Tropical tropopause dynamics (TTD) campaigns over Indian region: An overview

Highlights

• Intensive campaigns on ‘tropical tropopause dynamics’ have been conducted.

• Importance of a sustained study on the tropical tropopause is brought out.

• Expected to provide important contributions to the international campaigns.

Abstract

It is widely accepted that the tropical tropopause is closely linked to climate change. Several campaigns have already been conducted and also are being planned to address various issues related to the tropical tropopause layer (TTL). Despite many campaigns, several scientific issues still remain unexplained including Indian summer monsoon dynamics, cirrus clouds and the trace gas distribution across the tropopause etc. In order to address some of the issues, particularly over the Indian region, intensive observational campaigns called ‘tropical tropopause dynamics (TTD)’ are being conducted since December 2010 at two stations namely Gadanki (13.5°N, 79.2°E) and Trivandrum (8.5°N, 76.9°E) under CAWSES India Phase-II programme. This overview article aims to bring out the current understanding on the tropical tropopause, issues addressed through the TTD campaigns and the details of the data collected in these campaigns using collocated instruments as well as complementary satellite data. So far 32 campaigns have been completed successfully and in this paper main focus is given for describing the systematic data collected using various techniques (MST radar, Mie lidar, Radiosonde, ozonesonde) simultaneously in each month. In general, over the study region affected by the monsoon, a prominent updraft prevails in the middle and upper troposphere regions covering TTL affecting the transport of minor species across the tropopause. The behavior of the cold point tropopause (CPT) at Gadanki and Trivandrum reveals that there are significant differences in the CPT characteristics even within the monsoon region. Cold point tropopause shows stronger sub-daily scale variation over Trivandrum than Gadanki though no indication of deep convection is present at the former location particularly in winter.

Introduction

In recent years the ‘tropical tropopause’ has received immense scientific attention because of its sensitivity to anthropogenic activities leading to climate change. The focus is mainly on chemistry, dynamics, radiation and transport through the tropical tropopause layer (TTL). There is an immediate need to study the link between the TTL processes and climate change, which requires consideration of radiation and chemistry (Fueglistaler et al., 2009, Randel and Jensen, 2013). Understanding the physical and dynamical processes governing this region (e.g., processes determining the location and magnitude of cold point temperature) is critical to evaluate and model climate sensitivity (Atticks. and Robinson, 1983, Seidel et al., 2001). Convection in the tropics plays a key role in redistributing trace gases and aerosols in the upper troposphere–lower stratosphere (UTLS) region and the interaction between cloud dynamics, radiation and microphysics is of fundamental importance in these processes (Holton et al., 1995). Among these, convective detrainment and the effect of overshooting convection on the heat balance of the TTL (Danielsen, 1993, Sherwood and Dessler, 2003, Randel and Jensen, 2013) remain to be explored in detail.

Despite the wealth of knowledge we gained about the TTL during the ‘Several Accentuated Tropical Years for Analysis’ (SATYA) ‘a new golden age’ for TTL observations’ (Gettelman et al., 2013), several issues related to tropical tropopause remain unexplained due to limitations in the available data. These include the effects of (a) convection and extra-tropical pump (Holton et al., 1995, Haynes et al., 1991, Plumb and Eluszkiewicz, 1999, Norton, 2006), (b) Indian summer monsoon dynamics i.e., tropical easterly jet (TEJ) (Jayaraman et al., 2010), (c) the presence of cirrus and (d) trace gases (ozone and water vapor) distribution across tropopause (STE processes) on the tropopause and TTL (Fujiwara et al., 2009, Shibata et al., 2012, Thompson et al., 2012, Inai et al., 2013).

With a view to address the important and complex interplay of physical processes taking place in TTL, several focused field experiments have been conducted in the past and a number of campaigns across the globe with special emphasis over the Asian and Pacific region are being planned in the coming years (Gettelman et al., 2013). See Table 1 of SPARC Newsletter (issue number 40) for details of the campaigns being planned. Unfortunately, no campaigns were conducted over the Indian region using high resolution observations until recently. The Indian region is affected by both the summer monsoon (south-west monsoons) and the north-east monsoon (winter). The latter is active mainly over the south east peninsular region. Thus, well planned scientific campaigns on the troposphere and lower stratospheric regions will provide valuable inputs for the overall understanding of the physical processes taking place. Further, the Indian region is one of the potential source regions for much of the air entering into the lower stratosphere and several other complex processes happening mainly due to the peculiar weather related to Indian Summer Monsoon dynamics (e.g., Highwood and Hoskins, 1998, Vernier et al., 2009, Nishi et al., 2010).

To explore TTL issues over the Indian region, an intensive campaign, namely ‘tropical tropopause dynamics (TTD)’ Experiment, was conducted under the CAWSES-India phase-I program during the period 2004 to 2008 over Gadanki (13.5°N, 79.2°E) region. During the first phase, the main issues addressed were (1) identification of the tropical convective tropopause and its association with cold point tropopause, (2) variations of tropical tropopause over Indian monsoon region, (3) characteristics of multiple tropopauses in the tropics, (4) characteristics of semitransparent cirrus (STC) clouds in the upper troposphere and their effect on tropical tropopause, (5) characteristic differences between TTL and non-TTL semi-transparent cirrus (STC), (6) relation between tropospheric turbulence and STC, (7) aerosol stratification associated with stratified turbulence and (8) influence of STC on tropospheric temperature.

Key results obtained in the Phase 1 included devising a method to identify the major convective outflow level using MST radar derived divergence profile, the identified convective outflow level is well correlated with that identified using potential temperature lapse rate minimum (Gettelman and Forester, 2002) representing the convective tropopause (COT) (Mehta et al., 2008). Further, sub-daily scale variations of the tropopause characteristics and their relation to the tropical convection were investigated (Mehta et al., 2010). Most of the tropopause features observed over the Indian region are found to differ from those reported for the western Pacific (Mehta et al., 2011a) reported by Reid and Gage (1985). We also emphasized the limitations of the WMO definition for identifying the complex structures like multiple tropopauses (MTs) and developed alternate criteria to delineate the MTs over tropics effectively (Mehta et al., 2011b, Mehta et al., 2013). It was found that 42% of the time the cold point tropopause (CPT) is lower than the core of TEJ, suggesting that meridional temperature gradient between the warm Tibetan high and cool Indian Ocean is an important factor significantly affecting the tropopause (Ratnam et al., 2011).

The role of upper troposphere (UT) dynamics in the formation and persistence of high altitude tropical cirrus was studied using the MST radar and lidar located at Gadanki. It was found that the prevailing conditions in TTL are conducive for the formation and persistence of cirrus. A strong association between tropospheric turbulence and STC was observed (Parameswaran et al., 2003). In general the TTL-top remains fairly steady on short time scales while the TTL-base governed by the convective outflow shows significant variations (Mehta et al., 2008). An important event during the CAWSES phase-I was the commissioning of the Indian geostationary satellite KALPANA-1 positioned at 74°E over the equator (Rajeev et al., 2008) which provided spatial distribution of different types of clouds including cirrus.

As several issues remained unexplored in the Phase 1 programme mainly due to limitations of the data, it was felt that a more comprehensive approach is needed to better understand these issues. Major unresolved issues include (1) tropopause variability on short time scales and its relation with convection and TEJ, (2) mechanisms for the occurrence of multiple tropopauses—are they due to planetary scale waves or horizontal advection or cirrus clouds?, (3) stable layers in the tropopause region and (4) effect of cirrus clouds on tropopause, tropospheric thermal structure and turbulence in the TTL. (5) The altitude structure of turbulence in the TTL region, and (6) influence of atmospheric waves on the altitude structure of cirrus and the thermal structure in the UTLS region. To explore these, a much expanded TTD Experiment Campaign has been undertaken in CAWSES India-Phase II Programme making use of high resolution measurements of vertical velocity, TEJ, temperature lapse rate, stability, water vapor mixing ratio and ozone mixing ratio. In this phase, an additional station Trivandrum (8.5°N, 76.9°E) was included. New additions, which were not available in the first phase, are launching good quality radiosonde and ozonesonde with high vertical resolution simultaneously from both the stations. The details of the instruments used and their mode of operation at the two stations are elaborated in Section 4.