An anomalous atmospheric river linked to the late June 2021 western North America heatwave

Atmospheric rivers (ARs) are long and narrow bands of enhanced water vapour flux concentrated in the lower troposphere. Many studies have documented the important role of cold-season ARs in producing heavy precipitation and triggering extreme flooding in many parts of the world. However, relatively little research has been conducted on the warm-season ARs and their impacts on extreme heatwave development. Here we show an anomalous warm-season AR moving across the North Pacific and its interaction with the western North American heatwave in late June 2021. We call it an “oriental express’’ to highlight its capability to transport tropical moisture to the west coast of North America from sources in Southeast Asia. Its landfall over the Alaska Panhandle lasted for more than two days and resulted in significant spillover of moisture into western Canada. We provide evidence that the injected water vapour was trapped under the heat dome and may have formed a positive feedback mechanism to regulate the heatwave development in western North America.

Modelling plant transpiration and leaf climate using CFD

Research into vertical farms or plant factories is steadily increasing over the years, as the demand for sustainable food production and a shift to more environmental friendly food production is occurring. Modelling plant climate in these confined spaces is therefore essential to guarantee optimal growing conditions. Modelling of plant climate has already been done in greenhouses, but at length scales much bigger than individual leaves. In this study, one single plant will be modelled, using computational fluid dynamics and by incorporating additional source terms in the relevant transport equations. Plants are modelled using the big leaf approach, where a plant is modelled as one artificial leaf. Water vapour flux in plants is controlled by two resistances in series, the aerodynamic resistance, which is a function of the boundary layer around the leaves and the stomatal resistance, which is the resistance against water vapour transport in leaves. Two different plants are studied, impatiens pot plant and basil plants. Values of stomatal resistance for these crops are obtained from literature or were measured. Evapotranspiration was compared with the Penman-Monteith equation.

Moisture transport during large snowfall events in the New Zealand Southern Alps: The role of atmospheric rivers

Synoptic-scale moisture transport during large snowfall events in the New Zealand Southern Alps is largely unknown due to a lack of long-term snow observations. In this study, records from three recently developed automatic weather stations (Mahanga, Mueller Hut and Mt Larkins) near the Main Divide of the Southern Alps were used to identify large snowfall events between 2010 and 2018. The large snowfall events are defined as those events with daily snow depth increase by greater than the 90th percentile at each site. ERA-Interim reanalysis data were used to characterize the hydrometeorological features of the selected events. Our findings show that large snowfall events in the Southern Alps generally coincide with strong fields of integrated vapour transport (IVT) within a north-westerly airflow and concomitant deepening low pressure systems. Considering the frequency of large snowfall events, approximately 61% of such events at Mahanga were associated with narrow corridors of strong water vapour flux, known as atmospheric rivers (ARs). The contributions of ARs to the large snowfall events at Mueller Hut and Mt Larkins were 70% and 71%, respectively. Analysis of the vertical profiles of moisture transport dynamics during the passage of a landfalling AR during 11-12th October 2016 revealed the key characteristics of a snow-generating AR in the Southern Alps. An enhanced presence of low and mid-level moisture between 700-850 hPa and pronounced increases of wind velocities (more than 30 m s-1) with high values of the meridional component between 750-850 hPa were identified over the Southern Alps during the event.

Climate change impacts on crop water productivity in Africa using a multi-model inter-comparison

<p>Evapotranspiration (ET) or the water vapour flux is an important component in the water cycle and is widely studied due to its implications in disciplines ranging from hydrology to agricultural and climate sciences. In the recent past, growing attention has been given to estimating ET fluxes at regional and global scales. However, estimation of ET at large scales has been a difficult task due to direct measurement of ET being possible only at point locations, for example using flux towers. For the African continent, only a limited number of flux tower data are openly available for use, which makes verification of regional and global ET products very difficult. Recent advances in satellite based products provide promising data to fill these observational gaps.</p><p>In this study we propose to investigate the Climate Change (CC) impact on crop water productivity across Africa using ET and crop yield predictions of different crop models for future climate scenarios. Different model outputs are evaluated including models from both the ISI-MIP 2a and 2b protocols. Considering the problem of direct observations of ET being difficult to obtain, especially over Africa, we use ET estimates from several remotely sensed derived products as a references to evaluate the crop models (maize) in terms of magnitude, spatial patterns and variations between models. The crop model results for crop yield are compared to FAO reported crop yields at country scale. The results show a very strong disagreement between the different crop models of the baseline scenario and when compared with ET and crop yield data.  Also, a very large uncertainty is obtained for the climate change predictions. It is hence recommended to improve the crop models for application in Africa.</p>

How comets work: nucleus erosion versus dehydration

ABSTRACT We develop an activity model based on ice sublimation and gas diffusion inside cm-sized pebbles making-up a cometary nucleus. Our model explains cometary activity assuming no free parameters and fixing the nucleus surface temperature Ts, its gradient below the nucleus surface at thermal equilibrium, the pressure inside the porous pebbles, and the gas flux from them. We find that (i) the nucleus erosion rate and water vapour flux are independent of the nucleus refractory-to-ice ratio, which affects the dehydration rate only; (ii) water-driven dust ejection occurs in thermal quasi-equilibrium at Ts > 205 K; (iii) the smallest and largest ejected dust sizes depend on the nucleus surface temperature and its gradient at depths of few cm; and (iv) the water-driven nucleus erosion rate is independent of the water vapour flux. Regarding comet 67P/Churyumov–Gerasimenko, we find that (i) during the northern and southern polar summers, the nucleus active areas are ≈5 km2; (ii) >95 per cent of the southern pristine nucleus has a refractory-to-water-ice mass ratio >5; and (iii) the different temperature dependences of the dehydration and erosion rates explain the seasonal cycle: at perihelion, dm-sized chunks ejected by the sublimation of CO2 ices are rapidly enveloped by an insulating crust, preserving most water ice up to their fallout on the northern dust deposits; the inbound water-driven activity at low temperatures triggers a complete erosion of the fallout if its water-ice mass fraction is >0.1 per cent.

Low-cost eddy covariance: a case study of evapotranspiration over agroforestry in Germany

Heterogeneous land surfaces require multiple measurement units for spatially adequate sampling and representative fluxes. The complexity and cost of traditional eddy covariance (EC) set-ups typically limits the feasible number of sampling units. Therefore, new low-cost eddy covariance systems provide ideal opportunities for spatially replicated sampling. The aim of this study was to test the performance of a compact, low-cost pressure, temperature and relative humidity sensor for the application of evapotranspiration measurements by eddy covariance over agroforestry and conventional agriculture in Germany. We performed continuous low-cost eddy covariance measurements over agroforestry and conventional agriculture for reference at five sites across northern Germany over a period of 2 years from 2016 to 2017. We conducted side-by-side measurements using a roving enclosed-path eddy covariance set-up to assess the performance of the low-cost eddy covariance set-up. Evapotranspiration measured with low-cost eddy covariance compared well with fluxes from conventional eddy covariance. The slopes of linear regressions for evapotranspiration comparing low-cost and conventional eddy covariance set-ups ranged from 0.86 to 1.08 for 5 out of 10 sites, indicating a 14 % flux underestimation and a 8 % flux overestimation relative to the conventional eddy covariance set-up, respectively. Corresponding coefficients of determination, R2, ranged from 0.71 to 0.94 across sites. The root-mean-square error for differences between latent heat fluxes obtained by both set-ups were small compared to the overall flux magnitude, with a mean and standard deviation of 34.23±3.2 W m−2, respectively, across sites. The spectral response characteristics of the low-cost eddy covariance set-up were inferior to the eddy covariance set-up in the inertial sub-range of the turbulent spectrum. The water vapour flux co-spectrum of the low-cost eddy covariance set-up underestimated the theoretical slope of -4/3, stronger than the conventional eddy covariance set-up. This underestimation was mainly caused by the limited response time of the low-cost thermohygrometer being longer than 1 s. We conclude that low-cost eddy covariance sensors are an alternative to conventional eddy covariance sensors when, first, replicates are required and, second, the spatial variability of fluxes of the ecosystems of interest is larger than above-reported set-up-specific differences in fluxes.

Observation and modelling of snow at a polygonal tundra permafrost site: spatial variability and thermal implications

The shortage of information on snow properties in high latitudes places a major limitation on permafrost and more generally climate modelling. A dedicated field program was therefore carried out to investigate snow properties and their spatial variability at a polygonal tundra permafrost site. Notably, snow samples were analysed for surface-normal thermal conductivity (Keff−z) based on X-ray microtomography. Also, the detailed snow model SNOWPACK was adapted to these Arctic conditions to enable relevant simulations of the ground thermal regime. Finally, the sensitivity of soil temperatures to snow spatial variability was analysed. Within a typical tundra snowpack composed of depth hoar overlain by wind slabs, depth hoar samples were found more conductive (Keff-z=0.22±0.05 W m−1 K−1) than in most previously published studies, which could be explained by their high density and microstructural anisotropy. Spatial variations in the thermal properties of the snowpack were well explained by the microtopography and ground surface conditions of the polygonal tundra, which control depth hoar growth and snow accumulation. Our adaptations to SNOWPACK, phenomenologically taking into account the effects of wind compaction, basal vegetation, and water vapour flux, yielded realistic density and Keff−z profiles that greatly improved simulations of the ground thermal regime. Also, a density- and anisotropy-based parameterization for Keff−z lead to further slight improvements. Soil temperatures were found to be particularly sensitive to snow conditions during the early winter and polar night, highlighting the need for improved snow characterization and modelling over this period.

The concurrence of atmospheric rivers and explosive cyclogenesis in the North Atlantic and North Pacific basins

The explosive cyclogenesis of extratropical cyclones and the occurrence of atmospheric rivers are characteristic features of a baroclinic atmosphere, and are both closely related to extreme hydrometeorological events in the mid-latitudes, particularly on coastal areas on the western side of the continents. The potential role of atmospheric rivers in the explosive cyclone deepening has been previously analysed for selected case studies, but a general assessment from the climatological perspective is still missing. Using ERA-Interim reanalysis data for 1979–2011, we analyse the concurrence of atmospheric rivers and explosive cyclogenesis over the North Atlantic and North Pacific basins for the extended winter months (ONDJFM). Atmospheric rivers are identified for almost 80 % of explosive deepening cyclones. For non-explosive cyclones, atmospheric rivers are found only in roughly 40 % of the cases. The analysis of the time evolution of the high values of water vapour flux associated with the atmospheric river during the cyclone development phase leads us to hypothesize that the identified relationship is the fingerprint of a mechanism that raises the odds of an explosive cyclogenesis occurrence and not merely a statistical relationship. These new insights on the relationship between explosive cyclones and atmospheric rivers may be helpful to a better understanding of the associated high-impact weather events.

Observation and modelling of snow at a polygonal tundra permafrost site: spatial variability and thermal implications

The shortage of information on snow properties in high latitudes places a major limitation on permafrost and more generally climate modelling. A dedicated field program was therefore carried out to investigate snow properties and their spatial variability at a polygonal tundra permafrost site. Notably, snow samples were analysed for surface-normal thermal conductivity (Keff-z) based on X-ray microtomography. Also, the detailed snow model SNOWPACK was adapted to these Arctic conditions to enable relevant simulations of the ground thermal regime. Finally, the sensitivity of soil temperatures to snow spatial variability was analysed. Our depth hoar samples were found more conductive (Keff-z = 0.22 ± 0.05 W m−1 K−1) than in most previously published studies, which could be explained by their high density and anisotropy. Spatial variations in the thermal properties of the snowpack were well explained the micro-topography and ground surface conditions of the polygonal tundra, which control depth hoar growth and snow accumulation. Our adaptations to SNOWPACK, phenomenologically taking into account the effects of wind compaction, basal vegetation and water vapour flux, yielded realistic density and Keff-z profiles that greatly improved simulations of the ground thermal regime. The potential of an anisotropy and density-based formulation of Keff-z in snow models was shown. Soil temperatures were found to be particularly sensitive to snow conditions during the dark part of winter, highlighting the need for improved snow characterization and modelling over this period.