Sensitivities of modelled water vapour in the lower stratosphere: temperature uncertainty, effects of horizontal transport and small-scale mixing

Abstract

Abstract. Water vapour (H2O) in the upper troposphere and lower stratosphere (UTLS) has a significant role for global radiation. A realistic representation of H2O is therefore critical for accurate climate model predictions of future climate change. In this paper we investigate the effects of current uncertainties in tropopause temperature, horizontal transport and small-scale mixing on simulated H2O in the lower stratosphere (LS). To assess the sensitivities of simulated H2O, we use the Chemical Lagrangian Model of the Stratosphere (CLaMS). First, we examine CLaMS, which is driven by two reanalyses, from the European Centre of Medium-Range Weather Forecasts (ECMWF) ERA-Interim and the Japanese 55-year Reanalysis (JRA-55), to investigate the robustness with respect to the meteorological dataset. Second, we carry out CLaMS simulations with transport barriers along latitude circles (at the Equator, 15 and 35∘ N/S) to assess the effects of horizontal transport. Third, we vary the strength of parametrized small-scale mixing in CLaMS. Our results show significant differences (about 0.5 ppmv) in simulated stratospheric H2O due to uncertainties in the tropical tropopause temperatures between the two reanalysis datasets, JRA-55 and ERA-Interim. The JRA-55 based simulation is significantly moister when compared to ERA-Interim, due to a warmer tropical tropopause (approximately 2 K). The transport barrier experiments demonstrate that the Northern Hemisphere (NH) subtropics have a strong moistening effect on global stratospheric H2O. The comparison of tropical entry H2O from the sensitivity 15∘ N/S barrier simulation and the reference case shows differences of up to around 1 ppmv. Interhemispheric exchange shows only a very weak effect on stratospheric H2O. Small-scale mixing mainly increases troposphere–stratosphere exchange, causing an enhancement of stratospheric H2O, particularly along the subtropical jets in the summer hemisphere and in the NH monsoon regions. In particular, the Asian and American monsoon systems during a boreal summer appear to be regions especially sensitive to changes in small-scale mixing, which appears crucial for controlling the moisture anomalies in the monsoon UTLS. For the sensitivity simulation with varied mixing strength, differences in tropical entry H2O between the weak and strong mixing cases amount to about 1 ppmv, with small-scale mixing enhancing H2O in the LS. The sensitivity studies presented here provide new insights into the leading processes that control stratospheric H2O, which are important for assessing and improving climate model projections.

Diagnosing Observed Stratospheric Water Vapor Relationships to the Cold Point Tropical Tropopause

“…This influence of the monsoon regions on lower stratosphere water vapor can be viewed clearly in monthly average MLS data, as shown in the evolution during August-October 2017 in Figure 6. This reflects the contribution of the monsoon regions to the "moist phase" of the tape recorder signal, as has been inferred in modeling studies of water vapor, for example, Gettelman et al (2004), Poshyvailo et al (2018), and Nützel et al (2019). While it is difficult to quantify the contributions from monsoon versus the freezedrying of air masses ascending across the tropical tropopause from observations, this process undoubtedly contributes to the weaker NH correlations following boreal summer seen in Figure 5.…”

“…Time‐lag correlations demonstrate the rapid propagation of the near‐equatorial H 2 O anomalies to high latitudes in the lower stratosphere, approximately following (sloping) isentropic levels near 400 K, along with upward propagation in the tropics. These are well‐known aspects of H 2 O transport as seen in satellite observations (Gilford et al, ; Mote et al, ; Randel et al, ; Rosenlof et al, ) and model simulations (e.g., Poshyvailo et al, ). Our results have quantified the seasonally varying behavior of the H 2 O transport and T CP influences: During boreal winter coherent H 2 O variations extend to both hemispheres in the lower stratosphere and to altitudes above 30 km in the tropics (Figures and ).…”

“…Beyond the annual cycle, observations have also shown a close link between interannual changes in water vapor and tropical cold point temperature (hereafter T CP ;), for example, Randel et al (), Fujiwara et al (), Liang et al (), Fueglistaler et al (), Kawatani et al (), and Tweedy et al (). The coupling of H 2 O entry with T CP is furthermore confirmed by realistic simulations of water vapor using chemical transport models or Lagrangian trajectory calculations, where seasonal and interannual variations in imposed T CP are reflected in water vapor behavior (Avery et al, ; Fueglistaler et al, ; Fueglistaler & Haynes, ; Hegglin et al, ; Poshyvailo et al, ; Schiller et al, ; Schoeberl & Dessler, ; Schoeberl et al, ; Tao et al, ; Schoeberl et al, ; Wang et al, ). Coupling of T CP and stratospheric water vapor on interannual time scales was also highlighted in the modeling studies of Giorgetta and Bengston () and Geller et al ().…”

Tropospheric mixing and parametrization of unresolved convective updrafts as implemented in the Chemical Lagrangian Model of the Stratosphere (CLaMS v2.0)

“…Similar to WACCM, the artificial e90 tracer, with a constant e-folding lifetime of 90 d, is set to 150 ppb everywhere in the lowest layer of CLaMS. The e90 tracer is suitable to diagnose typical timescale of transport from the planetary boundary layer (PBL) into the lower stratosphere (Prather et al, 2011;Abalos et al, 2017). Figure 1 shows that CLaMS in the current version significantly underestimates the upward transport if compared with the WACCM model and that this comparison improves if the new version of transport is included.…”

Evaluation of the IAGOS-Core GHG package H<sub>2</sub>O measurements during the DENCHAR airborne inter-comparison campaign in 2011

“…In this process self-broadening effects must be taken into account, which broaden the line shape and hence decrease the peak height as the water vapor level increases. To avoid an underestimation of the water vapor mole fraction, a quadratic correction is implemented in the Picarro analyzer (Rella, 2010): (Winderlich et al, 2010). The calibration constant obtained in this experiment was transferred to all greenhouse gas CRDS instruments manufactured by Picarro Inc. (Rella, 2010):…”