scholarly journals Aircraft observations of cold pools under marine stratocumulus

2013 ◽  
Vol 13 (19) ◽  
pp. 9899-9914 ◽  
Author(s):  
C. R. Terai ◽  
R. Wood
Keyword(s):  
Boundary Layer ◽  
Dimethyl Sulfide ◽  
Potential Temperature ◽  
Static Stability ◽  
Cold Pool ◽  
Sulfide Concentration ◽  
Marine Stratocumulus ◽  
Cold Pools ◽  
Pressure Perturbations ◽  
Temperature Depression

Abstract. Although typically associated with precipitating cumuli, cold pools also form under shallower stratocumulus. This study presents cold-pool observations as sampled by the NSF/NCAR C-130, which made cloud and boundary-layer measurements over the southeast Pacific stratocumulus region at an altitude of approximately 150 m during the VOCALS Regional Experiment. Ninety edges of cold pools are found in the C-130 measurements by identifying step-like changes in the potential temperature. Examination of their mesoscale environment shows that the observed cold pools tend to form under heavier precipitation, thicker clouds, and in cleaner environments. Cold pools are also found to form under clouds with high LWP values over the night of or before sampling. When they form, cold pools often form in clusters or on top of each other, rather than as separate, individual entities. Their sizes range from 2 km to 16 km (middle 50th percentile), where the largest of cold pools are associated with the greatest drops in temperature. Composites of various observed thermodynamic and chemical variables along the cold-pool edges indicate increased humidity, equivalent potential temperature, coarse-mode aerosol, and dimethyl sulfide concentration inside cold pools. The enhancements inside cold pools are consistent with increased static stability that traps fluxes from the ocean surface in the lowest levels of the boundary layer. By using pressure perturbations, the average cold pool is estimated to be approximately 300 m deep. The temperature depression in cold pools also leads to density-driven flows that drive convergence of horizontal winds and measurable, mechanically driven vertical wind velocity at the edges of cold pools.

scholarly journals Aircraft observations of cold pools under marine stratocumulus

2013 ◽  
Vol 13 (4) ◽  
pp. 11023-11069 ◽  
Author(s):  
C. R. Terai ◽  
R. Wood
Keyword(s):  
Boundary Layer ◽  
Dimethyl Sulfide ◽  
Potential Temperature ◽  
Static Stability ◽  
Cold Pool ◽  
Sulfide Concentration ◽  
Marine Stratocumulus ◽  
Cold Pools ◽  
Pressure Perturbations ◽  
Temperature Depression

Abstract. Although typically associated with precipitating cumuli, cold pools also form under shallower stratocumulus. The NSF/NCAR C-130 made cloud and boundary layer measurements over the southeast Pacific stratocumulus region at an altitude of approximately 150 m during the VOCALS Regional Experiment. Ninety edges of cold pools are found in the C-130 measurements by identifying step-like decreases in the potential temperature. Examination of their mesoscale environment shows that the observed cold pools tend to form under heavier precipitation, thicker clouds, and in cleaner environments. Cold pools are also found to form under clouds with high LWP values over the night of or before sampling. When they form, cold pools often form in clusters or on top of each other, rather than as separate, individual entities. Their sizes range from 2 km to 16 km (middle 50th percentile), where the largest of cold pools are associated with the greatest drops in temperature. Composites of various observed thermodynamic and chemical variables along the cold pool edges indicate increased humidity, equivalent potential temperature, coarse-mode aerosol, and dimethyl sulfide concentration inside cold pools. The enhancements inside cold pools are consistent with increased static stability that traps fluxes from the ocean surface in the lowest levels of the boundary layer. By using pressure perturbations, the average cold pool is estimated to be approximately 300 m deep. The temperature depression in cold pools leads to density-driven flows that drive convergence of horizontal winds and measurable, mechanically-driven vertical wind velocity at the edges of cold pools.

Cold Pools and Their Influence on the Tropical Marine Boundary Layer

2017 ◽  
Vol 74 (4) ◽  
pp. 1149-1168 ◽  
Author(s):  
Simon P. de Szoeke ◽  
Eric D. Skyllingstad ◽  
Paquita Zuidema ◽  
Arunchandra S. Chandra
Keyword(s):  
Boundary Layer ◽  
Marine Boundary Layer ◽  
Sensible Heat Flux ◽  
Surface Flux ◽  
Active Phase ◽  
Surface Fluxes ◽  
Cold Pool ◽  
Madden Julian Oscillation ◽  
Cold Pools ◽  
Central Indian Ocean

Abstract Cold pools dominate the surface temperature variability observed over the central Indian Ocean (0°, 80°E) for 2 months of research cruise observations in the Dynamics of the Madden–Julian Oscillation (DYNAMO) experiment in October–December 2011. Cold pool fronts are identified by a rapid drop of temperature. Air in cold pools is slightly drier than the boundary layer (BL). Consistent with previous studies, cold pools attain wet-bulb potential temperatures representative of saturated downdrafts originating from the lower midtroposphere. Wind and surface fluxes increase, and rain is most likely within the ~20-min cold pool front. Greatest integrated water vapor and liquid follow the front. Temperature and velocity fluctuations shorter than 6 min achieve 90% of the surface latent and sensible heat flux in cold pools. The temperature of the cold pools recovers in about 20 min, chiefly by mixing at the top of the shallow cold wake layer, rather than by surface flux. Analysis of conserved variables shows mean BL air is composed of 51% air entrained from the BL top (800 m), 22% saturated downdrafts, and 27% air at equilibrium with the ocean surface. The number of cold pools, and their contribution to the BL heat and moisture, nearly doubles in the convectively active phase compared to the suppressed phase of the Madden–Julian oscillation.

Surface Characteristics of Observed Cold Pools

2008 ◽  
Vol 136 (12) ◽  
pp. 4839-4849 ◽  
Author(s):  
Nicholas A. Engerer ◽  
David J. Stensrud ◽  
Michael C. Coniglio
Keyword(s):  
Life Cycle ◽  
Potential Temperature ◽  
Surface Characteristics ◽  
Life Cycle Stage ◽  
Cold Pool ◽  
Convective System ◽  
Cold Pools ◽  
Equivalent Potential ◽  
Life Cycle Stages ◽  
The Mean

Abstract Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

scholarly journals Dynamics of Lower-Tropospheric Vorticity in Idealized Simulations of Tropical Cyclone Formation

2019 ◽  
Vol 76 (3) ◽  
pp. 707-727 ◽  
Author(s):  
Yaping Wang ◽  
Christopher A. Davis ◽  
Yongjie Huang
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mass Flux ◽  
Cloud Model ◽  
Vertical Wind Shear ◽  
Lower Troposphere ◽  
Cold Pool ◽  
Surface Drag ◽  
Near Surface ◽  
Cold Pools

Abstract Idealized simulations are conducted using the Cloud Model version 1 (CM1) to explore the mechanism of tropical cyclone (TC) genesis from a preexisting midtropospheric vortex that forms in radiative–convective equilibrium. With lower-tropospheric air approaching near saturation during TC genesis, convective cells become stronger, along with the intensifying updrafts and downdrafts and the larger area coverage of updrafts relative to downdrafts. Consequently, the low-level vertical mass flux increases, inducing vorticity amplification above the boundary layer. Of interest is that while surface cold pools help organize lower-tropospheric updrafts, genesis still proceeds, only slightly delayed, if subcloud evaporation cooling and cold pool intensity are drastically reduced. More detrimental is the disruption of near saturation through the introduction of weak vertical wind shear. The lower-tropospheric dry air suppresses the strengthening of convection, leading to weaker upward mass flux and much slower near-surface vortex spinup. We also find that surface spinup is similarly inhibited by decreasing surface drag despite the existence of a nearly saturated column, whereas larger drag accelerates spinup. Increased vorticity above the boundary layer is followed by the emergence of a horizontal pressure gradient through the depth of the boundary layer. Then the corresponding convergence resulting from the gradient imbalance in the frictional boundary layer causes vorticity amplification near the surface. It is suggested that near saturation in the lower troposphere is critical for increasing the mass flux and vorticity just above the boundary layer, but it is necessary yet insufficient because the spinup is strongly governed by boundary layer dynamics.

scholarly journals Why is the Tropical Cyclone Boundary Layer Not “Well Mixed”?

2016 ◽  
Vol 73 (3) ◽  
pp. 957-973 ◽  
Author(s):  
Jeffrey D. Kepert ◽  
Juliane Schwendike ◽  
Hamish Ramsay
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mixed Layer ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Static Stability ◽  
Layer Depth ◽  
Turbulent Dissipation ◽  
Unstable Layer ◽  
Tropical Cyclone Boundary Layer

Abstract Plausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the well-mixed layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer but is not without limitations. These results may have implications for boundary layer parameterizations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.

Sub-mesoscale cold-pool observations during FESST@HH 2020

2021 ◽  
Author(s):  
Bastian Kirsch ◽  
Cathy Hohenegger ◽  
Daniel Klocke ◽  
Felix Ament
Keyword(s):  
Strong Relationship ◽  
Blind Spot ◽  
Cold Pool ◽  
Density Currents ◽  
Horizontal Structure ◽  
Cold Pools ◽  
Local Perturbations ◽  
Large Eddy ◽  
Operational Networks ◽  
Pressure Perturbations

<p>Cold pools are areas of cool downdraft air, that form through evaporation underneath precipitating clouds and spread on the surface as density currents. Their importance for the development and maintenance of convection is long known. Modern Large-Eddy simulations with a grid spacing of 1 km or less explicitly resolve cold pools, however, they lack reference data for an adequate validation. Available operational networks are too coarse and, therefore, miss the horizontal structure and dynamics of cold pools.</p><p>The pioneering field experiment FESST@HH aims to shed light on this observational blind spot. During summer 2020 a dense network of 102 ground-based stations covering the greater area of Hamburg (Germany) realized meteorological measurements at sub-mesoscale resolution (Δx < 2 km, Δt ≤ 10 s), that provide novel insights into previously unobserved features of cold pools. Over three months more than 30 cold-pool events of different strength and size from various types of convection were detected. Analyses of prominent cases suggest a strong relationship between the local perturbations in air temperature and pressure within a cold pool, that allows inference about its vertical depth based on the hydrostatic assumption. Furthermore, temporary decoupling of horizontal variability in these signals reveal the presence of local non-hydrostatic pressure perturbations caused by convective downdrafts. The presented work will help to better understand the characteristics and life cycle of cold pools and to identify potential biases in convection-permitting simulations.</p>

Cold pool driven convective initiation: How can we improve its representation in km-scale models?

2020 ◽  
Author(s):  
Mirjam Hirt ◽  
George Craig
Keyword(s):  
Positive Impact ◽  
Potential Temperature ◽  
Horizontal Wind ◽  
Cold Pool ◽  
Model Resolution ◽  
Convective Initiation ◽  
Cold Pools ◽  
Scale Models ◽  
Gust Fronts ◽  
Gust Front

<p>Cold pools are essential for organizing convection and play a particular role in convective initiation in the afternoon and evening. Both aspects are deficient in current convection-permitting models and a better representation of cold pools is likely necessary to overcome these deficiencies. In a recent investigation, we identified several sensitivities of cold pool driven convective initiation to model resolution within hectometer simulations. In particular, a causal graph analysis has revealed that the dominant impact of model resolution on convective initiation is due to too weak gust front vertical velocities. This implies that cold pool gust fronts in km-scale models are too weak to trigger sufficient new convection.</p><p>To address this deficiency, we develop a parameterization for the convection-permitting COSMO model to improve the representation of cold pool gust fronts. We use the potential temperature gradient to identify cold pool gust fronts and enhance vertical wind tendencies within these gust front regions.  Also, we perturb horizontal wind tendencies to yield 3d non-divergent perturbations.  This parameterization strengthens gust front circulations and thereby enhances cold pool driven convective initiation. Consequently, precipitation is amplified and becomes more organized in the afternoon and evening. This improves the diurnal cycle of precipitation and also has some positive impact on the spatial distribution as quantified by the fraction skill score. Furthermore, cold pools themselves are strengthened, which can further enhance the gust front circulations, giving rise to a feedback loop. </p>

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

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

<p>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.</p><p>Nine model results of the DYAMOND collection are remapped into 0.1˚ × 0.1˚ regular grid system. Cold pool cluster is defined as a cluster with an area larger than ~64 km<sup>2</sup> (4 grids), with the perturbation virtual (density) potential temperature below 2 K and the maximum precipitation rate greater than 1 mm h<sup>–1</sup>. 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.</p><p>During simulated period (40 days from 1 August 2016), model averaged total detected cold pool number is 5.59 h<sup>–1</sup>. Although more number of cold pool clusters are detected over low topographic area (1.34 h<sup>–1</sup> and 4.25 h<sup>–1</sup> over high and low area, respectively), area weighted cold pool cluster number is 3.82 times larger over high topographic area (17.55 h<sup>–1</sup> and 4.60 h<sup>–1</sup> 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.</p>

scholarly journals A Characterization of Cold Pools in the West African Sahel

2016 ◽  
Vol 144 (5) ◽  
pp. 1923-1934 ◽  
Author(s):  
M. Provod ◽  
J. H. Marsham ◽  
D. J. Parker ◽  
C. E. Birch
Keyword(s):  
Potential Temperature ◽  
West African ◽  
Energy Deficit ◽  
Squall Line ◽  
Cold Pool ◽  
The West ◽  
Early Season ◽  
African Monsoon ◽  
Cold Pools

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.

scholarly journals Thermal Submesoscale Motions in the Nocturnal Stable Boundary Layer. Part 1: Detection and Mean Statistics

2021 ◽  
Author(s):  
Lena Pfister ◽  
Karl Lapo ◽  
Larry Mahrt ◽  
Christoph K. Thomas
Keyword(s):  
Boundary Layer ◽  
Stable Boundary Layer ◽  
Static Stability ◽  
Detection Algorithm ◽  
Cold Pool ◽  
Stable Boundary ◽  
Novel Approach ◽  
Order Of Magnitude ◽  
Air Layers ◽  
Objective Detection

AbstractSubmesoscale motions within the stable boundary layer were detected during the Shallow Cold Pool Experiment conducted in the Colorado plains, Colorado, U.S.A. in 2012. The submesoscale motion consisted of two air layers creating a well-defined front with a sharp temperature gradient, and further-on referred to as a thermal submesofront (TSF). The semi-stationary TSFs and their advective velocities are detected and determined by the fibre-optic distributed-sensing (FODS) technique. An objective detection algorithm utilizing FODS measurements is able to detect the TSF boundary, which enables a detailed investigation of its spatio–temporal statistics. The novel approach in data processing is to conditionally average any parameter depending on the distance between a TSF boundary and the measurement location. By doing this, a spatially-distributed feature like TSFs can be characterized by point observations and processes at the TSF boundary can be investigated. At the TSF boundary, the air layers converge, creating an updraft, strong static stability, and vigorous mixing. Further, the TSF advective velocity of TSFs is an order of magnitude lower than the mean wind speed. Despite being gentle, the topography plays an important role in TSF formation. Details on generating mechanisms and implications of TSFs on the stable boundary layer are discussed in Part 2.

Sign in / Sign up

Export Citation Format

Share Document