Comparisons of tropopause derived from COSMIC measurements at Nanjing since August 2006
Introduction
The tropopause, lying between troposphere and stratosphere, attracted increasing consideration recently. The World Meteorological Organization (WMO) (1957) has defined the tropopause as “the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km”, which is also known as thermal tropopause. Its properties serve as a hint of climate change. Both observations and numerical models show that tropopause altitude has increased since 1979 (Santer et al., 2003a, Santer et al., 2003b). The tropopause also plays a key role in the upper troposphere and lower stratosphere (UTLS) by affecting chemical tracers exchange (Holton et al., 1995).
In subtropics, the tropopause is of high oscillation due to frequent occurrence of double tropopause (Randel et al., 2007a). It is also sensitive to the expansion of the Hadley Cell (HC) because the boundary of the HC locates there (Lu et al., 2007). Unlike in the tropical and high latitudinal region, is fluctuating rapidly in subtropics with maximum gradient at ~35° (Son et al., 2011), where the NNR data sets underestimate.
To precisely identify tropopause properties in subtropics, we use the Global Positioning System (GPS) Radio occultation (RO) observations. This technique has been applied to study the atmosphere utilizing its high vertical resolution, high precision and global coverage (Kursinski et al., 1997, Nishida et al., 2000, Wickert et al., 2001, Hajj et al., 2002, Randel et al., 2003, Anthes, 2011, Guo et al., 2011a, Guo et al., 2011b). Kishore et al. (2009) validated the first year of COSMIC temperature observations by comparing them with three operational analyses. They found the largest differences in the polar region and the maximum differences (2–4 °C) in the tropics tropopause. He et al. (2009) demonstrated a very low mean difference between COSMIC and Vaisala-RS92 and Shanghai radiosonde systems in the UTLS. They also found diurnal radiative effects might cause large temperature biases. In addition, the synoptic scale variability increases biases between COSMIC and radiosonde data for collocation time and distance mismatch (Sun et al., 2010).
Both of andare also available from the NNR, which have been widely used (Hoerling et al., 1991, Hoinka, 1998, Hoinka, 1999, Highwood et al., 2000, Randel et al., 2007b). Randel et al. (2000) once examined the tropopause of radiosonde and the NNR data in the tropical. Their correlation coefficients of interannual anomalies were about 0.64–0.65 over the period 1957–1997. Birner (2010) argues that underlying reasons are poor centered differences and partial WMO definition without thickness criterion. Son et al. (2011) shows global distributions of tropopause differences between NNR and COSMIC data. They point that the largest bias exists in the subtropics.
The main objective of this paper is to extend the comparison between the COSMIC data, NNR reanalysis and ESRL observations from August 2006 to December 2011 on three timescales. Data sets and method are described in Section 2. Results are discussed in Section 3. In Section 4, we draw a conclusion.
Section snippets
Data and methodology
The Constellation Observing System for Meteorology, Ionosphere and Climate (COSMIC) data set is of the prime significance to examine the tropopause. In 2006, the National Aeronautics and Space Administration (NASA) and Taiwan's National Space Organization (NSPO) jointly launched six low-orbit satellites producing 2000 occultations or so per day (see Fig.1, left panel). The precision of each radio occultation (RO) profile could reach ∼0.05 °C in the UTLS (Anthes et al., 2008). Hence, it is
Ten-day average
Fig.2 presents 10-day mean tropopause properties derived from the COSMIC, NNR and ESRL data. Generally, these three data sets are in good agreement with each other, though tropopause properties are varying dramatically in every year. The COSMIC reaches maxima of −52 to −42 °C in winter and minima of −78 to −74 °C in summer. Its correspondingvaries greatly in winter from 250 to 320 hPa but keeps at ∼100 hPa in summer. Such changes are attributed to the double tropopause in this region (Randel et
Conclusions
In this paper, we compare the and obtained from the COSMIC, NNR and ESRL data sets covering a period from August 2006 to December 2011 at Nanjing. The conclusions produced by statistic methods are as follows:
- 1.
The ten-day average pressure and temperature differences between the COSMIC and radiosonde data sets are 5.3 hPa and 0.2 °C, which are smaller than that of the COSIMC minus NNR data by 12.2 hPa and 1.0 °C, respectively. Although correlation coefficients of ten-day mean tropopause show
Acknowledgments
We thank the COSMIC Data Analysis and Archive Center (CDAAC) for providing data archive and the ESRL for rendering radiosonde data. The NNR and NOAA OLR data were provided by the NOAA/OAR/ESRL PSD, Boulder, Colorado (http://www.esrl.noaa.gov/psd/). The work was partly supported by the National Natural Science Foundation of China (Grant No. 41105013), the National Natural Science Foundation of Jiangsu, China (Grant No. BK2011122), the Jiangsu Key Laboratory of Meteorological Observation and
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