Impact of land convection on temperature diurnal variation in the tropical lower stratosphere inferred from COSMIC GPS radio occultations

cess Atmospheric Chemistry and Physics Open Access Atmospheric Measurement Techniques Open Access Atmos. Chem. Phys., 13, 6391–6402, 2013 www.atmos-chem-phys.net/13/6391/2013/ doi:10.5194/acp-13-6391-2013 © Author(s) 2013. CC Attribution 3.0 License. Sciences Open Access Impact of land convection on temperature diurnal variation in the tropical lower stratosphere inferred from COSMIC GPS radio Biogeosciences occultations S. M. Khaykin, J.-P. Pommereau, and A. Hauchecorne Earth System Dynamics Open Access Correspondence to: S. Khaykin () Climate of the Past Open Access LATMOS, CNRS-Université de Versailles St Quentin, UMR8190, Guyancourt, France Received: 22 October 2012 – Published in Atmos. Chem. Phys. Discuss.: 2 January 2013 Revised: 8 May 2013 – Accepted: 3 June 2013 – Published: 5 July 2013 1 Introduction The tropical tropopause layer (TTL) has long been recognized as the stratospheric “kitchen”, setting the boundGeoscientific ary conditions for tropospheric tracers entering the stratosphere. However, the potential impact of the troposphereInstrumentation to-stratosphere transport (TST) is highlyand dependent on the Methods timescale of the processes involved. Indeed, still debated is Systemsconvective upthe global contribution ofData fast overshooting drafts on a timescale of hours, compared to the slow ascent by radiative heating of the layer above the level of neutral buoyancy (LNB) Geoscientific on a timescale of months (Sherwood and Dessler, 2000, Gettelman et al. 2002; Corti et al. Model Development 2005; Fueglistaler et al. 2009). The occurrence of convective overshoots reaching 20 km with local upward velocities of up to 50–60 m s−1 has long been known (e.g. Vonnegut and Moore, 1958; Burnham, 1970; Roach and James, 1972; Hydrology and Cornford and Spavins, 1973; Fujita, 1992; Danielsen, 1982, Earth System 1993). Whereas the existence of cross-tropopause transport by deep overshooting is generally accepted, its role in TST Sciences had not been assessed until recently since major attention in this context has been paid to the region of the warm pool in the west Pacific instead, where the vast majority of field observations has been carried out. This region was characterOcean Science ized by large-scale slow ascent and minimum outgoing longwave radiation (OLR) used as a proxy for deep convection. However as demonstrated by Alcala and Dessler (2002) from the observations of the precipitation radar (PR) of the Tropical Rainfall Measurement Mission (TRMM), OLR is not a good indicator of cloud penetration the stratosphere. InSolidinto Earth deed, in contrast, the TRMM PR indicates higher and more frequent “overshooting precipitation features” (OPFs) above Open Access Open Access Open Access Open Access Abstract. Following recent studies evidencing the influence of deep convection on the chemical composition and thermal structure of the tropical lower stratosphere, we explore its impact on the temperature diurnal variation in the upper troposphere and lower stratosphere using the high-resolution COSMIC GPS radio-occultation temperature measurements spanning from 2006 through 2011. The temperature in the lowermost stratosphere over land during summer displays a marked diurnal cycle characterized by an afternoon cooling. This diurnal cycle is shown collocated with most intense land convective areas observed by the Tropical Rainfall Measurement Mission (TRMM) precipitation radar and in phase with the maximum overshooting occurrence frequency in late afternoon. Two processes potentially responsible for that are identified: (i) non-migrating tides, whose physical nature is internal gravity waves, and (ii) local cross-tropopause mass transport of adiabatically cooled air by overshooting turrets. Although both processes can contribute, only the lofting of adiabatically cooled air is well captured by models, making it difficult to characterize the contribution of non-migrating tides. The impact of deep convection on the temperature diurnal cycle is found larger in the southern tropics, suggesting more vigorous convection over clean rain forest continents than desert areas and polluted continents in the northern tropics. Open Access The Cryosphere Open Access Published by Copernicus Publications on behalf of the European Geosciences Union. M 6392 S. M. Khaykin et al.: Impact of land convection on temperature diurnal variation the tropopause over Africa, South America and the Indonesian islands than above oceans (Liu and Zipser, 2005, hereinafter LZ05). Furthermore, the diurnal variation of OPFs displays a marked maximum in the late afternoon over land as opposed to maritime convection, which shows very little diurnal change. A considerable amount of observational evidence for cross-tropopause mass transport through convective overshooting has been made available by recent field campaigns in South America, Australia and Africa, revealing penetration of tropospheric air and ice crystals up to 19–20 km over land convective systems (Nielsen et al., 2007; Corti et al., 2008; Khaykin et al., 2009; de Reus et al., 2009; Schiller et al., 2009). Such convective updrafts of adiabatically cooled air and ice crystals across the tropopause are well captured by mesoscale cloud resolving models (Chaboureau et al., 2007; Jensen et al., 2007; Grosvenor et al., 2007; Chemel et al., 2009; Liu et al., 2010), but, because of their non-hydrostatic nature, they are missed by global meteorological and climate models. The relative importance of the contribution of such continental convective updrafts compared to their oceanic counterparts is suggested by the higher concentration of tropospheric trace gases in the TTL above continents reported by space-borne N2 O, CH4 and CO profiles measurements (Ricaud et al., 2007, 2009). Another indication of the importance of these is provided by the fast cleansing of stratospheric aerosols up to 20–21 km altitude resulting from the injection of clean tropospheric air during the Southern Hemisphere convective season observed by the CALIPSO lidar (Vernier et al., 2009, 2011). The influence of deep convection on the thermal structure of the upper troposphere/lower stratosphere (UTLS) has been studied by many authors. Gettelman and Birner (2007) characterized the regional convective temperature signal as warming of the upper troposphere due to latent heat release and cooling at TTL levels. Holloway and Neelen (2007) described the cooling at TTL levels as a natural response to latent heating, producing hydrostatic pressure gradients and forcing ascent and adiabatic cooling. A large-scale cold anomaly near the tropopause is commonly associated with convectively coupled equatorial waves (e.g. Randel and Wu, 2003; Sherwood et al., 2003; Norton, 2006; Kiladis et al., 2009) or the Madden–Julian oscillation (Kiladis et al., 2005; Zeng et al., 2012). There is a considerable amount of observational evidence of local cooling of the lower stra (...truncated)