitudes. UTLS clouds, which are retrieved from the Atmospheric Infrared Sounder (AIRS) and used as a proxy for deep convection in the tropics and high-altitude ice cloud sources at midlatitudes, have the highest correlations with SICs in the monsoon regions and the central United States. Gravity waves are mostly related to SICs at midlatitudes, especially over Patagonia and the Drake Passage. However, the second-highest correlation coefficients show that the cold tropopause temperature, the occurrence of double tropopauses, high stratospheric aerosol loading, frequent UTLS clouds, and gravity waves are highly correlated with the SICs locally. The long-term anomaly analyses show that interannual anomalies of SICs are correlated with the tropopause temperature and stratospheric aerosols instead of the UTLS clouds and gravity waves. The overlapping and similar correlation coefficients between SICs and all processes mentioned above indicate strong associations between those processes themselves. Due to their high inherent correlations, it is challenging to disentangle and evaluate their contributions to the occurrence of SICs on a global scale. However, the correlation coefficient analyses between SICs and all abovementioned processes (tropopause temperature, double tropopauses, clouds in the upper troposphere and lower stratosphere (UTLS), gravity waves, and stratospheric aerosols) in this study help us better understand the sources of SICs on a global scale.

Highlights

Stratospheric ice clouds (SICs) play an important role in regulating the water vapor in the upper troposphere and lower strato35 sphere (UTLS), i. e., ice cloud formation and sedimentation may dehydrate the UTLS (Jensen and Pfister, 2004; Schoeberl and Dessler, 2011; Schoeberl et al, 2019), while injection of convective clouds and sublimation of ice in the lower stratosphere would hydrate stratosphere (Dinh et al, 2012; Jain et al, 2013; Avery et al, 2017)
The weakest signal of stratospheric ice clouds (SICs) over the tropics occurs in boreal summer (JJA), when the hotspots of SICs are shifted to the north of the equator over the Asian Monsoon and North American Monsoon
A SIC is defined as an ice cloud with a cloud top height 0.25 km above the first thermal tropopause derived from ERA5 temperatures in this study

Summary

Introduction

Stratospheric ice clouds (SICs) play an important role in regulating the water vapor in the upper troposphere and lower strato sphere (UTLS), i. e., ice cloud formation and sedimentation may dehydrate the UTLS (Jensen and Pfister, 2004; Schoeberl and Dessler, 2011; Schoeberl et al, 2019), while injection of convective clouds and sublimation of ice in the lower stratosphere would hydrate stratosphere (Dinh et al, 2012; Jain et al, 2013; Avery et al, 2017). Stratospheric ice clouds (SICs) play an important role in regulating the water vapor in the upper troposphere and lower strato sphere (UTLS), i. SICs are important indicators for better understanding the vertical temperature struc ture in the UTLS, transport between troposphere and stratosphere, intensity and dynamics of deep convection (Liou, 1986; Corti et al, 2006; Mace et al, 2006; Jensen et al, 2011; Kärcher, 2017). More and more studies have demonstrated the existence of SICs from in-situ measurements, satellite measurements and ground-based lidar observations (Wang et al, 1996; Keckhut et al, 2005; De Reus et al, 2009; Dessler, 2009; Spang et al, 2015; Bartolome Garcia et al, 2021). Several cases of ice clouds were discovered above convective anvils reaching up to the lower stratosphere from the Geostationary Operational Environmental Satellite (GOES) and the Generation Weather Radar (NEXRAD) program Weather Surveillance Radar-1988 Doppler 55 (WSR-88D) network (Homeyer et al, 2017)

Results

Discussion

Conclusion