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1. Introduction

Recently, scientific attention to the roles of the stratosphere in the climate system has sparked renewed interest in the tropopause, particularly in the tropics. The transport of water vapor and other trace constituents through the tropical tropopause affects their concentration and distribution in the stratosphere. Long-term changes in the characteristics of the tropical tropopause may be associated with changes in stratospheric water vapor concentrations. Because increases in stratospheric water vapor, as demonstrated by balloon-borne frost-point hygrometer observations [Oltmans and Hofmann, 1995], have been postulated as playing a role in cooling the lower stratosphere [Forster and Shine, 1999] and depletion of ozone in the Arctic stratosphere [Kirk-Davidoff et al., 1999], a better understanding of tropical tropopause changes could clarify the causes of global and regional temperature and stratospheric ozone trends.

The passage of half a century since the main expansion of the radiosonde network, and the availability of new aerological datasets, afford the opportunity to develop a more temporally and spatially comprehensive depiction of the tropopause than ever before. Daily radiosonde data offer considerably better vertical resolution near the tropopause than either monthly radiosonde data, such as were employed by Highwood and Hoskins [1998], or model-based (re)analyses, as in the studies of Hoinka [1998, 1999], whose vertical resolution is similar to that of the mandatory radiosonde reporting levels. [The mandatory levels near the tropical tropopause are 150, 100, 70, and 50 hPa.] Furthermore, the use of daily soundings eliminates biases associated with using monthly data to vertically interpolate the tropopause level and to calculate variables, such as water vapor saturation mixing ratios, that are non-linearly related to observed temperature and heights at fixed pressure levels.

Here we present a new analysis of the climatological features of the tropical tropopause based on a thirty-year radiosonde dataset. We examine several "tropopause" levels including:

- the lapse-rate tropopause (LRT), formally defined by the World Meteorological Organization as "the lowest level at which the lapse rate decreases to 2 deg. C. per km or less, provided also the average lapse rate between this level and all higher levels within 2 km does not exceed 2 deg. C. per km" [WMO, 1957]

- the cold-point tropopause (CPT), identified as the location of the coldest point in the temperature sounding. This definition is preferred in studies of the cross-tropopause flux of water vapor in the tropics.

- the 100 hPa level, often used as a surrogate for the tropopause in the tropics

- the level at which water vapor saturation volume mixing ratio with respect to ice (qs) reaches a minimum value (qsmin).