Journal of Geophysical Research Atmospheres

Garcia, Matthew; Townsend, Philip A.

doi: 10.1002/2016JD025190pmid: N/A

We assess long‐term climatological means, trends, and interannual variability around the western end of Lake Superior during 1984–2013 by using available weather station data. Our results focus on changes in basic and derived climate indicators from seasonal and annual temperature and precipitation, to the traditionally defined frost‐free season, to a novel definition of the climatological growing season. We describe seasonal and year‐to‐year climate variability that influences forest phenology, using an alternative growing season metric that is based on the warm‐season plateau in accumulated chilling days as an indicator of environmental triggers for vegetation growth and senescence. Our results indicate +0.56°C regional warming during our 30 year study period, with cooler springs (−1.26°C) and significant autumn warming (+1.54°C). The duration of the climatological growing season has increased +0.27 d/yr, extending primarily into autumn. Summer precipitation in our study area has declined by an average −0.34 cm/yr, potentially leading to moisture stress that impairs vegetation carbon uptake rates and can render the forest more vulnerable to disturbance. Many changes in temperature, precipitation, and climatological growing season are most prominent in locations where Lake Superior exerts a strong hydroclimatological influence, especially the Minnesota shoreline and in forest areas downwind (southeast) of the lake. Observed trends in lake temperature and ice phenology have also changed, coincident with a large‐scale climatological regime shift around 1998. A number of factors are likely altering forest phenology and the role of the forest in the climate system of this ecologically important and highly varied forest‐and‐lake region.

Carro‐Calvo, L.; Hoose, C.; Stengel, M.; Salcedo‐Sanz, S.

doi: 10.1002/2016JD025552pmid: N/A

The phase partitioning between supercooled liquid water and ice in clouds in the temperature range between 0 and −37°C influences their optical properties and the efficiency of precipitation formation. Passive remote sensing observations provide long‐term records of the cloud top phase at a high spatial resolution. Based on the assumption of a cumulative Gaussian distribution of the ice cloud fraction as a function of temperature, we quantify the cloud glaciation temperature (CGT) as the 50th percentile of the fitted distribution function and its variance for different cloud top pressure intervals, obtained by applying an evolutionary algorithm (EA). EAs are metaheuristics approaches for optimization, used in difficult problems where standard approaches are either not applicable or show poor performance. In this case, the proposed EA is applied to 4 years of Pathfinder Atmospheres‐Extended (PATMOS‐x) data, aggregated into boxes of 1° × 1° and vertical layers of 5.5 hPa. The resulting vertical profile of CGT shows a characteristic sickle shape, indicating low CGTs close to homogeneous freezing in the upper troposphere and significantly higher values in the midtroposphere. In winter, a pronounced land‐sea contrast is found at midlatitudes, with lower CGTs over land. Among this and previous studies, there is disagreement on the sign of the land‐sea difference in CGT, suggesting that it is strongly sensitive to the detected and analyzed cloud types, the time of the day, and the phase retrieval method.

doi: 10.1002/jgrd.52490pmid: N/A

No abstract is available for this article.

Keune, Jessica; Gasper, Fabian; Goergen, Klaus; Hense, Andreas; Shrestha, Prabhakar; Sulis, Mauro; Kollet, Stefan

doi: 10.1002/2016JD025426pmid: N/A

The impact of 3D groundwater dynamics as part of the hydrologic cycle is rarely considered in regional climate simulation experiments. However, there exists a spatial and temporal connection between groundwater and soil moisture near the land surface, which can influence the land surface‐atmosphere feedbacks during heat waves. This study assesses the sensitivity of bedrock‐to‐atmosphere simulations to groundwater representations at the continental scale during the European heat wave 2003 using an integrated fully coupled soil‐vegetation‐atmosphere model. The analysis is based on the comparison of two groundwater configurations: (1) 3D physics‐based variably saturated groundwater dynamics and (2) a 1D free drainage (FD) approach. Furthermore, two different subsurface hydrofacies distributions (HFD) account for the uncertainty of the subsurface hydraulic characteristics, and ensemble simulations address the uncertainty arising from different surface‐subsurface initial conditions. The results show that the groundwater representation significantly impacts land surface‐atmosphere processes. Differences between the two groundwater configurations follow subsurface patterns, and the largest differences are observed for shallow water table depths. While the physics‐based setup is less sensitive to the HFD, the parameterized FD simulations are highly sensitive to the hydraulic characteristics of the subsurface. An analysis of variance shows that both, the groundwater configuration and the HFD, induce variability across all compartments with decreasing impact from the subsurface to the atmosphere, while the initial condition has only a minor impact.