The deep atmospheric boundary layer and its significance to the stratosphere and troposphere exchange over the Tibetan Plateau

Abstract

In this study the depth of the atmospheric boundary layer (ABL) over the Tibetan Plateau was measured during a regional radiosonde observation campaign in 2008 and found to be deeper than indicated by previously measurements. Results indicate that during fair weather conditions on winter days, the top of the mixed layers can be up to 5 km above the ground (9.4 km above sea level). Measurements also show that the depth of the ABL is quite distinct for three different periods (winter, monsoon-onset, and monsoon seasons). Turbulence at the top of a deep mixing layer can rise up to the upper troposphere. As a consequence, as confirmed by trajectory analysis, interaction occurs between deep ABLs and the low tropopause during winter over the Tibetan Plateau.

Figure 1. Profiles of (a) temperature, (b) potential temperature, (c) wind speed, (d) water vapor content on 25 Feb 2008.

Profiles were recorded at 01∶00 (dark line), 07∶00 (red line), 13∶00 (blue line), and 19∶00 (magenta line) BST. The horizontal dashed lines show the corresponding tops of the CBL, and horizontal solid lines show the positions of the tropopause. The stable layer (SL), residual layer (RL), and mixed layer (ML) are also marked.

Figure 2. Variation of CBL depth and height of LRT during the three IOPs.

Figure 3. Meridional cross-section at 84.25° E (over Gerze site) at 20∶00 BST, for the period 25 Feb. to 28 Feb. 2008, derived from ERA interim data, including zonal winds (cyan contours, m/s), potential vorticity (yellow lines, contours of 1, 2, 4 PV units), ozone (solid color, ×10⁶ kg/kg) and potential temperature (red contours, K).

The color bar is the scale of ozone concentration. The area in black shows the cross section of the Tibetan Plateau terrain. The red triangles and circles show the position of the LRT and the top of CBL.

Figure 4. GOME-2 ozone results.

Top: location of GOME-2 orbits over the Tibetan Plateau, with the altitude scale in km. Bottom left: GOME-2 ozone profiles in units of 10¹² molecules/cm³, with ERA-Interim number density contours superimposed. Bottom right: GOME-2 ozone profiles with ERA-Interim horizontal wind speed contours superimposed. The arrows below the x-axis in the bottom two plots show the extent of the Tibetan Plateau as illustrated in the top plot. The dashed line is the position of the thermal tropopause, according to the WMO definition.

Figure 5. Meridional cross-section at 84.25°E (over Gerze site) at 20∶00 BST, for period 7 Jul. 2008 to 10 Jul. 2008 BST.

As Figure 3, but for a period in IOP2.

Figure 6. A schematic illustration of the correspondence between tropopause folds, westerly jet displacement and convective boundary layer variations during winter and summer time.

The profile of water vapor (q) and potential temperature (θ) in the CBL and surface sensible heating (up green arrow) were included.

Figure 7. Clusters of trajectories for particles within a 10°×10° box around the station of Gerze and within the 10–14 km layer at 12∶00 UTC on 25 February 2008.

The smallest square shows the central date (12∶00 on 25 Feb 2008 GMT). Each point/diamond represents one time step (3 hours). The upper plots show the height-longitude representation: the solid black line represents the height of the tropopause for each time step as calculated by the FLEXPART model, and the diamond at the lowest level corresponds to the earliest time simulated. Colors correspond to the mean PV of the air mass: orange represents 2.5–3.0 PVU, yellow 2–2.5 PVU, green 1.5–2 PVU. The lower plots show the latitude-longitude representation: the dark grey part represents earlier times in the simulation, while the light grey part represents later times.