Review of “Diurnal cycle of clouds extending above the tropical tropopause observed by spaceborne lidar” by Dauhut et al:

The presence of clouds above the tropopause over tropical convection centers has so far been documented by spaceborne instruments that are either sun-synchronous or insensitive to thin cloud layers. Here we document, for the first time through direct observation by spaceborne lidar, how the tropical cloud fraction evolves above the tropopause throughout the day. After confirming previous studies that found such clouds most frequently above convection centers, we show that stratospheric clouds and their vertical extent above the tropopause follow a diurnal rhythm linked to convective activity. The diurnal cycle of the stratospheric clouds displays two maxima: one in the early night (19:00– 20:00 LT) and a later one (00:00–01:00 LT). Stratospheric clouds extend up to 0.5–1 km above the tropopause during nighttime, when they are the most frequent. The frequency and the vertical extent of stratospheric clouds is very limited during daytime, and when present they are found very close to the tropopause. Results are similar over the major convection centers (Africa, South America and the Warm Pool), with more clouds above land in DJF (December–January– February) and less above the ocean and in JJA (June–July– August). 1 Scientific context and objectives Low-stratospheric clouds impact the atmospheric system in several ways. First, their larger heating rate than in the clear sky (Corti et al., 2006) increases the upward mass flux and fosters the large-scale upward transport of water above the tropopause. At the hour timescale, the cloud particles penetrating the stratosphere via overshooting convection leads, on the one hand, to a direct stratospheric humidification (Schoeberl et al., 2019; Dauhut et al., 2018). On the other hand, these particles can serve as support for ice scavenging: under saturated conditions, the water vapor deposits on the particles, which grow and fall out (Corti et al., 2008), decreasing low-stratosphere humidity (Jensen et al., 2013). By all these effects the stratospheric clouds modulate the stratospheric water vapor concentrations (Iwasaki et al., 2015) and affect the overall dynamical structure near the tropopause (Corti et al., 2006), at timescales down to 1 h. This is why it is important to understand the formation and the sub-daily evolution of such clouds. The presence of ice clouds near the tropical tropopause has long been documented by in situ measurements (e.g., Thomas et al., 2002; Jensen et al., 2013; Frey et al., 2014). Detecting occurrences of clouds extending above the tropopause by remote sensing requires documenting the vertical cloud profile with a fine resolution and a high sensitivity to optically thin clouds, which few instruments can reach. Lidar measurements are able to document such occurrences (e.g., Nee et al., 1998; Dupont et al., 2010; Gouveia et al., 2017), but for a long time they were limited to local case studies. Dessler (2009) was the first to use the cloud detections by the CALIPSO lidar (Cloud-Aerosol Lidar Infrared Pathfinder Satellite Observations) to investigate how clouds extend above the tropopause on a global scale. Pan and Munchak (2011) refined the results by using an advanced tropopause dataset. Both studies found that clouds extending into the stratosphere are frequent above seasonal deep-convection centers and rarely elsewhere, especially in midlatitudes. Both studies deplored that the fixed overpass local time of the CALIPSO dataset is far from the late afPublished by Copernicus Publications on behalf of the European Geosciences Union. 3922 T. Dauhut et al.: The diurnal cycle of stratospheric clouds ternoon, when land convection is at its maximum. More recently, Wang et al. (2019) documented the presence of laminar cirrus in 10 years of CALIPSO data and reported a nonnegligible cloud amount above the tropopause. Because of the sun-synchronous orbit of CALIPSO, none of these studies were able to document the diurnal cycle of the stratospheric clouds. The diurnal evolution of the high-altitude cirrus clouds has been documented over some specific sites using groundbased lidars (Sassen et al., 2003; Dupont et al., 2010; Gouveia et al., 2017). Gouveia et al. (2017) documented the evolution of the integrated cloud fraction (no vertical distribution) over Amazonia; Sassen et al. (2003) documented the diurnal evolution of the composition of cirrus clouds over Salt Lake City; and Dupont et al. (2010) did the same over four observatories in France and in the United States. However, using ground-based lidar to document optically thin clouds extending above the tropopause is difficult for two reasons: (1) as the studies based on CALIPSO observations show, these clouds occur primarily in regions where operational ground-based sites are absent or very few (Pacific Ocean, Equatorial Africa and South America), and (2) these clouds are mainly associated with deep convection, which implies the presence of optically thick cloud systems in the troposphere beneath that will make the successful probing of optically thin clouds near the tropopause impossible in most cases due to the attenuation of lidar signals. This explains why the ground-based lidars do not document the diurnal cycle of the stratospheric clouds with a satisfying spatial and temporal coverage. Describing the diurnal evolution of the high-altitude clouds from a global perspective becomes possible with the CATS (Cloud-Aerosol Transport System) lidar operated from the International Space Station (ISS) between February 2015 and November 2017 (McGill et al., 2015). Thanks to the ISS non-synchronous orbit, CATS was able to probe the vertical cloud distribution of a particular region at different times of the day (not only at 01:30 and 13:30 local time like the instruments on CALIPSO). Aggregating CATS detections over a region of interest and over enough time provides a statistical overview of the diurnal evolution of cloud vertical profiles over that region (Noel et al., 2018). Our work aims at using CATS observations to describe and better understand the diurnal evolution of the cloud fraction in the tropical stratosphere. Finding the processes responsible for the formation of tropical stratospheric clouds proves difficult, just like with high-tropospheric clouds (Reverdy et al., 2012). Two processes have been mainly proposed. Overshooting convection can lead to the injection of ice crystals into the stratosphere (Dauhut et al., 2018; Lee et al., 2019). Stratospheric cooling triggered by gravity waves (Pfister et al., 2010) could also lead to so-called cloud in situ formation (Pan and Munchak, 2011). The ratio of stratospheric clouds that are formed in situ has not been estimated yet. The current study does not provide a further estimate, but by describing the spatiotemporal evolution of the stratospheric clouds, it highlights how important the convective activity is to drive the stratospheric cloudiness and how the twice-daily sampling by lidars on board sun-synchronous platforms can miss the highest and largest stratospheric cloud fraction over certain regions. In this paper, we document for the first time the diurnal cycle of clouds above the tropopause in the Tropics and the extent of their penetration in the stratosphere thanks to the high vertical and temporal resolution of the cloud detection by the CATS lidar. After describing CATS cloud data and the method to retrieve the tropopause heights used to detect clouds extending in the stratosphere (Sect. 2), we present maps of stratospheric clouds and document their diurnal cycle in regions of interest (Sect. 3). We then summarize our results and conclude (Sect. 4).