Metabolite or substrate channeling is a direct transfer of metabolites from one enzyme to the next enzyme in a cascade. Among many potential advantages of substrate channeling, acceleration of the total reaction rate is considered as one of the most important and self-evident. However, using a simple model, supported by stochastic simulations, we show that it is not always the case; particularly at long times (i.e. in steady state) and high substrate concentrations, a channeled reaction cannot be faster, and can even be slower, than the original non-channeled cascade reaction. In addition we show that increasing the degree of channeling may lead to an increase of the metabolite pool size. We substantiate that the main advantage of channeling likely lies in protecting metabolites from degradation or competing side reactions.

The sensitivity studies presented here provide new insights into the leading processes that control stratospheric H 2 O, important for assessing and improving climate model projections.

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.

reported catastrophic impacts with an increase of cardiovascular and respiratory diseases leading to premature mortality. Climate change adaptation strategies, tailored to each regional context and vulnerability, still require more reliable regional projections of such extreme events (Jacob et al., 2020). Data from regional climate models (RCMs) are a major source to investigate heat waves (

Abstract. Due to climate change, years with positive temperature anomalies are becoming more frequent in Europe, requiringhigh-resolution climate data to plan for climate change mitigation and adaptation. However, many regional climate models(RCMs) simplify the representation of groundwater processes, leading to biases in simulated extreme heat events. Here, westudy the characteristics of summer heat events in a unique dataset from the regional Terrestrial Systems Modeling Platform(TSMP) simulations, compared to an ensemble of EURO-CORDEX climate change scenario control simulations, for the historicaltime period 1976–2005. Our results show that in TSMP, the impact of groundwater coupling on the frequency of hot summer days depends on theconsidered time period and the region, associated with respective evaporative regime. An increasing trend of the frequencyof hot summer days averaged across Europe is the lowest in TSMP compared to the other RCMs considered. Groundwatercoupling has a systematic effect on the duration and intensity of heat events: summer heat events with long duration and highintensity are less frequent in TSMP compared to the CORDEX ensemble. In particular, extended heat events with a durationexceeding 6 days, i.e. heat waves, occur on average in Europe about 1.5–8 times less often in TSMP, while single-day heatevents happen slightly more often in TSMP compared to the CORDEX ensemble. The frequency of high-intensity heat wavesin TSMP is up to 12 times lower on average in Europe compared to the CORDEX ensemble. Thus, an explicit groundwaterrepresentation in RCMs may lead to rarer and weaker heat waves in Europe also in climate projections. The findings of thiswork indicate an existing discrepancy in the ensemble of EURO-CORDEX climate change scenario control simulations andemphasize the importance of groundwater representation in RCMs.