A Characterization of Cold Pools in the West African Sahel

Cold pools are integral components of squall-line mesoscale convective systems and the West African monsoon, but are poorly represented in operational global models. Observations of 38 cold pools made at Niamey, Niger, during the 2006 African Monsoon Multidisciplinary Analysis (AMMA) campaign (1 June–30 September 2006), are used to generate a seasonal characterization of cold pool properties by quantifying related changes in surface meteorological variables. Cold pools were associated with temperature decreases of 2°–14°C, pressure increases of 0–8 hPa, and wind gusts of 3–22 m s−1. Comparison with published values of similar variables from the U.S. Great Plains showed comparable differences. The leading part of most cold pools had decreased water vapor mixing ratios compared to the environment, with moister air, likely related to precipitation, approximately 30 min behind the gust front. A novel diagnostic used to quantify how consistent observed cold pool temperatures are with saturated or unsaturated descent from midlevels [fractional evaporational energy deficit (FEED)] shows that early season cold pools are consistent with less saturated descents. Early season cold pools were relatively colder, windier, and wetter, consistent with drier midlevels, although this was only statistically significant for the change in moisture. Late season cold pools tended to decrease equivalent potential temperature from the pre–cold pool value, whereas earlier in the season changes were smaller, with more increases. The role of cold pools may therefore change through the season, with early season cold pools more able to feed subsequent convection.

Download Full-text

Detection and Characterization of Cold Pools over Germany in the DYAMOND Simulations

10.5194/egusphere-egu2020-9711 ◽

2020 ◽

Author(s):

Jaemyeong Mango Seo ◽

Cathy Hohenegger

Keyword(s):

Wind Speed ◽

General Circulation ◽

Potential Temperature ◽

Squall Line ◽

Cold Pool ◽

Characteristic Parameters ◽

Pressure System ◽

Low Area ◽

Convective Clouds ◽

Cold Pools

Cold pool generated by convective clouds is an evaporatively cooled dry region which spreads out near the surface. Studying the cold pool characteristics enhances our understanding about convective clouds such as shallow-to-deep transition of convective clouds, long-lived squall line, and triggering secondary convection. In this study, cold pools over Germany are detected and characterized using phase 0 results of DYAMOND (stands for DYnamics of the Atmospheric general circulation Modeled On Non-hydrostatic Domains) intercomparison project. We aim to understand how the cold pool characteristics over Germany depend on topographic height, accompanying cloud size, and model.Nine model results of the DYAMOND collection are remapped into 0.1&#730; &#215; 0.1&#730; regular grid system. Cold pool cluster is defined as a cluster with an area larger than ~64 km2 (4 grids), with the perturbation virtual (density) potential temperature below 2 K and the maximum precipitation rate greater than 1 mm h&#8211;1. Detected cold pools are re-categorized by the topographic height to decompose cold pools related to orographic precipitation and by the accompanying cloud size to decompose cold pools related to large cloud system.During simulated period (40 days from 1 August 2016), model averaged total detected cold pool number is 5.59 h&#8211;1. Although more number of cold pool clusters are detected over low topographic area (1.34 h&#8211;1 and 4.25 h&#8211;1 over high and low area, respectively), area weighted cold pool cluster number is 3.82 times larger over high topographic area (17.55 h&#8211;1 and 4.60 h&#8211;1 over high and low area, respectively). Most of cold pool clusters are accompanied by larger clouds than themselves (78 %) and 9 % of cold pools are detected outside of cloud cover. Except for the cold pools accompanied by clouds of synoptic low pressure system, most of cold pools are detected in the daytime. Cold pool clusters over high topographic area are larger, more non-circular shaped, colder, and with lower wind speed than those over low topographic area. Cold pool clusters accompanied by small clouds are colder, drier, with higher wind speed, and with stronger precipitation than those accompanied by large clouds. In this study, relationship between cold pool characteristic parameters in each category is also investigated. To understand how cold pool feature varies from model to model, the cold pool characteristic parameters in each DYAMOND model result are compared and analyzed.

Download Full-text

Squall Line Response to Coastal Mid-Atlantic Thermodynamic Heterogeneities

Journal of the Atmospheric Sciences ◽

10.1175/jas-d-20-0044.1 ◽

2020 ◽

Vol 77 (12) ◽

pp. 4143-4170

Author(s):

Kelly Lombardo

Keyword(s):

Gravity Waves ◽

Potential Temperature ◽

Atlantic Coastal Plain ◽

Squall Line ◽

Cold Pool ◽

Ambient Conditions ◽

Storm Intensity ◽

Cold Pools ◽

Squall Lines ◽

The Impact

AbstractIdealized 3D numerical simulations are used to quantify the impact of moving marine atmospheric boundary layers (MABLs) on squall lines in an environment representative of the U.S. mid-Atlantic coastal plain. Characteristics of the MABL, including depth and potential temperature, are varied. Squall lines are most intense while moving over the deepest MABLs, while the storm encountering no MABL is the weakest. Storm intensity is only sensitive to MABL temperature when the MABL is sufficiently deep. Collisions between the storm cold pools and MABLs transition storm lift from surface-based cold pools to wavelike features, with the resulting ascent mechanism dependent on MABL density, not depth. Bores form when the MABL is denser than the cold pool and hybrid cold pool–bores form when the densities are similar. While these features support storms over the MABL, the type of lifting mechanism does not control storm intensity alone. Storm intensity depends on the amplification and maintenance of these features, which is determined by the ambient conditions. Isolated convective cells form ahead of squall lines prior to the cold pool–MABL collision, resulting in a rain peak and the eventual discrete propagation of the storms. Cells form as storm-generated high-frequency gravity waves interact with gravity waves generated by the moving marine layers, in the presence of reduced stability by the squall line itself. No cells form in the presence of the storm or the MABL alone.

Download Full-text

Intraseasonal Variability of the Saharan Heat Low and Its Link with Midlatitudes

Journal of Climate ◽

10.1175/2010jcli3093.1 ◽

2010 ◽

Vol 23 (10) ◽

pp. 2544-2561 ◽

Cited By ~ 65

Author(s):

Fabrice Chauvin ◽

Romain Roehrig ◽

Jean-Philippe Lafore

Keyword(s):

North Africa ◽

Potential Temperature ◽

West African Monsoon ◽

West African ◽

Maximum Temperature ◽

Temperature Structure ◽

The West ◽

African Monsoon ◽

The Mediterranean ◽

Heat Low

Abstract The Saharan heat low (SHL) is thought to be a key feature of the West African monsoon, and its variations during the summer season have not yet been systematically assessed. To characterize the intraseasonal variations of the SHL, real and complex empirical orthogonal function analyses were applied to the 850-hPa potential temperature field over northern Africa and the Mediterranean, using NCEP–Department of Energy (DOE) Atmospheric Model Intercomparison Project (AMIP-II) reanalysis results. A robust quasi-propagative mode was highlighted over North Africa and the Mediterranean. This mode consists of two phases. The west phase corresponds to a maximum temperature over the coast of Morocco–Mauritania, propagating southwestward, and a minimum between Libya and Sicily, propagating southeastward. The east phase corresponds to the opposite temperature structure, which propagates as in the west phase. A lag-composite analysis revealed that this SHL mode was preceded by large-scale, midlatitude, intraseasonal fluctuations of the atmosphere. The southward penetration of a Rossby wave disturbance over Europe and North Africa generates modulations of the three-dimensional atmospheric structure. The low-level ventilations and harmattan-like circulation are particularly impacted, as are the subtropical westerlies and the polar jets in the upper troposphere. The west phase is concomitant with an enhanced convective signal over the Darfur region, which propagates westward, as far as the middle of the Atlantic, at a speed similar to that of the well-known African easterly waves. The SHL appears to be a bridge between the midlatitudes and the West African monsoon, which may offer promising sources of predictability over the Sahel on an intraseasonal time scale.

Download Full-text

On What Scale Can We Predict the Agronomic Onset of the West African Monsoon?

Monthly Weather Review ◽

10.1175/mwr-d-15-0274.1 ◽

2016 ◽

Vol 144 (4) ◽

pp. 1571-1589 ◽

Cited By ~ 6

Author(s):

Rory G. J. Fitzpatrick ◽

Caroline L. Bain ◽

Peter Knippertz ◽

John H. Marsham ◽

Douglas J. Parker

Keyword(s):

West Africa ◽

Noise Analysis ◽

West African Monsoon ◽

West African ◽

New Method ◽

The West ◽

African Monsoon ◽

Spatial Consistency ◽

Local Stakeholders ◽

Local Rainfall

Abstract Accurate prediction of the commencement of local rainfall over West Africa can provide vital information for local stakeholders and regional planners. However, in comparison with analysis of the regional onset of the West African monsoon, the spatial variability of the local monsoon onset has not been extensively explored. One of the main reasons behind the lack of local onset forecast analysis is the spatial noisiness of local rainfall. A new method that evaluates the spatial scale at which local onsets are coherent across West Africa is presented. This new method can be thought of as analogous to a regional signal against local noise analysis of onset. This method highlights regions where local onsets exhibit a quantifiable degree of spatial consistency (denoted local onset regions or LORs). It is found that local onsets exhibit a useful amount of spatial agreement, with LORs apparent across the entire studied domain; this is in contrast to previously found results. Identifying local onset regions and understanding their variability can provide important insight into the spatial limit of monsoon predictability. While local onset regions can be found over West Africa, their size is much smaller than the scale found for seasonal rainfall homogeneity. A potential use of local onset regions is presented that shows the link between the annual intertropical front progression and local agronomic onset.

Download Full-text

A Lagrangian perspective on stable water isotopes during the West African Monsoon

10.1002/essoar.10506628.1 ◽

2021 ◽

Author(s):

Christopher Johannes Diekmann ◽

Matthias Schneider ◽

Peter Knippertz ◽

Andries Jan de Vries ◽

Stephan Pfahl ◽

...

Keyword(s):

West African Monsoon ◽

West African ◽

Water Isotopes ◽

Stable Water Isotopes ◽

The West ◽

African Monsoon

Download Full-text

A hydrological onset and withdrawal index for the West African monsoon

Theoretical and Applied Climatology ◽

10.1007/s00704-008-0022-8 ◽

2008 ◽

Vol 96 (1-2) ◽

pp. 179-189 ◽

Cited By ~ 7

Author(s):

G. A. Dalu ◽

M. Gaetani ◽

M. Baldi

Keyword(s):

West African Monsoon ◽

West African ◽

The West ◽

African Monsoon

Download Full-text

Assessing Bypassed Hydrocarbons

10.2118/207086-ms ◽

2021 ◽

Author(s):

Stanley Oifoghe ◽

Nora Alarcon ◽

Lucrecia Grigoletto

Keyword(s):

Continental Shelf ◽

Reservoir Characterization ◽

Gamma Ray ◽

West African ◽

The Novel ◽

The West ◽

Present Case Study ◽

Novel Model

Abstract Hydrocarbons are bypassed in known fields. This is due to reservoir heterogeneities, complex lithology, and limitations of existing technology. This paper seeks to identify the scenarios of bypassed hydrocarbons, and to highlight how advances in reservoir characterization techniques have improved assessment of bypassed hydrocarbons. The present case study is an evaluation well drilled on the continental shelf, off the West African Coastline. The targeted thin-bedded reservoir sands are of Cenomanian age. Some technologies for assessing bypassed hydrocarbon include Gamma Ray Spectralog and Thin Bed Analysis. NMR is important for accurate reservoir characterization of thinly bedded reservoirs. The measured NMR porosity was 15pu, which is 42% of the actual porosity. Using the measured values gave a permeability of 5.3mD as against the actual permeability of 234mD. The novel model presented in this paper increased the porosity by 58% and the permeability by 4315%.

Download Full-text