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Temp Vs. Height Theta Vs. Height Theta Lapse Rate Vs. Height

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Temp Vs. Height Theta Vs. Height Theta Lapse Rate Vs. Height 10.8 DIAGNOSES OF THE VERTICAL STRUCTURE OF THE TROPICAL TROPOPAUSE REGION Andrew Gettelman* National Center for Atmospheric Research, Boulder, CO 1. INTRODUCTION Temp vs. Height 25 20 The tropical tropopause layer (TTL) is a transition 15 layer between the convective equilibrium of the tropical ¦ 10 troposphere and the radiative equilibrium of the 5 stratosphere. As reviewed by Holton et al. (1995), the Altitude (km) 0 ¢ ¡ ¢ ¡ ¢ stratospheric circulation is driven by extratropical wave 100¡ 150 200 250 300 350 o forcing. While anectodal evidence of convection at the Temperature£¥¤ ( K) tropopause exists, recent work (Gettelman et al., 2001) indicates that such convection is rare. In this work we Theta vs. Height 25 examine the relative roles of tropospheric convection 20 and large scale diabatic heating at the tropopause and 15 ¦ try to define the coupling between the troposphere and 10 the stratosphere. 5 We find that the level of main convective influence Altitude (km) 0 ¡ ¡ ¡ ¡ ¡ in the upper troposphere is at 10−12km, which we 200¡ 300 400 500 600 700 define as the base of the tropopause layer. Above this Potential Temeprature (K) level, as noted by Folkins et al. (1999), the impact of convection drops off rapidly as air approaches Theta Lapse § Rate vs. Height 25 stratospheric stability. The level of maximum 20 convection varies in space and time, which may have 15 ¦ important implications for the chemistry of the TTL. 10 5 2. THE TROPICAL TROPOPAUSE LAYER Altitude (km) 0 © ¡ -20¨ -10 0 10 20 30 -1 The coupling between the stratosphere and Lapse rate (K km ) troposphere occurs in the tropopause layer, as the Figure 1: Temperature vs. Height (top Panel), Potential influence of convection tails off. The theoretical basis Teperature (Theta) vs. Height (middle panel) and for the coupling between the stratosphere and the Potential Temperature Lapse Rate vs. Height (bottom troposphere is illustrated in Figure 1. The top panel panel). Dashed line: sothermal profile (dT/dz=0), Dash− presents several different temperature profiles: an dot line: adiabatic profile (dT/dz= Γd, the dry adiabatic isothermal profile (dT/dz=0), an adiabatic profile lapse rate), Dotted line: a radiative equilibrium taken (dT/dz= Γd, the dry adiabatic lapse rate), and two from Manabe and Wetherald (1967), Thick solid line: profiles representative of the stability regimes, a convective equilibrium profile (dθe/dz=0, constant radiative equilibrium taken from Manabe and Wetherald equivalent potential temperature) (1967) and a convective equilibrium profile (dθe/dz=0, constant equivalent potential temperature). Note that profile, but in the radiative profile it increases rapidly the combination of a convective equilibrium profile from (Figure 1 bottom panel). The difference in lapse rate the surface to the troposphere and a radiative signature provides a clear transition between the equilibrium profile in the stratosphere (top panel) looks stability regimes which can be precisely located. The very much like an atmospheric sounding in the tropical minimum in the lapse rate profile in the upper western Pacific (Figure 2 top panel). troposphere marks the maximum impact of convection. Furthermore, the potential temperature approaches The altitude of this minimum for these soundings is a constant value in a convective equilibrium profile as between 10−14km, significantly below the tropopause all the water is removed (Figure 1 middle panel). Again, at 16km (bottom panel of figure 2). the combination of convective equilibrium and radiative This minimum lapse rate on a profile is the level of equilibrium resembles actual tropical soundings (Figure the maximum influence of convection, and we will use 2 middle panel). At tropopause levels (15km) the this to define the bottom boundary of the tropical potential temperature lapse rate (temperature lapse tropopause region, hereafter the tropical tropopause rate works just as well), goes to zero in a convective layer (TTL). High vertical resolution and long temporal resolution * Corresponding author address: Andrew Gettelman, records from radiosondes are available to explore the National Center for Atmospheric Research, Box 3000, details and variability of the tropopause region at Boulder, CO, 80307−3000; e−mail: [email protected] individual locations. We also illustrate how these Koror Sep 1996 Temp Soundings To validate the use of the minimum lapse rate as the base of the TTL we first use ozone soundings. High 100 vertical resolution soundings from the SHADOZ archive indicate the altitude of the minimum lapse rate lies just below the altitude of the ozone minimum. The ozone 39 Soundings minimum is created when convection lofts air with low Pressure (hPa) ozone from the surface into the upper troposphere. ¢ ¨ -90 -80 -70 -60 -50 -40 -30 -20 Above this level, photochemical production of ozone Temperature (degC) during slow ascent and mixing with the extratropical Koror Sep 1996 Potential Temp Soundings stratosphere increase the ozone concentration. These processes led Folkins et al. (1999) to argue for a 100 ’mixing barrier’ at the point where ozone begins to increase, above which there is little convective activity. The signature of convection also supports the interpretation of the minimum lapse rate as the base of Pressure (hPa) the TTL. Analysis of data at operational radiosonde ¨ ¡ ¨ 300¡ 320 340 360 380 400 420 Potential Temperature (K) stations shows that the level of minimum lapse rate rises and falls monthly with OLR. The correlation of Koror Theta Lapse Rate monthly OLR and altitude of minimum lapse rate is high 20 (0.7−0.9) at all stations. When OLR is low, the height of the lapse rate minimum is high, and vice versa. 15 ¦ Stations in each hemisphere show coherent variations, 10 with altitude maxima generally in the summer Altitude(km) 5 hemisphere when convection is a minimum. This is indicated in the climatology in Figure 3. Note that for all ¢ 0¡ 5 10 15 20 Gamma (K km-1) stations the change in height of the cold point Figure 2: Temperature (top panel), Potential tropopause has the same seasonal cycle between Temperature (middle panel) and Potential Temperature hemispheres, higher in January than in July. This Lapse Rate (bottom panel) profiles from Koror (7N, seasonal cycle is that expected from the stratospheric 134E) for September 1996. Vertical Scale is the same residual circulation (Holton et al., 1995). The seasonal for all panels: marked in pressure in top two and cycle of the lower boundary of the TTL, the minimum altitude in the bottom panel. lapse rate, is not consistent between regions. The western Pacific, central Pacific and Indian regions have signatures are consistent with observed proxies of more stations in the southern hemisphere, so that the convection (Outgoing Longwave Radiation) and the average altitude of the minimum lapse rate is higher in chemical signature of the layer from ozonesondes. SH summer (December−February) when convection is Important regional differences are noted in the active. However, 3 of 4 stations over Africa are in the structure of the TTL, which may affect the coupling of northern hemisphere, with strong convection in April− the stratosphere and the troposphere. June. Finally, over South America there are stations spread throughout the tropics, including along the 3. CLIMATOLOGY OF THE TTL equator, so that a double peak is seen in spring and fall as convection crosses the equator. Thus the seasonal Using sounding data from 22 selected operational variability in the altitude of the minimum lapse rate is tropical radiosonde stations and ozonesondes from the controlled by local convective processes. SHADOZ ozonesonde archive (Thompson and Witte, Furthermore, analysis of interannual variations of 1999), we will define the tropopause layer at each the base height of the TTL illustrates significant station. The upper level of the layer is defined as the correlations with convective activity. For stations in the cold point tropopause, near 17km in the tropics. The western Pacific, the height of the lapse rate minimum is bottom of the TTL is defined as the location of the anomalously low during El−Nino warm events when minimum lapse rate. Operational radiosonde data allow convection moves out into the central Pacific, and it is these locations to be discerned with several hundred anomalously high during La−Nina cold events when meter vertical resolution, and thousands of soundings. convection is concentrated in the western Pacific. In To supplement these data we will use 100 and 50m the central Pacific anomalies have the opposite vertical resolution soundings of temperature and ozone correlation with El−Nino, as expected from anomalies in from the SHADOZ archive. This data will be convection. All these factors indicate that the height of supplemented with an independent proxy of convective the minimum lapse rate is a good indicator of activity; daily Outgoing Longwave Radiation (OLR) data convective activity. interpolated by the National Oceanic and Atmospheric The overall climatology of the TTL from these Administration (NOAA), and provided by the NOAA analyses is illustrated in Figure 3, for several stations Climate Diagnostics Center, Boulder, Colorado averaged over each region of the globe. The cold point (http://www.cdc.noaa.gov/). tropopause height is virtually the same across the Tropical TTL layer 20 near 345 K. The 345 K potential temperature isentrope is located around 10−12km in the tropics, in good agreement with Figure 3. The equivalent potential Cold Point Tropopause 18 temperature at the surface is the potential temperature level of the level of neutral buoyancy assuming W.Pac undiluted ascent. This also provides an explanation for 16 C.Pac the regional differences in the TTL. The differences are ! S.Amer " Africa a function of the distribution of equivalent potential # Ind temperature at the surface, which is higher over the Altitude (km) 14 warm waters of the tropical Pacific, and lower (on Minimum Lapse Rate average) over tropical continents, which have a 12 reduced supply of moisture.
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