Tropopause sharpening by data assimilation

Abstract

Data assimilation was recently suggested to smooth out the sharp gradients that characterize the tropopause inversion layer (TIL) in systems that did not assimilate TIL‐resolving observations. We investigate whether this effect is present in the ERA‐Interim reanalysis and the European Centre for Medium‐Range Weather Forecasts (ECMWF) operational forecast system (which assimilate high‐resolution observations) by analyzing the 4D‐Var increments and how the TIL is represented in their data assimilation systems. For comparison, we also diagnose the TIL from high‐resolution GPS radio occultation temperature profiles from the COSMIC satellite mission, degraded to the same vertical resolution as ERA‐Interim and ECMWF operational analyses. Our results show that more recent reanalysis and forecast systems improve the representation of the TIL, updating the earlier hypothesis. However, the TIL in ERA‐Interim and ECMWF operational analyses is still weaker and farther away from the tropopause than GPS radio occultation observations of the same vertical resolution.

New insights into Rossby wave packet properties in the extratropical UTLS using GNSS radio occultations

“…However, the meridional temperature gradients in the lowermost stratosphere are of low magnitude, so it is not clear why the RWP temperature anomalies maximize so close to the tropopause and not higher up. Pilch Kedzierski et al (2017) also reported wave amplitudes maximizing within a short height range close to the tropopause.…”

“…Alexander and Shepherd (2010) studied planetary wave activity in the Arctic and Antarctic lower and mid-stratosphere, while Shepherd and Tsuda (2008) studied planetary waves in the SH polar summer at 30 km height. Pilch Kedzierski et al (2017) used GNSS-RO to study extratropical tropopause modulation by waves, although only separating wave activity by timescale and propagation direction, without describing wave properties. Our study is a first attempt to describe RWP properties in the extratropics with the use of GNSS-RO.…”

“…The method to grid GNSS-RO profiles in our study is a refinement of Pilch Kedzierski et al (2017) and Pilch Kedzierski et al (2016a), with better horizontal resolution and with ground-based instead of tropopause-based averaging. Both were developed after the method by Randel and Wu (2005).…”

New insights into Rossby wave packet properties in the extratropical UTLS using GNSS radio occultations

“…7 and 9 may be partially under-represented by GCMs. In fact, GCMs, reanalyses and forecast models are known to produce too smooth temperature gradients in the tropopause region (Gettelman et al, 2010;Hegglin et al, 2010;Birner et al, 2006;Pilch Kedzierski et al, 2016b). The tendency of forecast models of smoothing the tropopause with lead time (Gray et al, 2014) is partially compensated by diabatic and parameterized processes in the modelled RWPs (Saffin et al, 2017).…”

Global static stability and its relation to gravity waves in the middle atmosphere

“…Recent studies have shown that the TIL and the associated static stability enhancement occur at all latitudes, as revealed by satellite and ground‐based observations, as well as reanalysis data and model simulations (Grise et al, 2010; Gettelman et al, 2011; Zhang YH et al, 2015, 2019; Liu HL, 2017). The enhanced static stability above the tropopause is closely related to the wind shear and gravity waves (GWs) in this region (Liu HL, 2007, 2017; Sunilkumar et al, 2015, 2017; Zhang YH et al, 2015, 2019; Pilch Kedzierski et al, 2016). Through model simulations, Liu HL (2017) showed that large wind shears exist in the TIL region.…”