ALBERT

Description: Atmospheric water vapour tracers (WVTs) are an elegant tool to determine source–sink relations of moisture "online" in atmospheric general circulation models (AGCMs). However, it is sometimes desirable to establish such relations "offline" based on already existing atmospheric data (e.g. reanalysis data). One simple and frequently applied offline method is 2-D moisture tracing. It makes use of the "well-mixed" assumption, which allows for treating the vertical dimension integratively. Here we scrutinise the "well-mixed" assumption and 2-D moisture tracing by means of analytical considerations in combination with AGCM-WVT simulations. We find that vertically well-mixed conditions are seldom met. Due to the presence of vertical inhomogeneities, 2-D moisture tracing (i) neglects a significant degree of fast-recycling, and (ii) results in erroneous advection where the direction of the horizontal winds varies vertically. The latter is not so much the case in the extratropics, but in the tropics this can lead to large errors. For example, computed by 2-D moisture tracing, the fraction of precipitation in the western Sahel that originates from beyond the Sahara is ~40%, whereas the fraction that originates from the tropical and Southern Atlantic is only ~4%. According to full (i.e. 3-D) moisture tracing, however, both regions contribute roughly equally, showing that the errors introduced by the 2-D approximation can be substantial.

Description: Atmospheric water vapour tracers (WVTs) are an elegant tool to determine source-sink relations of moisture "online" in atmospheric general circulation models (AGCMs). However, it is sometimes desireable to establish such relations "offline" based on already existing atmospheric data (e.g. reanalysis data). One simple and frequently applied offline method is 2-D moisture tracing. It makes use of the "well-mixed" assumption, which allows to treat the vertical dimension integratively. Here we scrutinise the "well-mixed" assumption and 2-D moisture tracing by means of analytical considerations in combination with AGCM-WVT simulations. We find that vertically well-mixed conditions are seldomly met. Due to the presence of vertical inhomogeneities, 2-D moisture tracing (I) neglects a significant degree of fast-recycling, and (II) results in erroneous advection where the direction of the horizontal winds varies vertically. The latter is not so much the case in the extratropics, but in the tropics this can lead to large errors. For example, computed by 2-D moisture tracing, the fraction of precipitation in the Western Sahel that originates from beyond the Sahara is ~40%, whereas the fraction that originates from the tropical and Southern Atlantic is only ~4%. Full (i.e. 3-D) moisture tracing however shows that both regions contribute roughly equally, which reveals the results of an earlier study as spurious. Moreover, we point out that there are subtle degrees of freedom associated with the implementation of WVTs into AGCMs because the strength of mixing between precipitation and the ambient water vapour is not completely provided by such models. We compute an upper bound for the resulting uncertainty and show that this uncertainty is smaller than the errors associated with 2-D moisture tracing.

Description: Moisture recycling estimates are diagnostic measures that could ideally be used to deduce the response of precipitation to modified land-evaporation. Recycling estimates are based on moisture-budget considerations in which water is treated as a passive tracer. But in reality water is a thermodynamically active component of the atmosphere. Accordingly, recycling estimates are applicable to deduce the response to a perturbation only if other mechanisms by which evaporation affects climate do not dominate the response – a condition that has not received sufficient attention in the literature. In our analysis of what moisture recycling estimates tell us, we discuss two such additional mechanisms that result from water's active role. These are (I) local coupling, by which precipitation is affected locally via the thermal structure of the atmosphere, and (II) the atmospheric circulation, by which precipitation is affected on a large spatial scale. We perform two global climate model experiments: One with and another without continental evaporation. By this extreme perturbation we test the predictive utility of a certain type of recycling measure, the "continental recycling ratio". Moreover, by such a strong perturbation the whole spectrum of possible responses shows up simultaneously, giving us the opportunity to discuss all concurrent mechanisms jointly. The response to this extreme perturbation largely disagrees with the hypothesis that moisture recycling is the dominant mechanism. Instead, most of the response can be attributed to changes in the atmospheric circulation, while the contributions to the response by moisture recycling as well as local coupling, though noticeable, are smaller. By our case study it is not possible to give a general answer to the question posed in the title, but it demonstrates that recycling estimates do not necessarily mirror the consequences of land-use change for precipitation.

Description: On their journey over large land masses, water molecules experience a number of precipitation–evaporation cycles (recycling events). We derive analytically the frequency distributions of recycling events for the water molecules contained in a given air parcel. Given the validity of certain simplifying assumptions, the frequency distribution of recycling events is shown to develop either into a Poisson distribution or a geometric distribution. We distinguish two cases: in case (A) recycling events are counted since the water molecules were last advected across the ocean–land boundary. In case (B) recycling events are counted since the water molecules were last evaporated from the ocean. For case B we show by means of a~simple scale analysis that, given the conditions on earth, realistic frequency distributions may be regarded as a mixture of a Poisson distribution and a geometric distribution. By contrast, in case A the Poisson distribution generally appears as a reasonable approximation. This conclusion is consistent with the simulation results of an earlier study where an atmospheric general circulation model equipped with water vapor tracers was used. Our results demonstrate that continental moisture recycling can be interpreted as a Poisson process.

Description: Moisture evaporated from the continents (recycled moisture) contributes up to 80% to the total atmospheric moisture content and, hence, precipitation in some regions. Recycling estimates are traditionally used to indicate a region's rainfall-dependence on land-surface evaporation. Accordingly, recycling estimates are employed to deduce the hydrological consequences of land-cover change. However, moisture is not a passive but an active constituent of the atmosphere. Recent studies indicate that at small scales (up to 1000 km) local to regional evaporation-precipitation coupling by far dominates the atmospheric precipitation response, while the water-balance effect from moisture recycling in the traditional sense seems to be of minor importance. The value of moisture recycling estimates as indicator for consequences of land-cover change is therefore questionable. However, since atmospheric moisture is still subject to mass conservation, the relevance of moisture recycling may come into play at the continental scale. To explore the relevance of recycling estimates regarding land-cover change at the continental scale, we conduct two global experiments with an atmospheric general circulation model: (I) with present-day conditions and (II) with extreme land-cover change conditions, namely with totally suppressed continental evaporation. Using the simulated fields of moisture, wind, and evaporation from the present-day experiment, we quantify continental moisture recycling with a vertically integrating tracing scheme. We then compare the computed recycling patterns with the hydrological changes that follow the suppression of continental evaporation. While under present-day conditions the fraction of recycled moisture increases from continental upstream to downstream regions with respect to the prevailing winds, the suppression of continental evaporation leads to severe precipitation loss in almost all continental regions, no matter if situated upstream or downstream. Over the ocean the hydrological response is ambigious, even where under present-day conditions large fractions of the atmospheric moisture stem from continental evaporation. This suggests that continental moisture recycling can not act across large ocean basins. Over land the absence of evaporative cooling at the surface leads to substantial warming which acts to suppress precipitation. In large parts of the continents the precipitation decrease compensates for much of the missing evaporation, such that the continental moisture-sink is not much amplified. Consequently, the atmospheric moisture content is not systematically reduced in the evaporation-free experiment, as would be necessary for the traditional moisture recycling mechanism to be active. Noteworthy exceptions are continental regions that are substantial moisture sources for some time of the year, first of all tropical wet-dry climates during the dry season. Apart from these exceptions, our results challenge the relevance of moisture recycling estimates for the hydrological consequences of land-cover change even at the continental scale.

Description: On their journey across large land masses, water molecules experience a number of precipitation-evaporation cycles (recycling events). We derive analytically the frequency distributions of recycling events for the water molecules contained in a given air parcel. Given the validity of certain simplifying assumptions, continental moisture recycling is shown to develop either into a Poisson distribution or a geometric distribution. We distinguish two cases: in case (A) recycling events are counted since the water molecules were last advected across the ocean-land boundary. In case (B) recycling events are counted since the water molecules were last evaporated from the ocean. For case B we show by means of a simple scale analysis that, given the conditions on Earth, realistic frequency distributions may be regarded as a mixture of a Poisson distribution and a geometric distribution. By contrast, in case A the Poisson distribution generally appears as a reasonable approximation. This conclusion is consistent with the simulation results of an earlier study where an atmospheric general circulation model equipped with water vapor tracers was used. Our results demonstrate that continental moisture recycling can be interpreted as a Poisson process.

Description: Recent decades have seen significant developments in seasonal-to-interannual timescale climate prediction capabilities. However, until recently the potential of such systems to predict Arctic climate had not been assessed. This paper describes a multi-model predictability experiment which was run as part of the Arctic Predictability and Prediction On Seasonal to Inter-annual Timescales (APPOSITE) project. The main goal of APPOSITE was to quantify the timescales on which Arctic climate is predictable. In order to achieve this, a coordinated set of idealised initial-value predictability experiments, with seven general circulation models, was conducted. This was the first model intercomparison project designed to quantify the predictability of Arctic climate on seasonal to inter-annual timescales. Here we present a description of the archived data set (which is available at the British Atmospheric Data Centre) and an update of the project's results. Although designed to address Arctic predictability, this data set could also be used to assess the predictability of other regions and modes of climate variability on these timescales, such as the El Niño Southern Oscillation.