scholarly journals Meridional and vertical variations of the water vapour isotopic composition in the marine boundary layer over the Atlantic and Southern Ocean

2020 ◽  
Vol 20 (9) ◽  
pp. 5811-5835 ◽  
Author(s):  
Iris Thurnherr ◽  
Anna Kozachek ◽  
Pascal Graf ◽  
Yongbiao Weng ◽  
Dimitri Bolshiyanov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Time Series ◽  
Wind Speed ◽  
Southern Ocean ◽  
Water Vapour ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Sea Spray ◽  
Moisture Source

Abstract. Stable water isotopologues (SWIs) are useful tracers of moist diabatic processes in the atmospheric water cycle. They provide a framework to analyse moist processes on a range of timescales from large-scale moisture transport to cloud formation, precipitation and small-scale turbulent mixing. Laser spectrometric measurements on research vessels produce high-resolution time series of the variability of the water vapour isotopic composition in the marine boundary layer. In this study, we present a 5-month continuous time series of such ship-based measurements of δ2H and δ18O from the Antarctic Circumnavigation Expedition (ACE) in the Atlantic and the Southern Ocean in the time period from November 2016 to April 2017. We analyse the drivers of meridional SWI variations in the marine boundary layer across diverse climate zones in the Atlantic and Southern Ocean using Lagrangian moisture source diagnostics and relate vertical SWI differences to near-surface wind speed and ocean surface state. The median values of δ18O, δ2H and deuterium excess during ACE decrease continuously from low to high latitudes. These meridional SWI distributions reflect climatic conditions at the measurement and moisture source locations, such as air temperature, specific humidity and relative humidity with respect to sea surface temperature. The SWI variability at a given latitude is highest in the extratropics and polar regions with decreasing values equatorwards. This meridional distribution of SWI variability is explained by the variability in moisture source locations and its associated environmental conditions as well as transport processes. The westward-located moisture sources of water vapour in the extratropics are highly variable in extent and latitude due to the frequent passage of cyclones and thus widen the range of encountered SWI values in the marine boundary layer. Moisture loss during transport further contributes to the high SWI variability in the extratropics. In the subtropics and tropics, persistent anticyclones lead to well-confined narrow easterly moisture source regions, which is reflected in the weak SWI variability in these regions. Thus, the expected range of SWI signals at a given latitude strongly depends on the large-scale circulation. Furthermore, the ACE SWI time series recorded at 8.0 and 13.5 m above the ocean surface provide estimates of vertical SWI gradients in the lowermost marine boundary layer. On average, the vertical gradients with height found during ACE are -0.1‰m-1 for δ18O, -0.5‰m-1 for δ2H and 0.3 ‰ m−1 for deuterium excess. Careful calibration and post-processing of the SWI data and a detailed uncertainty analysis provide a solid basis for the presented gradients. Using sea spray concentrations and sea state conditions, we show that the vertical SWI gradients are particularly large during high wind speed conditions with increased contribution of sea spray evaporation or during low wind speed conditions due to weak vertical turbulent mixing. Although further SWI measurements at a higher vertical resolution are required to validate these findings, the simultaneous SWI measurements at several heights during ACE show the potential of SWIs as tracers for vertical mixing and sea spray evaporation in the lowermost marine boundary layer.

scholarly journals Meridional and vertical variations of the water vapour isotopic composition in the marine boundary layer over the Atlantic and Southern Ocean

2019 ◽  
Author(s):  
Iris Thurnherr ◽  
Anna Kozachek ◽  
Pascal Graf ◽  
Yongbiao Weng ◽  
Dimitri Bolshiyanov ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Time Series ◽  
Wind Speed ◽  
Southern Ocean ◽  
Water Vapour ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Sea Spray ◽  
Moisture Source

Abstract. Stable water isotopologues (SWIs) are useful tracers of moist diabatic processes in the atmospheric water cycle. They provide a framework to analyse moist processes on a range of time scales from large-scale moisture transport to cloud formation, precipitation, and small-scale turbulent mixing. Laser spectrometric measurements on research vessels produce high-resolution time series of the variability of the water vapour isotopic composition in the marine boundary layer. In this study, we present a five-month continuous time series of such ship-based measurements of δ2H and δ18O from the Antarctic Circumnavigation Expedition (ACE) in the Atlantic and the Southern Ocean in the time period from November 2016 to April 2017. We analyse the drivers of meridional SWI variations in the marine boundary layer across diverse climate zones in the Atlantic and Southern Ocean using Lagrangian moisture source diagnostics and relate vertical SWI differences to near-surface wind speed and ocean surface state. The median values of δ18O, δ2H and d-excess during ACE decrease continuously from low to high latitudes. These meridional SWI distributions reflect climatic conditions at the measurement and moisture source sites, such as air temperature, specific humidity, and relative humidity with respect to sea surface temperature. The SWI variability at a given latitude is highest in the extratropics and polar regions with decreasing values equatorwards. This meridional distribution of SWI variability is explained by the variability in moisture source locations and its associated environmental conditions as well as transport processes. The westward located moisture sources of water vapour in the extratropics are highly variable in extent and latitude due to the frequent passage of cyclones and thus widen the range of encountered SWI values in the marine boundary layer. Moisture loss during transport further contributes to the high SWI variability in the extratropics. In the subtropics and tropics, persistent anticyclones lead to well-confined narrow easterly moisture source regions, which is reflected in the low SWI variability in these regions. Thus, the expected range of SWI signals at a given latitude strongly depends on the large-scale circulation. Furthermore, the ACE SWI time series recorded at different heights above the ocean surface provide estimates of vertical SWI gradients in the lowermost marine boundary layer. On average, the vertical gradients with height found during ACE are −0.1 ‰ m−1 for δ18O, −0.5 ‰ m−1 for δ2H and 0.3 ‰ m−1 for d-excess. Careful calibration and post-processing of the SWI data and a detailed uncertainty analysis provide a solid basis for the presented gradients. Using sea spray concentrations and sea state conditions, we show that the vertical SWI gradients are particularly large during high wind speed conditions with increased contribution of sea spray evaporation or during low wind speed conditions due to weak vertical turbulent mixing and dominating effects from non-equilibrium fractionation. Although further SWI measurements at a higher vertical resolution are required to validate these findings, the simultaneous SWI measurements at several heights during ACE show the potential of SWIs as tracers for vertical mixing and sea spray evaporation in the lowermost marine boundary layer.

Iodine chemistry in the tropical and remote open ocean marine boundary layer

2020 ◽  
Author(s):  
Swaleha Inamdar ◽  
Liselotte Tinel ◽  
Rosie Chance ◽  
Lucy Jane Carpenter ◽  
Sabu Prabhakaran ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Wind Speed ◽  
Kinetic Model ◽  
Southern Ocean ◽  
Tropospheric Ozone ◽  
Marine Boundary Layer ◽  
Sea Surface ◽  
Iodine Species ◽  
Ocean Region ◽  
Inorganic Iodine

<p>Iodine chemistry plays an essential role in controlling the radiation budget by changing various atmospheric parameters. Iodine in the atmosphere is known to cause depletion of ozone via catalytic reaction cycles. It alters the atmospheric oxidation capacity, and lead to ultrafine particle formation that acts as potential cloud condensation nuclei. The ocean is the primary source of iodine; it enters the atmosphere through fluxes of gaseous reactive iodine species. At the ocean surface, seawater iodide reacts with tropospheric ozone (gas) to form inorganic iodine species in gaseous form. These species namely, hypoiodous acid (HOI) and molecular iodine (I<sub>2</sub>) quickly photolyse to release reactive iodine (I) in the atmosphere. This process acts as a significant sink for tropospheric ozone contributing to ~16% ozone loss throughout the troposphere. Reactive iodine released in the atmosphere undergoes the formation of iodine monoxide (IO) or higher oxides of iodine (I<sub>x</sub>O<sub>x</sub>) via self-recombination reactions. It is known that inorganic iodine fluxes (HOI and I<sub>2</sub>) contribute to 75% of the detected IO over the Atlantic Ocean. However, we did not observe this from ship-based MAX-DOAS studies between 2014-2017. At present, there are no direct observations of inorganic iodine (HOI; few for I<sub>2</sub>) and are estimated via empirical methods derived from the interfacial kinetic model by Carpenter et al. in 2013. Based on the kinetic model, estimation of HOI and I<sub>2</sub> fluxes depends on three parameters, namely, ozone concentration, surface iodide concentration, and the wind speed. This parameterisation for inorganic fluxes assumes a sea surface temperature (SST) of 293 K and has limiting wind speed conditions. Since the parameterisation conditions assumed SST of 293 K higher uncertainties due to errors in activation energy creeps in the estimation of HOI flux compared to I<sub>2</sub> as the flux of HOI is ~20 times greater than that of I<sub>2</sub>. For three consecutive expeditions in the Indian and Southern Ocean, we detected ~1 pptv of IO in the marine boundary layer. These levels are not explained by the calculated inorganic fluxes by using observed and predicted sea surface iodide concentrations. This method of iodine flux estimation is currently used in all global models, along with the MacDonald et al. 2014 iodide estimation method. Model output using these parameterisations have not been able to match the observed IO levels in the Indian and Southern Ocean region. This discrepancy suggests that the process of efflux of iodine to the atmosphere may not be fully understood, and the current parametrisation does not do justice to the observations. It also highlights that the flux of organic iodine may also play a role in observed IO levels, especially in the Indian Ocean region. A correlation of 0.7 was achieved above the 99% confidence limit for chlorophyll-a with observed IO concentration in this region. There is a need to carry more observations to improve the estimation technique of iodine sea-air flux thus improving model predictions of IO in the atmosphere.</p>

An Evaluation of WRF Simulations of Clouds over the Southern Ocean with A-Train Observations

2014 ◽  
Vol 142 (2) ◽  
pp. 647-667 ◽  
Author(s):  
Yi Huang ◽  
Steven T. Siems ◽  
Michael J. Manton ◽  
Gregory Thompson
Keyword(s):  
Boundary Layer ◽  
Southern Ocean ◽  
Large Scale ◽  
Marine Boundary Layer ◽  
Cloud Condensation Nuclei ◽  
Surface Fluxes ◽  
Shortwave Radiation ◽  
Microphysical Properties ◽  
Formidable Challenge ◽  
Weak Surface

Abstract The representation of the marine boundary layer (BL) clouds remains a formidable challenge for state-of-the-art simulations. A recent study by Bodas-Salcedo et al. using the Met Office Unified Model highlights that the underprediction of the low/midlevel postfrontal clouds contributes to the largest bias of the surface downwelling shortwave radiation over the Southern Ocean (SO). A-Train observations and limited in situ measurements have been used to evaluate the Weather Research and Forecasting Model, version 3.3.1 (WRFV3.3.1), in simulating the postfrontal clouds over Tasmania and the SO. The simulated cloud macro/microphysical properties are compared against the observations. Experiments are also undertaken to test the sensitivity of model resolution, microphysical (MP) schemes, planetary boundary layer (PBL) schemes, and cloud condensation nuclei (CCN) concentration. The simulations demonstrate a considerable level of skill in representing the clouds during the frontal passages and, to a lesser extent, in the postfrontal environment. The simulations, however, have great difficulties in portraying the widespread marine BL clouds that are not immediately associated with fronts. This shortcoming is persistent to the changes of model configuration and physical parameterization. The representation of large-scale conditions and their connections with the BL clouds are discussed. A lack of BL moisture is the most obvious explanation for the shortcoming, which may be a consequence of either strong entrainment or weak surface fluxes. It is speculated that the BL wind shear/turbulence may be an issue over the SO. More comprehensive observations are necessary to fully investigate the deficiency of the simulations.

Prediction of Atmospheric Turbulence Characteristics for Surface Boundary Layer using Empirical Spectral Methods

2020 ◽  
Author(s):  
Yagya Dutta Dwivedi ◽  
Vasishta Bhargava Nukala ◽  
Satya Prasad Maddula ◽  
Kiran Nair
Keyword(s):  
Boundary Layer ◽  
Time Series ◽  
Surface Roughness ◽  
Wind Speed ◽  
Atmospheric Turbulence ◽  
Surface Boundary ◽  
Low Frequencies ◽  
Surface Boundary Layer ◽  
Power Spectral ◽  
Mean Wind Speed

Abstract Atmospheric turbulence is an unsteady phenomenon found in nature and plays significance role in predicting natural events and life prediction of structures. In this work, turbulence in surface boundary layer has been studied through empirical methods. Computer simulation of Von Karman, Kaimal methods were evaluated for different surface roughness and for low (1%), medium (10%) and high (50%) turbulence intensities. Instantaneous values of one minute time series for longitudinal turbulent wind at mean wind speed of 12 m/s using both spectra showed strong correlation in validation trends. Influence of integral length scales on turbulence kinetic energy production at different heights is illustrated. Time series for mean wind speed of 12 m/s with surface roughness value of 0.05 m have shown that variance for longitudinal, lateral and vertical velocity components were different and found to be anisotropic. Wind speed power spectral density from Davenport and Simiu profiles have also been calculated at surface roughness of 0.05 m and compared with k−1 and k−3 slopes for Kolmogorov k−5/3 law in inertial sub-range and k−7 in viscous dissipation range. At high frequencies, logarithmic slope of Kolmogorov −5/3rd law agreed well with Davenport, Harris, Simiu and Solari spectra than at low frequencies.

scholarly journals Modeling microphysical effects of entrainment in clouds observed during EUCAARI-IMPACT field campaign

2013 ◽  
Vol 13 (16) ◽  
pp. 8489-8503 ◽  
Author(s):  
D. Jarecka ◽  
H. Pawlowska ◽  
W. W. Grabowski ◽  
A. A. Wyszogrodzki
Keyword(s):  
Boundary Layer ◽  
Turbulent Mixing ◽  
Marine Boundary Layer ◽  
Cloud Droplet ◽  
Droplet Radius ◽  
Subgrid Scale ◽  
Microphysics Scheme ◽  
Local Conditions ◽  
Field Campaign ◽  
Les Model

Abstract. This paper discusses aircraft observations and large-eddy simulation (LES) modeling of 15 May 2008, North Sea boundary-layer clouds from the EUCAARI-IMPACT field campaign. These clouds are advected from the northeast by the prevailing lower-tropospheric winds and featured stratocumulus-over-cumulus cloud formations. An almost-solid stratocumulus deck in the upper part of the relatively deep, weakly decoupled marine boundary layer overlays a field of small cumuli. The two cloud formations have distinct microphysical characteristics that are in general agreement with numerous past observations of strongly diluted shallow cumuli on one hand and solid marine stratocumulus on the other. Based on the available observations, a LES model setup is developed and applied in simulations using a novel LES model. The model features a double-moment warm-rain bulk microphysics scheme combined with a sophisticated subgrid-scale scheme allowing local prediction of the homogeneity of the subgrid-scale turbulent mixing. The homogeneity depends on the characteristic time scales for the droplet evaporation and for the turbulent homogenization. In the model, these scales are derived locally based on the subgrid-scale turbulent kinetic energy, spatial scale of cloudy filaments, mean cloud droplet radius, and humidity of the cloud-free air entrained into a cloud, all predicted by the LES model. The model reproduces contrasting macrophysical and microphysical characteristics of the cumulus and stratocumulus cloud layers. Simulated subgrid-scale turbulent mixing within the cumulus layer and near the stratocumulus top is on average quite inhomogeneous, but varies significantly depending on the local conditions.

scholarly journals Sea State and Boundary Layer Stability Limit Sea Spray Aerosol Lifetime over the Southern Ocean

2020 ◽  
Author(s):  
Sebastian Landwehr ◽  
Michele Volpi ◽  
Marzieh H Derkani ◽  
Filippo Nelli ◽  
Alberto Alberello ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Southern Ocean ◽  
Stability Limit ◽  
Sea Spray ◽  
Boundary Layer Stability ◽  
Sea State ◽  
Sea Spray Aerosol

LESbrary: A library of large eddy simulation data for the calibration and uncertainty quantification of ocean surface boundary layer turbulence parameterizations

2021 ◽  
Author(s):  
Gregory Wagner ◽  
Andre Souza ◽  
Adeline Hillier ◽  
Ali Ramadhan ◽  
Raffaele Ferrari
Keyword(s):  
Boundary Layer ◽  
Turbulent Mixing ◽  
Large Scale ◽  
Ocean Surface ◽  
Limited Area ◽  
Surface Boundary ◽  
Model Errors ◽  
Surface Boundary Layer ◽  
Large Eddy ◽  
Free Parameters

<p>Parameterizations of turbulent mixing in the ocean surface boundary layer (OSBL) are key Earth System Model (ESM) components that modulate the communication of heat and carbon between the atmosphere and ocean interior. OSBL turbulence parameterizations are formulated in terms of unknown free parameters estimated from observational or synthetic data. In this work we describe the development and use of a synthetic dataset called the “LESbrary” generated by a large number of idealized, high-fidelity, limited-area large eddy simulations (LES) of OSBL turbulent mixing. We describe how the LESbrary design leverages a detailed understanding of OSBL conditions derived from observations and large scale models to span the range of realistically diverse physical scenarios. The result is a diverse library of well-characterized “synthetic observations” that can be readily assimilated for the calibration of realistic OSBL parameterizations in isolation from other ESM model components. We apply LESbrary data to calibrate free parameters, develop prior estimates of parameter uncertainty, and evaluate model errors in two OSBL parameterizations for use in predictive ESMs.</p>

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