The role of convection in the momentum budget of ICON-LEM hindcasts over the North Atlantic

2020 ◽  
Author(s):  
Kevin Helfer ◽  
Louise Nuijens ◽  
Vishal Dixit ◽  
Pier Siebesma
Keyword(s):  
North Atlantic ◽  
Cloud Layer ◽  
Momentum Transport ◽  
Cloud Base ◽  
Relative Role ◽  
Shallow Convection ◽  
Near Surface ◽  
Momentum Budget ◽  
The Impact

<p>Motivated by the uncertain role of convective momentum transport from low clouds in setting patterns of wind in the trades, we discuss the impact of shallow convection on boundary-layer winds and its role in the overall momentum budget in the trades from large-domain large-eddy simulations. To this end, we analyse ICON-LEM hindcast simulations over the (sub)tropical North Atlantic during the NARVAL1 and NARVAL2 flight campaigns.</p><p>We describe that the character of the momentum flux profile differs significantly in regimes of shallow and deep convection and thus its influence on cloud-layer and near-surface winds. In particular, we establish that the momentum transport tendency is of similar importance as other terms in the momentum budget, and though the shape of the profile is remarkably insensitive to the horizontal resolution of the simulation, the relative role of subgrid and resolved fluxes changes with resolution. Furthermore, we find that counter-gradient transport occurs even in the absence of organisation, namely in the lower cloud layer, where cloudy updrafts carry slow momentum air upwards, which locally accelerates winds and may play a role at maintaining the cloud-base wind maximum.</p>

Convectively driven wind variability in connection to wind biases in the ECMWF operational weather model

2020 ◽  
Author(s):  
Louise Nuijens ◽  
Irina Sandu ◽  
Beatrice Saggiorato ◽  
Hauke Schulz ◽  
Mariska Koning ◽  
...  
Keyword(s):  
Model Development ◽  
Meridional Overturning Circulation ◽  
Momentum Transport ◽  
Cloud Base ◽  
Moist Convection ◽  
Shallow Convection ◽  
Wind Speeds ◽  
Wind Variability ◽  
Large Eddy

<p>Despite playing a key role in the atmospheric circulation, the representation of momentum transport by moist convection (cumulus clouds) has been largely overlooked by the model development community over the past decade, at least compared with diabatic and radiative effects of clouds. In particular, how shallow convection may influence surface and boundary layer winds is not thoroughly investigated. In this talk, we discuss the role of convective momentum transport (CMT) in setting low-level wind speed and its variability and evaluate its role in long-standing wind biases in the ECMWF IFS model.</p><p>We use high-frequency wind profiling measurements and high-resolution large-eddy simulations to inform our understanding of convectively driven wind variability. We do this at two locations: in the trades, using wind lidar and radiosonde measurements from the Barbados Cloud Observatory and the intensive EUREC4A field campaign, and over the Netherlands, using an observationally constrained reanalysis wind dataset and large-eddy simulation hindcasts.</p><p>At both locations we use the data and model output to investigate whether CMT can be responsible for a missing drag near the surface in the IFS model. Namely, at short leadtimes, the model produces stronger than observed easterly/westerly flow near the surface, while “a missing drag” produces weaker than observed wind turning. Consequently, the meridional overturning circulation in both the tropics and midlatitudes is weaker in the IFS and in ERA-Interim and ERA5 reanalysis products.</p><p>Comparing simulated and IFS wind tendencies at selected grid points at the above locations, and by turning off the process of CMT by shallow convection in the model, we gain insight in the role of CMT in explaining wind biases. We find that CMT alone does not explain a missing drag near the surface. CMT often acts to accelerate winds near the surface. But CMT plays a role in communicating biases in cloud base wind speeds towards the surface. In the trades, a strong jet near cloud base is determined by thermal wind and a strong flux of zonal momentum through cloud base, where “cumulus friction” minimizes. Near this jet, the presence of (counter-gradient) turbulent momentum fluxes produces most of the drag. Implications of these findings for CMT parameterization are discussed.</p>

Decomposing the time-mean Atlantic Meridional Overturning Circulation and its variability with latitude.

2021 ◽  
Author(s):  
Tomas Jonathan ◽  
Mike Bell ◽  
Helen Johnson ◽  
David Marshall
Keyword(s):  
Global Climate ◽  
Dominant Role ◽  
Meridional Overturning Circulation ◽  
Western Boundary ◽  
Relative Role ◽  
Overturning Circulation ◽  
Near Surface ◽  
Boundary Density ◽  
Meridional Overturning

<p>The Atlantic Meridional Overturning Circulations (AMOC) is crucial to our global climate, transporting heat and nutrients around the globe. Detecting  potential climate change signals first requires a careful characterisation of inherent natural AMOC variability. Using a hierarchy of global coupled model  control runs (HadGEM-GC3.1, HighResMIP) we decompose the overturning circulation as the sum of (near surface) Ekman, (depth-dependent) bottom velocity, eastern and western boundary density components, as a function of latitude. This decomposition proves a useful low-dimensional characterisation of the full 3-D overturning circulation. In particular, the decomposition provides a means to investigate and quantify the constraints which boundary information imposes on the overturning, and the relative role of eastern versus western contributions on different timescales. </p><p>The basin-wide time-mean contribution of each boundary component to the expected streamfunction is investigated as a function of depth, latitude and spatial resolution. Regression modelling supplemented by Correlation Adjusted coRrelation (CAR) score diagnostics provide a natural ranking of the contributions of the various components in explaining the variability of the total streamfunction. Results reveal the dominant role of the bottom component, western boundary and Ekman components at short time-scales, and of boundary density components at decadal and longer timescales.</p>

Evaluating the role of soil physical properties on simulated land-atmosphere interactions over South Africa using coupled atmosphere-hydrological modeling

2021 ◽  
Author(s):  
Zhenyu Zhang ◽  
Patrick Laux ◽  
Joël Arnault ◽  
Jianhui Wei ◽  
Jussi Baade ◽  
...  
Keyword(s):  
South Africa ◽  
Physical Properties ◽  
Land Degradation ◽  
Land Surface ◽  
Soil Physical Properties ◽  
Near Surface ◽  
Soil Database ◽  
Global Soil ◽  
The Impact

<p>Land degradation with its direct impact on vegetation, surface soil layers and land surface albedo, has great relevance with the climate system. Assessing the climatic and ecological effects induced by land degradation requires a precise understanding of the interaction between the land surface and atmosphere. In coupled land-atmosphere modeling, the low boundary conditions impact the thermal and hydraulic exchanges at the land surface, therefore regulates the overlying atmosphere by land-atmosphere feedback processes. However, those land-atmosphere interactions are not convincingly represented in coupled land-atmosphere modeling applications. It is partly due to an approximate representation of hydrological processes in land surface modeling. Another source of uncertainties relates to the generalization of soil physical properties in the modeling system. This study focuses on the role of the prescribed physical properties of soil in high-resolution land surface-atmosphere simulations over South Africa. The model used here is the hydrologically-enhanced Weather Research and Forecasting (WRF-Hydro) model. Four commonly used global soil datasets obtained from UN Food and Agriculture Organization (FAO) soil database, Harmonized World Soil Database (HWSD), Global Soil Dataset for Earth System Model (GSDE), and SoilGrids dataset, are incorporated within the WRF-Hydro experiments for investigating the impact of soil information on land-atmosphere interactions. The simulation results of near-surface temperature, skin temperature, and surface energy fluxes are presented and compared to observational-based reference dataset. It is found that simulated soil moisture is largely influenced by soil texture features, which affects its feedback to the atmosphere.</p>

The role of subpolar North Atlantic as a source of predictability

2021 ◽  
Author(s):  
Shuting Yang ◽  
Tian Tian ◽  
Yiguo Wang ◽  
Torben Schmith ◽  
Steffen M. Olsen ◽  
...  
Keyword(s):  
North Atlantic ◽  
Decadal Variability ◽  
Model Systems ◽  
The Arctic ◽  
Ocean Temperature ◽  
High Latitudes ◽  
Cold Anomaly ◽  
Prediction Systems ◽  
The Impact

<p>The subpolar North Atlantic (SPNA) is a region experiencing substantial decadal variability, which has been linked to extreme weather impacts over continents. Recent studies have suggested that the connectivity with the SPNA may be a key to predictions in high latitudes. To understand the impact of the SPNA on predictability of North Atlantic-European sectors and the Arctic, we use two climate<strong> </strong>prediction systems, EC-Earth3-CPSAI and NorCPM1, to perform ensemble pacemaker experiments with a focus on the subpolar extreme cold anomaly event in 2015. This 2015 cold anomaly event is generally underestimated by the decadal prediction systems. In order to force the model to better represent the observed anomaly in SPNA, we apply nudging in a region of the SPNA (i.e., 51.5°W - 13.0°W, 30.4°N - 57.5°N, and from surface to 1000 m depth in the ocean). Here ocean temperature and salinity is restored to observed conditions from reanalysis in both model systems. All other aspects of the setup of this pacemaker experiment follow the protocol for the CMIP6 DCPP-A hindcasts and initialized on November 1, 2014. The restoration is applied during the hindcasts from November 2014 to December 2019. Multi-member ensembles of 10-year hindcasts are performed with 10 members for the EC-Earth3-CPSAI and 30 members for the NorCPM1.</p><p>The time evolution of ensembles of the initialized nudging hindcasts (EXP1) is compared with the initialized DCPP-A hindcast ensembles (EXP2) and the uninitialized ensembles (EXP3). The prediction skills of the three sets of experiments are also assessed. It can be seen that restoring the ocean temperature and salinity in the SPNA region to the reanalysis improves the prediction in the region quickly after the simulation starts, as expected. On the interannual to decadal time scales, the areas with improved prediction skills extend to over almost the entire North Atlantic for both models. The improved skill over Nordic Seas is particularly significant, especially for EC-Earth3-CPSAI. For NorCPM, the regions with improved skills extend to the entire Arctic. Our results suggest the possible role of the SPNA as a source of skillful predictions on interannual to decadal time scale, especially for high latitudes. The ocean pathways are the critical source of skill whereas our results imply a limited role of coupled feedbacks through the atmosphere.  </p>

scholarly journals Comparing the cloud vertical structure derived from several methods based on radiosonde profiles and ground-based remote sensing measurements

2014 ◽  
Vol 7 (8) ◽  
pp. 2757-2773 ◽  
Author(s):  
M. Costa-Surós ◽  
J. Calbó ◽  
J. A. González ◽  
C. N. Long
Keyword(s):  
Remote Sensing ◽  
Vertical Structure ◽  
Cloud Layer ◽  
Cloud Base ◽  
Low Resolution ◽  
Telecommunication System ◽  
Perfect Agreement ◽  
Data Set ◽  
Global Telecommunication System ◽  
The Impact

Abstract. The cloud vertical distribution and especially the cloud base height, which is linked to cloud type, are important characteristics in order to describe the impact of clouds on climate. In this work, several methods for estimating the cloud vertical structure (CVS) based on atmospheric sounding profiles are compared, considering the number and position of cloud layers, with a ground-based system that is taken as a reference: the Active Remote Sensing of Clouds (ARSCL). All methods establish some conditions on the relative humidity, and differ in the use of other variables, the thresholds applied, or the vertical resolution of the profile. In this study, these methods are applied to 193 radiosonde profiles acquired at the Atmospheric Radiation Measurement (ARM) Southern Great Plains site during all seasons of the year 2009 and endorsed by Geostationary Operational Environmental Satellite (GOES) images, to confirm that the cloudiness conditions are homogeneous enough across their trajectory. The perfect agreement (i.e., when the whole CVS is estimated correctly) for the methods ranges between 26 and 64%; the methods show additional approximate agreement (i.e., when at least one cloud layer is assessed correctly) from 15 to 41%. Further tests and improvements are applied to one of these methods. In addition, we attempt to make this method suitable for low-resolution vertical profiles, like those from the outputs of reanalysis methods or from the World Meteorological Organization's (WMO) Global Telecommunication System. The perfect agreement, even when using low-resolution profiles, can be improved by up to 67% (plus 25% of the approximate agreement) if the thresholds for a moist layer to become a cloud layer are modified to minimize false negatives with the current data set, thus improving overall agreement.

scholarly journals Simulated retreat of Jakobshavn Isbræ since the Little Ice Age controlled by geometry

2018 ◽  
Vol 12 (7) ◽  
pp. 2249-2266 ◽  
Author(s):  
Nadine Steiger ◽  
Kerim H. Nisancioglu ◽  
Henning Åkesson ◽  
Basile de Fleurian ◽  
Faezeh M. Nick
Keyword(s):  
Little Ice Age ◽  
Ice Age ◽  
Climate Forcing ◽  
Relative Role ◽  
Regional Warming ◽  
History Of ◽  
The Impact ◽  
Term Response

Abstract. Rapid retreat of Greenland's marine-terminating glaciers coincides with regional warming trends, which have broadly been used to explain these rapid changes. However, outlet glaciers within similar climate regimes experience widely contrasting retreat patterns, suggesting that the local fjord geometry could be an important additional factor. To assess the relative role of climate and fjord geometry, we use the retreat history of Jakobshavn Isbræ, West Greenland, since the Little Ice Age (LIA) maximum in 1850 as a baseline for the parameterization of a depth- and width-integrated ice flow model. The impact of fjord geometry is isolated by using a linearly increasing climate forcing since the LIA and testing a range of simplified geometries. We find that the total length of retreat is determined by external factors – such as hydrofracturing, submarine melt and buttressing by sea ice – whereas the retreat pattern is governed by the fjord geometry. Narrow and shallow areas provide pinning points and cause delayed but rapid retreat without additional climate warming, after decades of grounding line stability. We suggest that these geometric pinning points may be used to locate potential sites for moraine formation and to predict the long-term response of the glacier. As a consequence, to assess the impact of climate on the retreat history of a glacier, each system has to be analyzed with knowledge of its historic retreat and the local fjord geometry.

The Strong Impact of Weak Horizontal Convergence on Continental Shallow Convection

2020 ◽  
Vol 77 (9) ◽  
pp. 3119-3137
Author(s):  
Marcin J. Kurowski ◽  
Wojciech W. Grabowski ◽  
Kay Suselj ◽  
João Teixeira
Keyword(s):  
Large Scale ◽  
Cloud Microphysics ◽  
Cloud Base ◽  
Small Scale ◽  
Strong Impact ◽  
Shallow Convection ◽  
Liquid Water Path ◽  
Strong Response ◽  
The Impact ◽  
Benchmark Solutions

Abstract Idealized large-eddy simulation (LES) is a basic tool for studying three-dimensional turbulence in the planetary boundary layer. LES is capable of providing benchmark solutions for parameterization development efforts. However, real small-scale atmospheric flows develop in heterogeneous and transient environments with locally varying vertical motions inherent to open multiscale interactive dynamical systems. These variations are often too subtle to detect them by state-of-the-art remote and in situ measurements, and are typically excluded from idealized simulations. The present study addresses the impact of weak [i.e., O(10−6) s−1] short-lived low-level large-scale convergence/divergence perturbations on continental shallow convection. The results show a strong response of shallow nonprecipitating convection to the applied weak large-scale dynamical forcing. Evolutions of CAPE, mean liquid water path, and cloud-top heights are significantly affected by the imposed convergence/divergence. In contrast, evolving cloud-base properties, such as the area coverage and mass flux, are only weakly affected. To contrast those impacts with microphysical sensitivity, the baseline simulations are perturbed assuming different observationally based cloud droplet number concentrations and thus different rainfall. For the tested range of microphysical perturbations, the imposed convergence/divergence provides significantly larger impact than changes in the cloud microphysics. Simulation results presented here provide a stringent test for convection parameterizations, especially important for large-scale models progressing toward resolving some nonhydrostatic effects.

Weathering the Soviet Countryside: The Impact of Climate and Agricultural Policies on Russian Grain Yields, 1958–2010

2013 ◽  
Vol 40 (1) ◽  
pp. 115-143 ◽  
Author(s):  
Nikolai M. Dronin ◽  
Andrei P. Kirilenko
Keyword(s):  
Large Scale ◽  
State Level ◽  
Relative Role ◽  
Agricultural Policies ◽  
Production Potential ◽  
Russian Agriculture ◽  
Land Privatization ◽  
History Of ◽  
The Impact

Agriculture in Russia has always had to contend with unfavorable climate. At the same time, large-scale socio-economic experiments have also strained the country’s food production potential throughout the 20th century. The relative role of climate and state agricultural policies in affecting production of cereals was studied for the period of 1958–2010. The study used statistical yield modeling to explain the variations in observed yields with slowly changing progress in technology and management and weather variability. The correlation between the actual and weather-explained yields is moderate to high: measured at the level of the entire country, Pearson’s r is 0.74 and Spearman’s rho is 0.68. Further, we suggest that the residual yield variability can be explained partially with the influence of large-scale changes in agricultural policies at the state level. Between these policies, we consider the following key periods in the history of Russian agriculture: “Virgin Lands” campaign (end of 1950s), Kosygin-Liberman initiatives (late 1960s), Brezhnev’s investment programmes in response of stagnation of agriculture (late 1970s – early 1980s), Gorbachev’s “Perestrojka” (1985–1991), and land privatization and price liberalization (1990s).

scholarly journals Comparing the cloud vertical structure derived from several methods based on measured atmospheric profiles and active surface measurements

2014 ◽  
Vol 7 (4) ◽  
pp. 3681-3725
Author(s):  
M. Costa-Surós ◽  
J. Calbó ◽  
J. A. González ◽  
C. N. Long
Keyword(s):  
Vertical Structure ◽  
Current Data ◽  
Cloud Layer ◽  
Cloud Base ◽  
Low Resolution ◽  
Telecommunication System ◽  
Perfect Agreement ◽  
Data Set ◽  
Global Telecommunication System ◽  
The Impact

Abstract. The cloud vertical distribution and especially the cloud base height, which is linked to cloud type, is an important characteristic in order to describe the impact of clouds on climate. In this work several methods to estimate the cloud vertical structure (CVS) based on atmospheric sounding profiles are compared, considering number and position of cloud layers, with a ground based system which is taken as a reference: the Active Remote Sensing of Clouds (ARSCL). All methods establish some conditions on the relative humidity, and differ on the use of other variables, the thresholds applied, or the vertical resolution of the profile. In this study these methods are applied to 193 radiosonde profiles acquired at the ARM Southern Great Plains site during all seasons of year 2009 and endorsed by GOES images, to confirm that the cloudiness conditions are homogeneous enough across their trajectory. The perfect agreement (i.e. when the whole CVS is correctly estimated) for the methods ranges between 26–64%; the methods show additional approximate agreement (i.e. when at least one cloud layer is correctly assessed) from 15–41%. Further tests and improvements are applied on one of these methods. In addition, we attempt to make this method suitable for low resolution vertical profiles, like those from the outputs of reanalysis methods or from the WMO's Global Telecommunication System. The perfect agreement, even when using low resolution profiles, can be improved up to 67% (plus 25% of approximate agreement) if the thresholds for a moist layer to become a cloud layer are modified to minimize false negatives with the current data set, thus improving overall agreement.

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