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A Climatology of the Tropical Tropopause Layer

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A Climatology of the Tropical Tropopause Layer Journal of the Meteorological Society of Japan, Vol. 80, No. 4B, pp. 911--924, 2002 911 A Climatology of the Tropical Tropopause Layer A. GETTELMAN National Center for Atmospheric Research, Boulder, CO, USA and P.M. de F. FORSTER Department of Meteorology, University of Reading, Reading, UK (Manuscript received 4 June 2001, in revised form 17 September 2001) Abstract The tropical tropopause layer (TTL) is a transition region between the troposphere and the strato- sphere. In this study the vertical extent of the TTL is diagnosed from radiosonde and ozonesonde profiles in the tropics and a climatology of this layer is presented. The radiative balance in the TTL is also characterized. The TTL is locally defined as extending from the level of the lapse rate minimum at 10– 12 km to the cold point tropopause (CPT) at 16–17 km. The minimum in lapse rate represents the level of maximum convective impact on upper tropospheric temperatures, which is found to closely correspond to a minimum in ozone. Variations in this level are correlated with convective activity as measured by satellite brightness temperatures and Outgoing Longwave Radiation (OLR). At the cold point, the TTL height is nearly uniform throughout the tropics, and has a pronounced annual cycle. There are regional variations in the altitude of the lower boundary of the TTL. Interannual variations of the TTL result from changes in the large scale organization of convective activity, such as from the El-Nin˜ o Southern Oscillation (ENSO). Over the last 40 years, records indicate an increase (200–400 m) in the height of both the cold point tropopause and the level of minimum lapse rate. To better understand vertical transport in the TTL, the clear sky radiative heating rate is diagnosed using a sophisticated radiative transfer scheme. The level of zero radiative heating occurs roughly 1 km below the CPT, implying that convection needs to loft air 4–5 km above the base of the TTL if the air is to eventually enter the stratosphere. 1. Introduction vectively dominated tropical troposphere and the radiatively controlled stratosphere. The tropical tropopause is not a material The TTL is important as the source region for surface. It has been recognized for some time much of the air entering the stratosphere (Hol- that a single valued definition of the tropo- ton et al. 1995). As discussed first by Brewer pause, designed for the extratropics, may not (1949), the boundary condition for stratospheric be appropriate in the tropics. It is perhaps bet- water vapor is likely set in this region. The in- ter to think of the the tropical tropopause layer teraction of convection, the stratospheric wave- (TTL) as a transition layer between the con- driven circulation and horizontal exchange with the midlatitude lower stratosphere gov- Corresponding author: A. Gettelman, National Center for Atmospheric Research, Box 3000, Boul- This manuscript was prepared with modified der, CO, 80307-3000, USA. AGU’s LATEX macros v3.0, with the extension E-mail: [email protected] package ‘AGUþþ’ by P.W. Daly, version 1.5e from ( 2002, Meteorological Society of Japan 1997/11/18, modified for AMS journals. 912 Journal of the Meteorological Society of Japan Vol. 80, No. 4B erns the transport of ozone, aerosols and other the equator as a ‘secondary tropopause’ at 10– short lived chemical species into the strato- 12 km. This concept was picked up on again by sphere. The relative roles of these processes in Palme´n and Newton (1969), and by Shapiro the TTL is unclear, and the boundaries of the (1980). Later, Atticks and Robinson (1983) TTL are defined qualitatively differently by noted from looking at tropical temperature different authors (see below). In this work, we soundings that the tropical tropopause was set out a definition for the vertical boundaries really a ‘transition region’ between the strato- of the TTL based on simple theoretical argu- sphere and the troposphere. ments and observations. The definition is not In this work we define the tropical tropo- the only one possible (see below), but allows us pause layer in more detail. We make a case to examine the upper and lower boundaries of that the level of minimum temperature or po- the TTL together using radiosonde and ozone- tential temperature lapse rate is a good marker sonde data. We use this definition to develop a for the maximum influence of convection on the climatology for the TTL, and then investigate temperature profile and hence the base of the the radiatively driven vertical motion in the TTL. The upper boundary of the TTL is well TTL. We conclude by refining the conceptual characterized by the cold point. We introduce picture of the TTL. the data in Section 2. We develop and justify The concept of a tropical tropopause layer our definition of the TTL in Section 3. Using rather than a tropopause surface has been this definition we then present a climatology of recently discussed by Highwood and Hoskins the TTL in Section 4. To better understand (1998) in the context of the large scale tropical vertical transport in the TTL, clear sky radia- circulation, extending from the main convective tive heating rates are calculated using a de- outflow to the cold point. Sherwood and Dessler tailed radiative code in Section 5. Discussion (2001) have defined the TTL as the region and the implications of this study are found in where convection overshoots its level of neutral Section 6. buoyancy, extending from about 150 hPa to 2. Data 50 hPa. Folkins et al. (1999) has noted a change in the stability and increase in ozone at For high vertical resolution information on about 14 km. Vo¨mel et al. (2002) went on to the tropopause layer, we use vertical sounding note there were important spatial and temporal information from radiosondes and ozonesondes. variations in this structure, and that the TTL These data have the advantage of relatively lower boundary can be defined by a local mini- high vertical resolution and fairly high preci- mum in relative humidity. Thuburn and Craig sion. In addition, there are sufficient observa- (2002) have recently explored the thermody- tions to capture many scales of variability and namic basis for the layer and defined it as a re- to minimize random errors. Radiosonde obser- gion where radiation is more important than vations are drawn from the standard upper air convection, but convection is more important Global Telecommunications System (GTS) net- than chemical production of ozone. work, collected by the National Centers for En- However, the concept of a tropopause layer is vironmental Prediction (NCEP) for 1989–1999. not new. There is a long tradition of defining To supplement this data (which may have tem- multiple tropopause levels in soundings by suc- poral gaps in individual station records), we cessive changes in lapse rate toward increasing also use data from a small subset of stations in stability. In the extratropics and sub tropics, the Comprehensive Aerological Research Data multiple tropopause structures go at least back Set (CARDS) radiosonde archive (Eskridge et to Bjerknes and Palme´n (1937), who called the al. 1995) over a longer time period (1958–1997). region between these multiple tropopauses the In addition to these records, we use high- ‘sub-stratosphere’. Willett (1944) also defined vertical resolution (50–100 m) ozonesonde data the tropopause as a layer rather than a sur- from the SHADOZ ozonesonde archive (Thomp- face, extending well into the subtropics. Multi- son and Witte 1999). This data set provides ple tropopause structures were discussed ex- several hundred soundings of temperature and tensively by Kochanski (1955), who noted the ozone from 10 stations in the southern hemi- extension of the extratropical tropopause to sphere tropics on an operational basis since August 2002 A. GETTELMAN and P.M. de F. FORSTER 913 1998. This data will be supplemented with two independent proxies of convective activity; satellite brightness temperatures and daily averaged Outgoing Longwave Radiation (OLR). Satellite brightness temperatures are derived from infrared radiances at high space and time resolution as described by Salby et al. (1991), and are available only for a few years. Daily averaged OLR is available for a much longer time record. OLR is derived from the same infrared radiances, interpolated by the Na- tional Oceanic and Atmospheric Administra- tion (NOAA), and provided by the NOAA Cli- mate Diagnostics Center, Boulder, Colorado (http://www.cdc.noaa.gov/). This study uses linear correlation analysis. Significance of linear correlations, unless other- wise noted, is reported assuming a 95% con- fidence level and a 2-sided T-test with appro- priate degrees of freedom (Santer et al. 2000), which for the trend analyses is typically one degree of freedom per year. 3. Definition of the TTL The transition between the stratosphere and troposphere occurs in the tropopause layer, as the influence of convection tails off with height. The theoretical basis for the TTL ‘tran- sition layer’ is illustrated in Fig. 1. Figure 1a illustrates several different temperature pro- files representative of stratospheric and tropo- spheric stability regimes. A radiative equilib- rium profile taken from Manabe and Wetherald (1967) represents the stratospheric regime. The model uses profiles of ozone from observations and fixed relative humidity and carbon dioxide in the vertical. It is broadly but not specifically applicable to the tropics. The tropospheric regime is defined as the region in which the Fig. 1. Theoretical profiles of A) temper- atmosphere is in recent contact with with the ature, B) potential temperature ðyÞ and boundary layer through convection (rapid ver- C) potential temperature lapse rate. tical motion). A convective equilibrium profile Four lines are shown. The solid line is represented by a profile of constant saturated is convective equilibrium or constant equivalent potential temperature ðdyes=dz ¼ 0Þ.
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