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  • Open access
  • 16 Reads
Synoptic climatology of cut-off low events that produced extreme precipitation in Valencia, Spain

A seasonal synoptic climatology of cut-off lows (COLs) that produced extreme precipitation in the Valencia region of Spain during 1998-2014 is presented. COLs were shown to be the main producer of extreme precipitation in the region, especially during the transition seasons. The strongest raining COL events occurred during September-November. Six-day composites of lower and upper tropospheric winds, geopotential, sea-level pressure and precipitation show that COLs that produce extreme rainfall in this region remain stationary over Spain for 2-3 days and produce rainfall over the Valencia region for at least two days. In the low levels these COLs are characterized by low pressure over the Mediterranean sea and winds with an easterly, onshore component. Another interesting aspect of the composites is that transition season COLs are characterized by the presence of a filament of moisture that extends from the tropical Atlantic to Spain suggesting a role for remote moisture transport to feed COL rainfall. Further analysis is needed to confirm this hypothesis.

  • Open access
  • 21 Reads
Climatic influence on atmospheric evaporative demand in Estonia (1951-2015)

Atmospheric evaporative demand (ADE) trends at global scale are important to understand the impact of global warming in the hydrological cycle. But there is no consensus, in the global scale studies, about ADE variability and many areas have been ignored in regional studies. This is the case of Estonia, located in the eastern coast of the Baltic Sea between 57.5 and 59.5°N. To shed light on the ADE variability in the country we have studied the spatial and temporal variability of ET0 from 1951 to 2015.

We have computed ET0 from 9 high-quality meteorological stations by Penman–Monteith equation. We have analysed the spatial and temporal variability of ET0 and its main drivers i.e. maximum temperature, minimum temperature, wind speed, sunshine duration, relative humidity and atmospheric pressure.

ET0, at annual scale and country level, shows a positive and significant trend with a magnitude of change during the studied period of 5.3 mm decade-1, with the highest values during the spring (4.1 mm decade-1).  The costal series show a higher magnitude of change (7.1 mm decade-1in average) than the inland series (4.3 mm decade-1 in average), principally because coastal areas show greater magnitude of change during the summer. High significant correlation (r=0.7-0.8) have been found among computed ET0 and observed evaporation measurements with evaporation pan and lysimeter during the period 1968-2005.

At annual scale, during spring and summer ET0 is highly correlated with sunshine (positive), relative humidity (negative) and maximum temperature (positive). Meanwhile sunshine has no significate trend, maximum temperature shows positive and significant trend in all the series and seasons and relative humidity shows significant negative trends in 8 of the 9 series studied during the spring.

  • Open access
  • 25 Reads
Role of Caribbean low-level jet and Choco jet in the transport of moisture patterns towards Central America

Central America (CAM) is a thin strip of land, whose climate is influenced by the presence of low-level jets, by transporting atmospheric moisture from the surrounding oceanic masses (Pacific and Atlantic Oceans). In this study, we analyzed the seasonal patterns of water vapor transport to this region and their interannual variability, with special emphasis on the role of the Caribbean low-level jet (CLLJ) and the Choco jet (CJ). The semi-lagrangian 2D model Dynamic Recycling Model (DRM) is implemented, using information from ERA-Interim reanalysis during the period 1980-2012. Our results suggest that approximately 72% of mean annual atmospheric moisture transported to Central America comes from the Atlantic Ocean, with a contribution of 35% from the Caribbean Sea, and 23 and 14% from the North Atlantic and the Tropical North Atlantic, respectively. This transport is closely related to the CLLJ, showing that a strong (weak) jet induces a greater (smaller) transport from the Atlantic to CAM. On the other hand, transport from the Pacific exhibits a very marked seasonality, responding to the intensity of the CJ, which during high intensity episodes stimulates an increase in transport of water vapor. Finally, at the interannual time scale, it is found that during the positive phase of the El Niño–Southern Oscillation (ENSO) more moisture from the Caribbean reaches CAM during boreal Spring and Autumn, in contrast to a marked decrease from the Pacific during Autumn. A comparison of our results with the work by Durán-Quesada et al. (2017) using the 3D FLEXPART model, suggests that the DRM has a bias in estimating the mean annual cycle of water vapor transport associated with the CLLJ wind shear. However, the DRM is able to capture the interannual variability of the moisture transport and its response to ENSO and anomalies in the low-level jets.

  • Open access
  • 17 Reads
Identification of moisture sources in the Atlantic Ocean for cyclogenesis processes

We used the Lagrangian model FLEXPART to identify the moisture sources of 110 tropical cyclones with cyclogenesis within an area comprised between 15 – 45 W and 8 – 20 N. The model computes changes in the specific humidity from a 10-day period before the day of cyclogenesis of each tropical cyclone is and its contribution to the moisture budget of the region of interest. We calculated the values of the anomalies of the moisture budget to identify the main regions of moisture sources: the African coasts in the North Atlantic, the continental region over western Africa and along the South African coast. We also calculated the contribution of moisture sources from the South Atlantic to the cyclogenesis. The frequency distribution of these percentages shows two main peaks, one in ~20% and other in ~50%. The result shows that the path of tropical cyclones with ~50% of moisture contribution from the South Atlantic made landfall more often than those with ~20%.  

  • Open access
  • 13 Reads
Tagging moisture sources with Eulerian and Lagrangian tracers: Application to an intense atmospheric river event.

One Eulerian and two Lagrangian tracers’ tools are evaluated for studies on atmospheric moisture sources and pathways. The first method has been recently implemented online into the Weather Research and Forecasting (WRF) mesoscale model (Insua-Costa and Míguez-Macho, 2017), while the Lagrangian methods are described here. In these methods, a moisture volume is assigned to each particle which is then advected by the wind flow. Usual Lagrangian methods consider this volume to remain constant and the particle follows exactly the stream lines of the flow (Stohl and James, 2004). On the other hand, the initial moisture volume can be thought to depend on time as the flow is advected due to thermodynamic processes (for example, pressure, and temperature changes). In this case, the drag on the tracer volume must be taken into account. Equations have been implemented and moisture convection (Forster et al., 2007) was taken into account for both Lagrangian models.

We apply these methods to evaluate the intense atmospheric river (AR; i.e., a narrow plume of strong water vapor flux) that devastated the Pacific North Western America with flooding rains and intense winds in early November 2006 (Neiman et al., 2008). We note that the usual Lagrangian method underestimate moisture availability on the continent while the active tracers (both Eulerian and Lagrangian) achieve better results.

 

Forster, C., Stohl, A. and Seibert, P. J. Applied Meteo. and Climatology. 46, 403 (2007).

Insua-Costa, D. and Míguez-Macho, G. Eath Syst. Dyn. (2017).

Neiman, P.J. et al. Monthly Wea. Rev. 136, 4398 (2008).

Stohl, A. and James, P. J. Hidrometeor. 5, 656 (2004).

  • Open access
  • 27 Reads
Monsoon Dynamics in the Ganges-Brahmaputra-Meghna Basin

A recently funded US National Science Foundation project seeks to investigate monsoon variability within the Ganges-Brahmaputra-Meghna (GBM) river basin as a potential predictor for annual shoreline erosion rates in the lower coastal delta region. Many previous studies have examined the interannual variability of South Asian precipitation either within political boundaries or across large spans of latitudes and longitudes, but few have taken a more hydrologic approach by analyzing the atmospheric-oceanic forcings that lead to precipitation falling only within the GBM basin.  The temporal climate patterns would likely be different from previous studies and are hypothesized to have a more direct effect on outlet discharge and erosion rates. In the present study, mean monsoon precipitation (June-July-August-September) for the 2309 0.25° grid boxes of the Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record (PERSIANN-CDR) was extracted using geospatial methods. A Principal Component (PC) analysis was performed over the period 1983 to 2015. The first PC explains 88.7% of the variance and resembles climatology with the center of action over Bangladesh.  The eigenvector shows a downward trend consistent with studies reporting a recent decline in monsoon rainfall. The second PC explains 2.9% of the variance and concentrates rainfall in the western portion of the basin. The 2nd component has greater temporal variability than the 1st component and an apparent decadal cycle. An analysis of global precipitation indicates that the rainfall patterns obtained within the GBM are localized. Surface and upper-air atmospheric height fields suggest the 2nd PC pattern is forced by a Rossby wave train stemming from the North Atlantic.

  • Open access
  • 33 Reads
Satellite observed solar induced fluorescence to monitor global plant stress

Biophysical feedbacks on climate depend on plant responses to stress conditions. Yet current land surface models (LSMs) still treat plant stress rudimentarily, and typically assume the same sensitivity to soil moisture for all vegetation types. There is a need therefore to investigate the dynamics of vegetation stress at the global scale, both to further understand the effect of land feedbacks on climate, as well as to improve the representation of these processes in LSMs. Solar induced fluorescence (SIF) is a subtle glow of energy emitted by vegetation during photosynthesis. Recently, satellite observations of SIF have been shown to closely mimic the spatiotemporal variability of photosynthesis. Given the nexus between photosynthesis and transpiration through the opening and closing of stomata, a link between SIF observations and evaporation can be hypothesised. Here, we introduce a novel index of evaporative stress (i.e. the ratio of actual to potential evaporation) based on satellite SIF observations, and we compare it to the estimates of evaporative stress by various LSMs from the Earth2Observe database (i.e. JULES, HTESSEL, ORCHIDEE). Results of validations against in situ evaporative stress – calculated from the FLUXNET2015 eddy-covariance archive – indicate that our SIF-based stress index outperforms the estimates of the LSMs across the majority of sites, with the exception of regions with sparse vegetation in which bare soil evaporation dominates the flux of vapour from land to atmosphere. SIF derived stress greatly outperforms over densely forested regions, and shows a high skill to capture leaf-out periods. Overall, this novel SIF application provides improvements for large-scale estimates of transpiration and can be used to further understand vegetation–atmosphere feedbacks from different ecosystem types. Furthermore, the implications of this research are relevant to (a) the hydrology and climate modelling communities, given the opportunity to utilise our SIF-based evaporative stress to benchmark model representation of the land control over the atmospheric demand for water, and (b) the remote sensing community, that will see how an observation originally intended for the study of the carbon cycle is valorized through its application to study water cycle dynamics as well.

  • Open access
  • 29 Reads
New insights on land surface-atmosphere feedbacks over tropical South America at interannual timescales

Using monthly data for the period 1979-2010, we study the dynamics and strength of land surface-atmosphere feedbacks (LAFs) among variables involved in the heat and moisture fluxes, at interannual timescales for Tropical South America (TropSA). The variables include precipitation, surface air temperature, specific humidity at 925 hPa, evaporation, and estimates of volumetric soil water content. Using a dimensional reduction, we apply a Maximum Covariance Analysis (MCA) to rank the relative contributions to LAFs and group the time series into Maximum Covariance States (MCS) with common mechanisms among variables. We estimate linear (Pearson correlations) and non-linear (information transfer and causality) coupling metrics among pairs of variables to configure the structure of linkages. The main MCS associated with LAFs over TropSA are strongly influenced by ENSO, and the meridional and equatorial SSTs modes over the Atlantic and Indian Oceans. ENSO favors a unimodal behavior, with center of action in the Amazon River basin, while SSTs over the Tropical North Atlantic result in a dipole between northern and southern TropSA. Results show that soil moisture plays a leading role in regulating heat and water anomalies, and provides the memory of the atmosphere-driven processes and their subsequent influence. Thus, soil moisture is fundamental and leads up to 9 month-lags whereby ENSO enhances the interannual connectivity and memory of LAFs in 25% with respect to the mode influenced by TNA. Within the identified multivariate structure, evaporation and soil moisture enhance the interannual connectivity of the whole set of variables since both variables exhibit more frequent two-way feedbacks with the remaining variables.

  • Open access
  • 11 Reads
Using thermal modelling to characterize the groundwater discharge towards a permanent pond (Doñana National Park, Spain)

In the present study we have modeled the groundwater inputs to the only permanent pond of Doñana Biological Reserve (Andalusia, Southern Spain) by means of the application of a thermal modelling methodology. During the study period (February-May 2017) a chain of thermistors obtained hourly temperature data from the water column and the sediment below. In addition, water level in the pond and in the aquifer near the shores was also monitored at similar time rates. Results showed that a net groundwater discharge is produced heterogeneously through the pond’s bed. The comparison of these outcomes with previous studies made in the pond (hydrological water balances, hydrogeological methods applying Darcy’s equation) is coherent and strengthens the existing hydrological knowledge of this water body. Finally, it has been detected that the pond is in a sharp relation with the alterations of the sand aquifer, even if such alterations are produced away from the pond an at high depths. The effects of groundwater extractions for urban supply of a nearby coastal resort are immediately affecting the evolution of the water level, although this affection is not necessarily traduced onto water level depletion. Most probably, the effect is produced by an elastic rebound of the groundwater in the aquifer.

  • Open access
  • 38 Reads
Patterns of atmospheric moisture transport linked to Southern Ocean Sea ice coverage changes

Moisture sources identification and Sea Ice Concentration (SIC) were calculated for the period 1980-2016 for the Southern Ocean Sea. Five sectors of the Southern Ocean Sea (King Hakon VII, East Antarctic, Ross/Amundsen, Amundsen and Bellingshausen, Weddell) were selected to calculate their moisture sources. The results show that the most important moisture sources (calculated as positive values of Evaporation minus Precipitation, E-P>0) for these five seas come from extratropical latitudes in the storm track trajectories. The main moisture sources and affected regional seas are: Southern Australia (SAUS) moisture source which affect mainly Ross/Amundsen and Amundsen and Bellingshausen seas; the Atlantic Ocean is the main source of moisture for Weddell and King Hakon VII; and the Pacific Ocean provides moisture to Ross/Amundsen, Weddell and Amundsen and Bellingshausen seas. For most of these seas it was identified positive trends of E-P>0 anomalies, while negative trends were identified only for the SAUS moisture source to Amundsen and Bellingshausen Sea. In terms of SIC, for the whole Antarctic the total anomalies are increasing, but no breaking points in this time serie were detected. Preliminary results also indicate some areas, which do not coincide exactly with the limit of the regional seas, where the increase of Sea Ice Extension (SIE) is statistically significant.

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