scholarly journals Density currents as a desert dust mobilization mechanism

2012 ◽  
Vol 12 (8) ◽  
pp. 21579-21614 ◽  
Author(s):  
S. Solomos ◽  
G. Kallos ◽  
E. Mavromatidis ◽  
J. Kushta
Keyword(s):  
Vertical Mixing ◽  
Convective Cloud ◽  
Dust Storms ◽  
Density Current ◽  
Density Currents ◽  
Near Surface ◽  
Dust Mobilization ◽  
Turbulence Mixing ◽  
Cloud Resolving Model ◽  
Dust Modeling

Abstract. The formation and propagation of density currents are well studied processes in fluid dynamics with many applications to other science fields. In the atmosphere, density currents are usually meso-β/γ mechanisms and are often associated with storm downdrafts. These storms are responsible for the formation of severe dust episodes (haboobs) over desert areas. In the present study, the formation of a convective cool pool and the associated dust mobilization is examined for a representative event over the western part of Sahara desert. The physical processes involved in the mobilization of dust are described in the framework of the integrated atmospheric-air quality RAMS/ICLAMS model. Dust is effectively produced due to the development of near surface vortices and increased turbulence mixing along the frontal line. Increased dust emissions and recirculation of the elevated particles inside the density current head result in the formation of a moving "dust wall". Transport of the uplifted dust in higher layers – outside of the density current – occurs mainly in three ways: (1) uplifting of preexisting dust over the frontal line with the aid of the strong updraft (2) entrainment at the upper part of the density current head due to turbulent mixing (3) vertical mixing after the dilution of the system. The role of the produced dust in the associated convective cloud system was found to be limited. Proper representation of convective processes and dust fluxes requires the use of high resolution (cloud resolving) model configuration and online parameterization of dust production. Haboob-type of dust storms are effective dust sources and should be treated accordingly in dust modeling applications.

scholarly journals Density currents as a desert dust mobilization mechanism

2012 ◽  
Vol 12 (22) ◽  
pp. 11199-11211 ◽  
Author(s):  
S. Solomos ◽  
G. Kallos ◽  
E. Mavromatidis ◽  
J. Kushta
Keyword(s):  
Turbulent Mixing ◽  
Vertical Mixing ◽  
Convective Cloud ◽  
Dust Storms ◽  
Dust Particles ◽  
Density Current ◽  
Density Currents ◽  
Near Surface ◽  
Dust Mobilization ◽  
Dust Modeling

Abstract. The formation and propagation of density currents are well studied processes in fluid dynamics with many applications in other science fields. In the atmosphere, density currents are usually meso-β/γ phenomena and are often associated with storm downdrafts. These storms are responsible for the formation of severe dust episodes (haboobs) over desert areas. In the present study, the formation of a convective cool pool and the associated dust mobilization are examined for a representative event over the western part of Sahara desert. The physical processes involved in the mobilization of dust are described with the use of the integrated atmospheric-air quality RAMS/ICLAMS model. Dust is effectively produced due to the development of near surface vortices and increased turbulent mixing along the frontal line. Increased dust emissions and recirculation of the elevated particles inside the head of the density current result in the formation of a moving "dust wall". Transport of the dust particles in higher layers – outside of the density current – occurs mainly in three ways: (1) Uplifting of preexisting dust over the frontal line with the aid of the strong updraft (2) Entrainment at the upper part of the density current head due to turbulent mixing (3) Vertical mixing after the dilution of the system. The role of the dust in the associated convective cloud system was found to be limited. Proper representation of convective processes and dust mobilization requires the use of high resolution (cloud resolving) model configuration and online parameterization of dust production. Haboob-type dust storms are effective dust sources and should be treated accordingly in dust modeling applications.

scholarly journals Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer*

2015 ◽  
Vol 143 (9) ◽  
pp. 3532-3555 ◽  
Author(s):  
Matt C. Wilbanks ◽  
Sandra E. Yuter ◽  
Simon P. de Szoeke ◽  
W. Alan Brewer ◽  
Matthew A. Miller ◽  
...  
Keyword(s):  
Marine Boundary Layer ◽  
Propagation Speed ◽  
Sensor Data ◽  
Density Current ◽  
Density Currents ◽  
Near Surface ◽  
Cold Pools ◽  
Southeast Pacific ◽  
The Mean ◽  
Rain Rates

Abstract Density currents (i.e., cold pools or outflows) beneath marine stratocumulus clouds are characterized using 30 days of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An air density increase criterion applied to the Improved Meteorological (IMET) sensor data identified 71 density current front, core (peak density), and tail (dissipating) zones. The similarity in speeds of the mean density current propagation speed (1.8 m s−1) and the mean cloud-level advection relative to the surface layer wind (1.9 m s−1) allowed drizzle cells to deposit elongated density currents in their wakes. Scanning Doppler lidar captured prefrontal updrafts with a mean intensity of 0.91 m s−1 and an average vertical extent of 800 m. Updrafts were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. The observed density currents were 5–10 times thinner and weaker than typical continental thunderstorm cold pools. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm day−1 over a 30-km diameter. Rather than peaking when rain rates were highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurred with shallow subcloud dry and stable layers. The dry layers may have contributed to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occurred in a large region of predominantly open cells but also occurred in regions of closed cells.

scholarly journals Dynamics of Biweekly Oscillations in the Equatorial Indian Ocean*

2006 ◽  
Vol 36 (5) ◽  
pp. 827-846 ◽  
Author(s):  
Toru Miyama ◽  
Julian P. McCreary ◽  
Debasis Sengupta ◽  
Retish Senan
Keyword(s):  
Indian Ocean ◽  
Numerical Models ◽  
Energy Levels ◽  
Vertical Mixing ◽  
Equatorial Indian Ocean ◽  
Higher Order ◽  
Near Surface ◽  
Higher Order Modes ◽  
Model Control ◽  
Quikscat Winds

Abstract Variability of the wind field over the equatorial Indian Ocean is spread throughout the intraseasonal (10–60 day) band. In contrast, variability of the near-surface υ field in the eastern, equatorial ocean is concentrated at biweekly frequencies and is largely composed of Yanai waves. The excitation of this biweekly variability is investigated using an oceanic GCM and both analytic and numerical versions of a linear, continuously stratified (LCS) model in which solutions are represented as expansions in baroclinic modes. Solutions are forced by Quick Scatterometer (QuikSCAT) winds (the model control runs) and by idealized winds having the form of a propagating wave with frequency σ and wavenumber kw. The GCM and LCS control runs are remarkably similar in the biweekly band, indicating that the dynamics of biweekly variability are fundamentally linear and wind driven. The biweekly response is composed of local (nonradiating) and remote (Yanai wave) parts, with the former spread roughly uniformly along the equator and the latter strengthening to the east. Test runs to the numerical models separately forced by the τx and τy components of the QuikSCAT winds demonstrate that both forcings contribute to the biweekly signal, the response forced by τy being somewhat stronger. Without mixing, the analytic spectrum for Yanai waves forced by idealized winds has a narrowband (resonant) response for each baroclinic mode: Spectral peaks occur whenever the wavenumber of the Yanai wave for mode n is sufficiently close to kw and they shift from biweekly to lower frequencies with increasing modenumber n. With mixing, the higher-order modes are damped so that the largest ocean response is restricted to Yanai waves in the biweekly band. Thus, in the LCS model, resonance and mixing act together to account for the ocean's favoring the biweekly band. Because of the GCM's complexity, it cannot be confirmed that vertical mixing also damps its higher-order modes; other possible processes are nonlinear interactions with near-surface currents, and the model's low vertical resolution below the thermocline. Test runs to the LCS model show that Yanai waves from several modes superpose to form a beam (wave packet) that carries energy downward as well as eastward. Reflections of such beams from the near-surface pycnocline and bottom act to maintain near-surface energy levels, accounting for the eastward intensification of the near-surface, equatorial υ field in the control runs.

scholarly journals Joint Modeling of Severe Dust Storm Events in Arid and Hyper Arid Regions Based on Copula Theory: A Case Study in the Yazd Province, Iran

Climate ◽  
2020 ◽  
Vol 8 (5) ◽  
pp. 64 ◽  
Author(s):  
Tayyebeh Mesbahzadeh ◽  
Maryam Mirakbari ◽  
Mohsen Mohseni Saravi ◽  
Farshad Soleimani Sardoo ◽  
Nir Y. Krakauer
Keyword(s):  
Wind Speed ◽  
Natural Disasters ◽  
Return Period ◽  
Vertical Velocity ◽  
Geopotential Height ◽  
Dust Storms ◽  
Maximum Wind Speed ◽  
Near Surface ◽  
Copula Theory ◽  
Maximum Wind

Natural disasters such as dust storms are random phenomena created by complicated mechanisms involving many parameters. In this study, we used copula theory for bivariate modeling of dust storms. Copula theory is a suitable method for multivariate modeling of natural disasters. We identified 40 severe dust storms, as defined by the World Meteorological Organization, during 1982–2017 in Yazd province, central Iran. We used parameters at two spatial vertical levels (near-surface and upper atmosphere) that included surface maximum wind speed, and geopotential height and vertical velocity at 500, 850, and 1000 hPa. We compared two bivariate models based on the pairs of maximum wind speed–geopotential height and maximum wind speed–vertical velocity. We determined the bivariate return period using Student t and Gaussian copulas, which were considered as the most suitable functions for these variables. The results obtained for maximum wind speed–geopotential height indicated that the maximum return period was consistent with the observed frequency of severe dust storms. The bivariate modeling of dust storms based on maximum wind speed and geopotential height better described the conditions of severe dust storms than modeling based on maximum wind speed and vertical velocity. The finding of this study can be useful to improve risk management and mitigate the impacts of severe dust storms.

The Decline In Summer Fallow In The Northern Plains Cooled Near-Surface Climate But Had Minimal Impacts On Precipitation

2021 ◽  
Author(s):  
Gabriel Bromley ◽  
Andreas F. Prein ◽  
Shannon E. Albeke ◽  
Paul C. Stoy
Keyword(s):  
Land Cover ◽  
Land Cover Change ◽  
Great Plains ◽  
Land Management ◽  
North American ◽  
Land Surface ◽  
Convective Cloud ◽  
Cloud Formation ◽  
Near Surface ◽  
Summer Fallow

Abstract Land management strategies can moderate or intensify the impacts of a warming atmosphere. Since the early 1980s, nearly 116,000 km2 of crop land that was once held in fallow during the summer is now planted in the northern North American Great Plains. To simulate the impacts of this substantial land cover change on regional climate processes, convection-permitting model experiments using the Weather Research and Forecasting (WRF) model were performed to simulate modern and historical amounts of summer fallow, and were extensively validated using multiple observational data products as well as eddy covariance tower observations. Results of these simulations show that the transition from summer fallow to modern land cover lead to ~1.5 °C cooler temperatures and decreased vapor pressure deficit by ~0.15 kPa during the growing season, which is consistent with observed cooling trends. The cooler and wetter land surface with vegetation leads to a shallower planetary boundary layer and lower lifted condensation level, creating conditions more conducive to convective cloud formation and precipitation. Our model simulations however show little widespread evidence of land surface changes effects on precipitation. The observed precipitation increase in this region is more likely related to increased moisture transport by way of the Great Plains Low Level Jet as suggested by the ERA5 reanalysis. Our results demonstrate that land cover change is consistent with observed regional cooling in the northern North American Great Plains but changes in precipitation cannot be explained by land management alone.

scholarly journals Effects of morphology in controlling propagation of density currents in a reservoir using uncalibrated three-dimensional hydrodynamic modeling

2020 ◽  
Author(s):  
Behnam Zamani ◽  
Manfred Koch ◽  
Ben R. Hodges
Keyword(s):  
Hydrodynamic Model ◽  
Large Scale ◽  
Three Dimensional ◽  
Density Current ◽  
Density Currents ◽  
Large Reservoir ◽  
Bottom Water Temperature ◽  
The Difference ◽  
Basin Morphology ◽  
Southwest Iran

In this study, effects of basin morphology are shown to affect density current hydrodynamics of a large reservoir using a three-dimensional (3D) hydrodynamic model that is validated (but not calibrated) with in situ observational data. The AEM3D hydrodynamic model was applied for 5-month simulations during winter and spring flooding for the Maroon reservoir in southwest Iran, where available observations indicated that large-scale density currents had previously occurred. The model results were validated with near-bottom water temperature measurements that were previously collected at five locations in the reservoir. The Maroon reservoir consists of upper and lower basins that are connected by a deep and narrow canyon. Analyses of simulations show that the canyon strongly affects density current propagation and the resulting differing limnological characteristics of the two basins. The evolution of the Wedderburn Number, Lake Number, and Schmidt stability number are shown to be different in the two basins, and the difference is attributable to the morphological separation by the canyon. Investigation of the background potential energy (BPE) changes along the length of the canyon indicated that a density front passes through the upper section of the canyon but is smoothed into simple filling of the lower basin. The separable dynamics of the basins has implications for the complexity of models needed for representing both water quality and sedimentation.

scholarly journals Enhanced tropospheric BrO over Antarctic sea ice in mid winter observed by MAX-DOAS on board the research vessel Polarstern

2007 ◽  
Vol 7 (12) ◽  
pp. 3129-3142 ◽  
Author(s):  
T. Wagner ◽  
O. Ibrahim ◽  
R. Sinreich ◽  
U. Frieß ◽  
R. von Glasow ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Vertical Mixing ◽  
Satellite Observations ◽  
Research Vessel ◽  
Optical Absorption Spectroscopy ◽  
Back Trajectory ◽  
First Year ◽  
Polar Regions ◽  
Near Surface

Abstract. We present Multi AXis-Differential Optical Absorption Spectroscopy (MAX-DOAS) observations of tropospheric BrO carried out on board the German research vessel Polarstern during the Antarctic winter 2006. Polarstern entered the area of first year sea ice around Antarctica on 24 June 2006 and stayed within this area until 15 August 2006. For the period when the ship cruised inside the first year sea ice belt, enhanced BrO concentrations were almost continuously observed. Outside the first year sea ice belt, typically low BrO concentrations were found. Based on back trajectory calculations we find a positive correlation between the observed BrO differential slant column densities (ΔSCDs) and the duration for which the air masses had been in contact with the sea ice surface prior to the measurement. While we can not completely rule out that in several cases the highest BrO concentrations might be located close to the ground, our observations indicate that the maximum BrO concentrations might typically exist in a (possibly extended) layer around the upper edge of the boundary layer. Besides the effect of a decreasing pH of sea salt aerosol with altitude and therefore an increase of BrO with height, this finding might be also related to vertical mixing of air from the free troposphere with the boundary layer, probably caused by convection over the warm ocean surface at polynyas and cracks in the ice. Strong vertical gradients of BrO and O3 could also explain why we found enhanced BrO levels almost continuously for the observations within the sea ice. Based on our estimated BrO profiles we derive BrO mixing ratios of several ten ppt, which is slightly higher than many existing observations. Our observations indicate that enhanced BrO concentrations around Antarctica exist about one month earlier than observed by satellite instruments. From detailed radiative transfer simulations we find that MAX-DOAS observations are up to about one order of magnitude more sensitive to near-surface BrO than satellite observations. In contrast to satellite observations the MAX-DOAS sensitivity hardly decreases for large solar zenith angles and is almost independent from the ground albedo. Thus this technique is very well suited for observations in polar regions close to the solar terminator. For large periods of our measurements the solar elevation was very low or even below the horizon. For such conditions, most reactive Br-compounds might exist as Br2 molecules and ozone destruction and the removal of reactive bromine compounds might be substantially reduced.

Three-Dimensional Structures in Experimental Density Currents

2007 ◽  
Author(s):  
B. Firoozabadi ◽  
H. Afshin ◽  
E. Safaaee
Keyword(s):  
Salt Solution ◽  
Chemical Elements ◽  
Three Dimensional ◽  
Lateral Spreading ◽  
Current Data ◽  
Density Current ◽  
Density Currents ◽  
Ambient Water ◽  
Data Set ◽  
Bed Slope

Density currents are continuous currents which move down-slope due to the fact that their density is greater than that of ambient water. The density difference is caused by temperature differences, chemical elements, dissolved materials, or suspended sediment. Many researchers have studied the density current structures, their complexities and uncertainties. However, there is not a detailed 3-D turbulent density current data set perfectly. In this work, the structure of 3-dimensional salt solution density currents is investigated. A laboratory channel was used to study the flow resulting from the release of salt solution into freshwater over an inclined bed. The experiments were conducted with different bottom slopes, inlet concentrations and flow rates. In these tests, the instantaneous velocities are measured by an ADV apparatus (Acoustic Doppler Velocimeter). Results show that by increasing the bed-slope and inlet concentrations, the height of the current decreases. As the density current moves downward the channel or by increasing the discharge, the height of the density current increases. Finally, the effects of different variables such as the bed slope, concentration and flow rate of entering fluid on the velocity profile in different distances from the entrance is studied. The entrainment coefficient, lateral spreading and drag coefficient of the bed and shear layer between salt solution and ambient water is discussed.

scholarly journals Satellite mapping of PM<sub>2.5</sub> episodes in the wintertime San Joaquin Valley: a “static” model using column water vapor

2020 ◽  
Vol 20 (7) ◽  
pp. 4379-4397 ◽  
Author(s):  
Robert B. Chatfield ◽  
Meytar Sorek-Hamer ◽  
Robert F. Esswein ◽  
Alexei Lyapustin
Keyword(s):  
Water Vapor ◽  
Mixed Layer ◽  
Atmospheric Correction ◽  
Vertical Mixing ◽  
San Joaquin Valley ◽  
Winter Period ◽  
Statistical Regression ◽  
Simple Method ◽  
Near Surface

Abstract. The use of satellite aerosol optical thickness (AOT) from imaging spectrometers has been successful in quantifying and mapping high-PM2.5 (particulate matter with a mass <2.5 µm diameter) episodes for pollution abatement and health studies. However, some regions have high PM2.5 but poor estimation success. The challenges in using AOT from imaging spectrometers to characterize PM2.5 worldwide was especially evident in the wintertime San Joaquin Valley (SJV). The SJV's attendant difficulties of high-albedo surfaces and very shallow, variable vertical mixing also occur in other significantly polluted regions around the world. We report on more accurate PM2.5 maps (where cloudiness permits) for the whole winter period in the SJV (19 November 2012–18 February 2013). Intensive measurements by including NASA aircraft were made for several weeks in that winter, the DISCOVER-AQ (Deriving Information on Surface Conditions from COlumn and VERtically Resolved Observations Relevant to Air Quality) California mission. We found success with a relatively simple method based on calibration and checking with surface monitors and a characterization of vertical mixing, and incorporating specific understanding of the region's climatology. We estimate PM2.5 to within ∼7 µg m−3 root mean square error (RMSE) and with R values of ∼0.9, based on remotely sensed multi-angle implementation of atmospheric correction (MAIAC) observations, and certain further work will improve that accuracy. Mapping is at 1 km resolution. This allows a time sequence of mapped aerosols at 1 km for cloud-free days. We describe our technique as a “static estimation.” Estimation procedures like this one, not dependent on well-mapped source strengths or on transport error, should help full source-driven simulations by deconstructing processes. They also provide a rapid method to create a long-term climatology. Essential features of the technique are (a) daily calibration of the AOT to PM2.5 using available surface monitors, and (b) characterization of mixed layer dilution using column water vapor (CWV, otherwise “precipitable water”). We noted that on multi-day timescales both water vapor and particles share near-surface sources and both fall to very low values with altitude; indeed, both are largely removed by precipitation. The existence of layers of H2O or aerosol not within the mixed layer adds complexity, but mixed-effects statistical regression captures essential proportionality of PM2.5 and the ratio variable (AOT ∕ CWV). Accuracy is much higher than previous statistical models and can be extended to the whole Aqua satellite data record. The maps and time series we show suggest a repeated pattern for large valleys like the SJV – progressive stabilization of the mixing height after frontal passages: PM2.5 is somewhat more determined by day-by-day changes in mixing than it is by the progressive accumulation of pollutants (revealed as increasing AOT).

scholarly journals Assessing the formation and evolution mechanisms of severe haze pollution in Beijing−Tianjin−Hebei region by using process analysis

2019 ◽  
Author(s):  
Lei Chen ◽  
Jia Zhu ◽  
Hong Liao ◽  
Yi Gao ◽  
Yulu Qiu ◽  
...  
Keyword(s):  
Radiative Forcing ◽  
Process Analysis ◽  
Vertical Mixing ◽  
Mixing Process ◽  
Aerosol Chemistry ◽  
Near Surface ◽  
Aerosol Radiative Forcing ◽  
Local Emission ◽  
Aerosol Radiative ◽  
Pm2.5 Concentration

Abstract. Fine–particle pollution associated with haze threatens human health, especially in the North China Plain, where extremely high PM2.5 concentrations were frequently observed during winter. In this study, the WRF–Chem model coupled with an improved integrated process analysis scheme was used to investigate the formation and evolution mechanisms of a haze event happened over Beijing–Tianjin–Hebei (BTH) in December 2015, including examining the contributions of local emission and outside transport to the absolute PM2.5 concentration in BTH, and the contributions of each detailed physical or chemical process to the variations in the PM2.5 concentration. The influence mechanisms of aerosol radiative forcing (including aerosol direct and indirect effects) were also examined by using the process analysis. During the aerosol accumulation stage (December 20–22, Stage_1), the average near–surface PM2.5 concentration in BTH was 250.0 µg m−3, which was contributed by local emission of 42.3 % and outside transport of 36.6 %. During the aerosol dispersion stage (December 23–27, Stage_2), the average concentration of PM2.5 was 107.9 µg m−3. The contribution of local emission increased to 50.9 %, while the contribution of outside transport decreased to 24.3 %. The 24–h change (23:00LST minus 00:00LST) in the near–surface PM2.5 concentration was +50.4 µg m−3 during Stage_1 and −41.5 µg m−3 during Stage_2. Contributions of aerosol chemistry process and vertical mixing process to the 24–h change were +43.8 (+17.9) µg m−3 and −161.6 (−221.6) µg m−3 for Stage_1 (Stage_2), respectively. Small differences in contributions from other processes were found between Stage_1 and Stage_2, such as advection process, cloud chemistry process, and so on. Therefore, the PM2.5 increase over BTH during haze formation stage (Stage_1) was mainly attributed to strong production by aerosol chemistry process and weak removal by vertical mixing process. When aerosol radiative feedback was considered, the 24–h PM2.5 increase was enhanced by 9.6 µg m−3 during Stage_1, which could be mainly attributed to the contributions of vertical mixing process (+39.8 µg m−3), advection process (−38.6 µg m−3) and aerosol chemistry process (+5.1 µg m−3). The restrained vertical mixing could be the primary reason for the enhancement in near–surface PM2.5 increase when aerosol radiative forcing was considered.

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