Milankovitch, the father of paleoclimate modelling

The origin of the long-term variations of the astronomical elements used by Milankovitch are first described, followed by the value of the astronomical periods. The detailed calculations by Milankovitch of the incoming solar radiation during the astronomical and caloric half-years are summarized, stressing the originality of the caloric ones. The second original contribution of Milankovitch to paleoclimate research was without any doubt his mathematical climate. How this model allowed him to give the caloric summer and winter insolation a climatological meaning is illustrated.

Assessing annual streamflow response to forest disturbance in the western US: A large-sample hydrology approach

<p>Forested watersheds across the western US have experienced recent widespread disturbance and tree mortality due to a combination of heat, drought, and epidemic insect and disease outbreaks. Hydrologic response has included both increases and decreases in the fraction of annual precipitation that is partitioned to streamflow versus evapotranspiration (ET). We used a large-sample hydrology approach to address two questions: First, how have water budget components changed during this period of high forest disturbance, and second, does streamflow response vary with disturbance severity, incoming solar radiation, and/or aridity? From previous studies, streamflow and runoff ratio are expected to increase with forest disturbance due to reduced ET, and conversely increases in forest density are expected to reduce streamflow. We statistically evaluated whether these expectations were met, and where and why contradictory responses occurred, using trend and regression analysis. We constructed annual water budgets for 211 watersheds in the western US from daily observations in the CAMELS dataset, which includes streamflow and climate data as well as watershed characteristics such as mean incoming solar radiation and aridity (i.e., ratio of mean annual potential ET to mean annual precipitation, or PET/P). Forest disturbance was quantified as percentage change in live tree volume and mean annual rate of tree mortality, using data collected by the US Forest Service’s Forest Inventory and Analysis program. While most water budget components and forcing variables did not exhibit consistent trends, many watersheds experienced significant increases in temperature and PET. Unexpected trends in runoff ratio occurred in two scenarios: First, runoff ratio decreased following forest disturbance in many water-limited watersheds (i.e., PET/P>1) of the southwestern US; and second, both runoff ratios and forest densities increased in some energy-limited watersheds of the Pacific Northwest. Water-limited watersheds and those with high solar radiation experienced more forest disturbance than energy-limited watersheds. We used hydrologic time trend analysis to quantify the magnitude of streamflow change. A linear regression model including precipitation and temperature as inputs was calibrated and validated using the pre-disturbance time period (1980-2006, odd years and even years, respectively; r<sup>2</sup><sub>val</sub>=0.954), and then applied to the post-disturbance time period (2007-2019), where model residuals are assumed to represent change in streamflow due to factors not included in the model, i.e., forest change. Among the 65 watersheds with significant streamflow change, the magnitude of change was moderately correlated with both disturbance severity and solar radiation. Decreased post-disturbance streamflow occurred mainly in watersheds with low to moderate tree mortality and high incoming solar radiation. We used multiple linear regression to identify important predictors of streamflow change. Pre-disturbance streamflow, change in precipitation and PET, solar radiation, and the interaction of solar radiation and tree mortality were all highly significant predictors (p</p>

Clouds and vegetation modulate shallow groundwater table depth

A 10-year simulation of shallow groundwater table (GWT) depth over a temperate region in northwestern Europe, using a physics based integrated hydrological model at km-scale, exhibits a strong seasonal cycle. This is also well captured in terms of near surface soil moisture anomalies, terrestrial water storage anomalies and shallow GWT depth anomalies from observations over the region. The modeled monthly anomaly of GWT depth exhibits a statistically significant (p<0.05)moderate positive/negative correlation with non-rain and rain affected monthly anomalies of incoming solar radiation. The vegetation cover also produces a strong local control on the variability of shallow GWT depth. Thus, much of the variability in the simulated seasonal cycle of shallow GWT depth could be linked to the distribution of clouds and vegetation.The spatiotemporal distribution of clouds, partly influenced by the Rhein Massif, modulates the seasonal variability of incoming solar radiation and precipitation, over the region. Particularly, the southwestern and northern part of the Rhien Massif divided by the Rhein valley exhibits a dipole behavior with relatively high/low shallow GWT depth fluctuations, associated with positive/negative anomaly of incoming solar radiation and negative/positive anomaly of precipitation.

Cloudiness Information Services for Solar Energy Management in West Africa

In West Africa (WA), interest in solar energy development has risen in recent years with many planned and ongoing projects currently in the region. However, a major drawback to this development in the region is the intense cloud cover that reduces the incoming solar radiation when present and causes fluctuations in solar power production. Therefore, understanding the occurrence of clouds and their link to the surface solar radiation in the region is important for making plans to manage future solar energy production. In this study, we use the state-of-the-art European Centre for Medium-range Weather Forecasts ReAnalysis (ERA5) dataset to examine the occurrence and persistence of cloudy and clear-sky conditions in the region. Then, we investigate the effects of cloud cover on the quantity and variability of the incoming solar radiation. The cloud shortwave radiation attenuation (CRASW↓) is used to quantify the amount of incoming solar radiation that is lost due to clouds. The results showed that the attenuation of incoming solar radiation is stronger in all months over the southern part of WA near the Guinea Coast. Across the whole region, the maximum attenuation occurs in August, with a mean CRASW↓ of about 55% over southern WA and between 20% and 35% in the Sahelian region. Southern WA is characterized by a higher occurrence of persistent cloudy conditions, while the Sahel region and northern WA are associated with frequent clear-sky conditions. Nonetheless, continuous periods with extremely low surface solar radiation were found to be few over the whole region. The analysis also showed that the surface solar radiation received from November to April only varies marginally from one year to the other. However, there is a higher uncertainty during the core of the monsoon season (June to October) with regard to the quantity of incoming solar radiation. The results obtained show the need for robust management plans to ensure the long-term success of solar energy projects in the region.

Observations and Modelling Results Help to Understand Global Hillslope Asymmetry

&lt;p&gt;Aspect-controlled vegetation over opposing hillslopes are driven by non-uniform distribution of incoming solar radiation in semi-arid ecosystems. This leads to variation in soil and vegetation characteristics. In mid- to high-latitudes where available soil moisture is a limiting factor for vegetation growth, poleward-facing slopes develop denser vegetation cover providing greater erosion protection than the equatorward-facing hillslopes. The variation in erosion rates across opposing hillslopes leads to the development of topographic asymmetry of hillslopes over long timescales. This asymmetry is quantified by the hillslope asymmetry index (HAI), a metric given as the ratio of the median slope angles of opposite hillslopes. We present a combined approach of modelling and observed data analysis to investigate the relationships of HAI with climatological, geomorphic, and ecologic variables at a global scale. We analysed these relationships using digital elevation topographic data to compute observed HAI for 80 different catchments across the world, where aspect-controlled vegetation has been reported in the literature. Further, we used the CHILD landscape evolution model (LEM), which uses the continuity equation for water, sediment, and biomass, to investigate the control of climatological, geomorphic, and ecologic variables on the development of hillslope asymmetry through a modelling approach,. The outcomes of the study highlights that latitude and mean topographic gradient are the two dominant factors affecting hillslope asymmetry due to their vital role in controlling vegetation density through the modulation of incoming solar radiation. These results improve our understanding on how different climatic variables and geographic properties affect the magnitude of hillslope asymmetry and their implications on landform evolution modelling.&lt;/p&gt;