scholarly journals Taklimakan Desert Nocturnal Low Level Jet: Climatology and Dust Emission

2016 ◽  
Author(s):  
J. M. Ge ◽  
H. Y. Liu ◽  
J. P. Huang ◽  
Q. Fu
Keyword(s):  
Inversion Layer ◽  
Dust Emission ◽  
Warm Season ◽  
Cold Season ◽  
Radiosonde Data ◽  
Taklimakan Desert ◽  
Convective Boundary ◽  
Low Level ◽  
Low Level Jets ◽  
Nocturnal Inversion

Abstract. While nocturnal Low-Level Jets (NLLJs) occur frequently in many parts of the world, the occurrence and other detailed characteristics of NLLJs over the Taklimakan Desert (TD) are not well known. This paper presents a climatology of NLLJs and coincident dust over the TD by analyzing multi-year ERA-Interim reanalysis and satellite observations. It is found that the ERA-Interim dataset can capture the NLLJs feature well by comparing with radiosonde data from two surface sites. The NLLJs occur in more than 60 % of nights, which are primarily easterly to east-northeasterly. They typically appear at 100 to 400 m above the surface with a speed of 4 to 10 ms−1. Most NLLJs are located above the nocturnal inversion during warm season while they are embedded in the inversion layer during cold season. NLLJs above the inversion have a strong annual cycle with a maximum frequency in August. We also quantify the convective boundary layer (CBL) height and construct an index to measure the magnitude of the momentum in the CBL. We find that the NLLJ contains more momentum than without NLLJ, and in warm season the downward momentum transfer process is more intense and rapid. The winds below the NLLJ core to the desert surface gain strength in summer and autumn, which are coincident with an enhancement of aerosol optical depth. It indicates that the NLLJ is an important mechanism for dust emission and transport during the warm season over the Taklimakan.

scholarly journals Taklimakan Desert nocturnal low-level jet: climatology and dust activity

2016 ◽  
Vol 16 (12) ◽  
pp. 7773-7783 ◽  
Author(s):  
Jin Ming Ge ◽  
Huayue Liu ◽  
Jianping Huang ◽  
Qiang Fu
Keyword(s):  
Inversion Layer ◽  
Warm Season ◽  
Cold Season ◽  
Lower Atmosphere ◽  
Radiosonde Data ◽  
Taklimakan Desert ◽  
Convective Boundary ◽  
Low Level ◽  
Low Level Jets ◽  
Nocturnal Inversion

Abstract. While nocturnal low-level jets (NLLJs) occur frequently in many parts of the world, the occurrence and other detailed characteristics of NLLJs over the Taklimakan Desert (TD) are not well known. This paper presents a climatology of NLLJs and coincident dust over the TD by analyzing multi-year ERA-Interim reanalysis and satellite observations. It is found that the ERA-Interim dataset can capture the NLLJs' features well by comparison with radiosonde data from two surface sites. The NLLJs occur in more than 60 % of nights, which are primarily easterly to east-northeasterly. They typically appear at 100 to 400 m above the surface with a speed of 4 to 10 m s−1. Most NLLJs are located above the nocturnal inversion during the warm season, while they are embedded in the inversion layer during the cold season. NLLJs above the inversion have a strong annual cycle with a maximum frequency in August. We also quantify the convective boundary layer (CBL) height and construct an index to measure the magnitude of the momentum in the CBL. We find that the magnitude of momentum in the lower atmosphere from the top of the surface layer to the top of mixed layer is larger for NLLJ cases than for non-NLLJ cases, and in the warm season the downward momentum transfer process is more intense and rapid. The winds below the NLLJ core to the desert surface gain strength in summer and autumn, and these summer and autumn winds are coincident with an enhancement of aerosol optical depth. This indicates that the NLLJ is an important mechanism for dust activity and transport during the warm season over the Taklimakan.

A Conceptual View on Inertial Oscillations and Nocturnal Low-Level Jets

2010 ◽  
Vol 67 (8) ◽  
pp. 2679-2689 ◽  
Author(s):  
B. J. H. Van de Wiel ◽  
A. F. Moene ◽  
G. J. Steeneveld ◽  
P. Baas ◽  
F. C. Bosveld ◽  
...  
Keyword(s):  
Inversion Layer ◽  
Geostrophic Wind ◽  
Wind Vector ◽  
Inertial Oscillations ◽  
Wind Speed Profile ◽  
Low Level ◽  
Low Level Jets ◽  
Low Level Jet ◽  
Nocturnal Inversion ◽  
Wind Profiles

Abstract In the present work Blackadar’s concept of nocturnal inertial oscillations is extended. Blackadar’s concept describes frictionless inertial oscillations above the nocturnal inversion layer. The current work includes frictional effects within the nocturnal boundary layer. It is shown that the nocturnal wind speed profile describes an oscillation around the nocturnal equilibrium wind vector, rather than around the geostrophic wind vector (as in the Blackadar case). By using this perspective, continuous time-dependent wind profiles are predicted. As such, information on both the height and the magnitude of the nocturnal low-level jet is available as a function of time. Preliminary analysis shows that the proposed extension performs well in comparison with observations when a simple Ekman model is used to represent the equilibrium state in combination with a realistic initial velocity profile. In addition to jet dynamics, backward inertial oscillations are predicted at lower levels close to the surface, which also appear to be present in observations. The backward oscillation forms an important mechanism behind weakening low-level winds during the afternoon transition. Both observational and theoretical modeling studies are needed to explore this phenomenon further.

scholarly journals Comparing two years of Saharan dust source activation obtained by regional modeling and satellite observations

2012 ◽  
Vol 12 (10) ◽  
pp. 27667-27691
Author(s):  
I. Tegen ◽  
K. Schepanski ◽  
B. Heinold
Keyword(s):  
Regional Scale ◽  
Dust Emission ◽  
Saharan Dust ◽  
Strong Increase ◽  
Dust Source ◽  
Low Level ◽  
Mountainous Regions ◽  
Emission Fluxes ◽  
Low Level Jets ◽  
The Difference

Abstract. A regional-scale dust model is used to simulate Saharan dust emissions and atmospheric distributions in the years 2007 and 2008. The model results are compared to dust source activation events compiled from infrared dust index imagery from the geostationary Meteosat Second Generation (MSG) satellite. The observed morning maximum in dust source activation frequencies indicates that the breakdown of nocturnal low-level jets is responsible for a considerable number of dust source activation events in the Sahara. The comparison shows that the time of the day of the onset of dust emission is delayed in the model compared to the observations. Also, the simulated number of dust emission events associated with nocturnal low level jets in mountainous regions is underestimated in the model. The MSG dust index observations indicate a strong increase in dust source activation frequencies in the year 2008 compared to 2007, the difference between the two years is less pronounced in the model. The quantitative comparison of simulated dust optical thicknesses with observations at stations of the sunphotometer network AERONET shows, however, good agreement for both years, indicating that the number of observed dust activation events is only of limited use for estimating actual dust emission fluxes in the Sahara.

scholarly journals Comparing two years of Saharan dust source activation obtained by regional modelling and satellite observations

2013 ◽  
Vol 13 (5) ◽  
pp. 2381-2390 ◽  
Author(s):  
I. Tegen ◽  
K. Schepanski ◽  
B. Heinold
Keyword(s):  
Regional Scale ◽  
Dust Emission ◽  
Saharan Dust ◽  
Strong Increase ◽  
Regional Modelling ◽  
Dust Source ◽  
Low Level ◽  
Mountainous Regions ◽  
Low Level Jets ◽  
The Difference

Abstract. A regional-scale dust model is used to simulate Saharan dust emissions and atmospheric distributions in the years 2007 and 2008. The model results are compared to dust source activation events compiled from infrared dust index imagery from the geostationary Meteosat Second Generation (MSG) satellite. The observed morning maximum in dust source activation frequencies indicates that the breakdown of nocturnal low level jets is an important mechanism for dust source activation in the Sahara. The comparison shows that the time of the day of the onset of dust emission is delayed in the model compared to the observations. Also, the simulated number of dust emission events associated with nocturnal low level jets in mountainous regions is underestimated in the model. The MSG dust index observations indicate a strong increase in dust source activation frequencies in the year 2008 compared to 2007. The difference between the two years is less pronounced in the model. Observations of dust optical thickness, e.g. at stations of the sunphotometer network AERONET, do not show such increase, in agreement with the model results. This indicates that the number of observed dust activation events is only of limited use for estimating actual dust emission fluxes in the Sahara. The ability to reproduce interannual variability of Saharan dust with models remains an important challenge for understanding the controls of the atmospheric dust load.

scholarly journals Convection Initiation Caused by Heterogeneous Low-Level Jets over the Great Plains

2018 ◽  
Vol 146 (8) ◽  
pp. 2615-2637 ◽  
Author(s):  
Joshua G. Gebauer ◽  
Alan Shapiro ◽  
Evgeni Fedorovich ◽  
Petra Klein
Keyword(s):  
Great Plains ◽  
Three Dimensional ◽  
Warm Season ◽  
The United States ◽  
Dimensional Structure ◽  
Nonuniform Heating ◽  
Low Level ◽  
Low Level Jets ◽  
Convection Initiation ◽  
Capping Inversion

AbstractObservations from three nights of the Plains Elevated Convection at Night (PECAN) field campaign were used in conjunction with Rapid Refresh model forecasts to find the cause of north–south lines of convection, which initiated away from obvious surface boundaries. Such pristine convection initiation (CI) is relatively common during the warm season over the Great Plains of the United States. The observations and model forecasts revealed that all three nights had horizontally heterogeneous and veering-with-height low-level jets (LLJs) of nonuniform depth. The veering and heterogeneity were associated with convergence at the top-eastern edge of the LLJ, where moisture advection was also occurring. As time progressed, this upper region became saturated and, due to its placement above the capping inversion, formed moist absolutely unstable layers, from which the convergence helped initiate elevated convection. The structure of the LLJs on the CI nights was likely influenced by nonuniform heating across the sloped terrain, which led to the uneven LLJ depth and contributed toward the wind veering with height through the creation of horizontal buoyancy gradients. These three CI events highlight the importance of assessing the full three-dimensional structure of the LLJ when forecasting nocturnal convection over the Great Plains.

scholarly journals Speciated mercury at marine, coastal, and inland sites in New England – Part 1: Temporal variability

2012 ◽  
Vol 12 (11) ◽  
pp. 5099-5112 ◽  
Author(s):  
H. Mao ◽  
R. Talbot
Keyword(s):  
New England ◽  
Marine Boundary Layer ◽  
Limit Of Detection ◽  
Inversion Layer ◽  
Warm Season ◽  
Elemental Mercury ◽  
Positive Trend ◽  
Gaseous Elemental Mercury ◽  
Mixing Ratios ◽  
Nocturnal Inversion

Abstract. A comprehensive analysis was conducted using long-term continuous measurements of gaseous elemental mercury (Hg0), reactive gaseous mercury (RGM), and particulate phase mercury (HgP) at coastal (Thompson Farm, denoted as TF), marine (Appledore Island, denoted as AI), and elevated inland (Pac Monadnock, denoted as PM) sites from the AIRMAP Observatories in southern New Hampshire, USA. Decreasing trends in background Hg0 were identified in the 7.5- and 5.5-yr records at TF and PM with decline rates of 3.3 parts per quadrillion by volume (ppqv) yr−1 and 6.3 ppqv yr−1, respectively. Common characteristics at these sites were the reproducible annual cycle of Hg0 with its maximum in winter-spring and minimum in fall, comprised of a positive trend in the warm season (spring – early fall) and a negative one in the cool season (late fall – winter). Year-to-year variability was observed in the warm season decline in Hg0 at TF varying from a minimum total (complete) seasonal loss of 43 ppqv in 2009 to a maximum of 92 ppqv in 2005, whereas variability remained small at AI and PM. The coastal site TF differed from the other two sites with its exceptionally low levels (as low as below 50 ppqv) in the nocturnal inversion layer possibly due to dissolution in dew water. Measurements of Hg0 at PM exhibited the smallest diurnal to annual variability among the three environments, where peak levels rarely exceeded 250 ppqv and the minimum was typically 100 ppqv. It should be noted that summertime diurnal patterns at TF and AI were opposite in phase indicating strong sink(s) for Hg0 during the day in the marine boundary layer, which was consistent with the hypothesis of Hg0 oxidation by halogen radicals there. Mixing ratios of RGM in the coastal and marine boundary layers reached annual maxima in spring and minima in fall, whereas at PM levels were generally below the limit of detection (LOD) except in spring. RGM levels at AI were higher than at TF and PM indicating a stronger source strength in the marine environment. Mixing ratios of HgP at AI and TF were close in magnitude to RGM levels and were mostly below 1 ppqv. Diurnal variation in HgP was barely discernible at TF and AI in spring and summer. Higher levels of HgP were observed during the day, while values that were smaller, but above the LOD, occurred at night.

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