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>

Sub-mesoscale observations of cold pools during FESSTVaL

2020 ◽  
Author(s):  
Bastian Kirsch ◽  
Felix Ament ◽  
Cathy Hohenegger ◽  
Daniel Klocke
Keyword(s):  
Low Cost ◽  
Cold Pool ◽  
Density Currents ◽  
Horizontal Structure ◽  
Cold Pools ◽  
Meteorological Observatory ◽  
First Results ◽  
Spatio Temporal ◽  
Operational Networks ◽  
Measurement Strategies

<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 are able to explicitly resolve cold pools, however, they lack reference data for an adequate validation. Available point measurements from operational networks are too coarse and, therefore, miss the horizontal structure and dynamics of cold pools.</p><p>The upcoming measurement campaign FESSTVaL (Field Experiment on Sub-mesocale Spatio-Temporal Variability in Lindenberg) aims to test novel measurement strategies for the observation of previously unresolved sub-mesoscale boundary layer structures and phenomena, such as cold pools. The key component of the experiment during this summer will be a dense network of ground-based measurements within 15 km around the Meteorological Observatory Lindenberg near Berlin. The network of 100 low-cost APOLLO (Autonomous cold POoL LOgger) stations allows to record air pressure and temperature with a spatial and temporal resolution of 100 m and 1 s, respectively. We present first results from a test campaign during last summer that successfully demonstrated the ability of the proposed network stations to observe cold pool dynamics on the sub-mesoscale.</p>

scholarly journals Illuminating the blind spot of sub-mesoscale phenomena with a dense station network

2021 ◽  
Author(s):  
Bastian Kirsch ◽  
Felix Ament ◽  
Cathy Hohenegger ◽  
Daniel Klocke
Keyword(s):  
Low Cost ◽  
Blind Spot ◽  
Cold Pool ◽  
Data Set ◽  
Additional Information ◽  
Cold Pools ◽  
Secondary Network ◽  
First Results ◽  
Urban Heat ◽  
Operational Networks

<p>Between June and August 2020 an observational network of 103 autonomous ground-based stations covered the greater area (50 km × 35 km) of Hamburg (Germany) within the framework of the FESST@HH field experiment. The purpose of the experiment was to conduct meteorological measurements at sub-mesoscale resolution (500 m to 5 km) to observe phenomena that typically remain unresolved in operational networks. The experimental design focuses on studying cold pools that form through evaporation underneath precipitating clouds and spread on the Earth’s surface.</p><p>During the experiment 82 low-cost APOLLO (Autonomous cold POoL LOgger) stations sampled air temperature and pressure at 1 s resolution to adequately capture the rapid signals of horizontally propagating cold-pool fronts. A secondary network of 21 autonomous weather stations with commercial sensors provided additional information on relative humidity, wind speed and precipitation at 10 s resolution. This work introduces the novel type of instruments, describes the generated data set, and presents first results of the experiment.</p><p>Over the three-month period the FESST@HH network experienced more than 30 cold-pool events of different strength and size. Case studies demonstrate that the observations allow to capture the internal structure and growth of a cold pool and to infer its vertical depth based on the hydrostatic assumption. The data set does not only provide novel insights into the life cycle of cold pools, but also opens new perspectives on phenomena like the urban heat island. Moreover, the experiment may serve as a prototype for the design of future observational networks, including citizen science approaches.</p>

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.

Impact of Graupel Parameterization Schemes on Idealized Bow Echo Simulations

2013 ◽  
Vol 141 (4) ◽  
pp. 1241-1262 ◽  
Author(s):  
Rebecca D. Adams-Selin ◽  
Susan C. van den Heever ◽  
Richard H. Johnson
Keyword(s):  
Complete Removal ◽  
Cold Pool ◽  
Size Distributions ◽  
Pressure Gradients ◽  
Cooling Rates ◽  
Cold Pools ◽  
Highly Sensitive ◽  
Bow Echo ◽  
Mean Size ◽  
Microphysical Processes

Abstract The effect of changes in microphysical cooling rates on bow echo development and longevity are examined through changes to graupel parameterization in the Advanced Research Weather Research and Forecasting Model (ARW-WRF). Multiple simulations are performed that test the sensitivity to different graupel size distributions as well as the complete removal of graupel. It is found that size distributions with larger and denser, but fewer, graupel hydrometeors result in a weaker cold pool due to reduced microphysical cooling rates. This yields weaker midlevel (3–6 km) buoyancy and pressure perturbations, a later onset of more elevated rear inflow, and a weaker convective updraft. The convective updraft is also slower to tilt rearward, and thus bowing occurs later. Graupel size distributions with more numerous, smaller, and lighter hydrometeors result in larger microphysical cooling rates, stronger cold pools, more intense midlevel buoyancy and pressure gradients, and earlier onset of surface-based rear inflow; these systems develop bowing segments earlier. A sensitivity test with fast-falling but small graupel hydrometeors revealed that small mean size and slow fall speed both contribute to the strong cooling rates. Simulations entirely without graupel are initially weaker, because of limited contributions from cooling by melting of the slowly falling snow. However, over the next hour increased rates of melting snow result in an increasingly more intense system with new bowing. Results of the study indicate that the development of a bow echo is highly sensitive to microphysical processes, which presents a challenge to the prediction of these severe weather phenomena.

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.

Peering into the internal structure of cold pools and their interactions with a dense observational network

2021 ◽  
Author(s):  
Cathy Hohenegger ◽  
Jaemyeong Seo ◽  
Hannes Nevermann ◽  
Bastian Kirsch ◽  
Nima Shokri ◽  
...  
Keyword(s):  
Internal Structure ◽  
Land Surface ◽  
Surface Fluxes ◽  
Cold Pool ◽  
Convective Clouds ◽  
Cold Pools ◽  
Soil Sensors ◽  
Modelling Studies ◽  
Surface Network ◽  
Shed Light

<p>Melting and evaporation of hydrometeors in and below convective clouds generates cold, dense air that falls through the atmospheric column and spreads at the surface like a density current, the cold pool. In modelling studies, the importance of cold pools in controlling the lifecycle of convection has often been emphasized, being through their organization of the cloud field or through their sheer deepening of the convection. Larger, longer-lived cold pools benefit convection, but little is actually known on the size and internal structure of cold pools from observations as the majority of cold pools are too small to be captured by the operational surface network.  One aim of the field campaign FESSTVaL was to peer into the internal structure of cold pools and their interactions with the underlying land surface by deploying a dense network of surface observations. This network consisted of 80 self-designed cold pool loggers, 19 weather stations and 83 soil sensors deployed in an area of 15 km around Lindenberg. FESSTVaL took place from 17 May to 27 August 2021.</p> <p>In principle, cold pool characteristics are affected both by the atmospheric state, which fuels cold pools through melting and evaporation of hydrometeors, and the land surface, which acts to destroy cold pools through friction and warming by surface fluxes. In this talk, the measurements collected during FESSTVaL will be used to shed light on these interactions.  We are particularly interested to assess how homogeneous the internal structure of cold pools is and whether heterogeneities of the land surface imprint themselves on this internal structure. The results will be compared to available model simulations.</p>

FESST@HH 2020: Ein dichtes Messnetz als Lupe für die Struktur konvektiver Cold Pools

2021 ◽  
Author(s):  
Bastian Kirsch ◽  
Cathy Hohenegger ◽  
Daniel Klocke ◽  
Felix Ament
Keyword(s):  
Cold Pool ◽  
Cold Pools ◽  
O 10

<p>Cold Pools sind mesoskalige Gebiete kalter und dichter Luftmassen, die durch Verdunstung von Hydrometeoren unterhalb regnender Wolken entstehen. Während die kalte Luft absinkt und sich als Dichteströmung an der Erdoberfläche ausbreitet, löst sie durch Hebung an ihrer Vorderseite häufig neue Konvektion aus oder forciert den Übergang von flacher zu tiefer Konvektion. Viele modellbasierte Arbeiten belegen die Bedeutung von Cold Pools für die Organisation von Konvektion. Operationelle Messnetze mit einer typischen Maschenweite von 25 km hingegen sind blind für sub-mesoskalige (O(100) m — O(10) km) Prozesse wie Cold Pools und erlauben somit weder die Untersuchung noch die Validierung ihrer raum-zeitlichen Struktur.</p> <p>Im Rahmen der Messkampagne FESST@HH wurde von Juni bis August 2020 im Großraum Hamburg (50 km × 35 km) ein dichtes Netz bestehend aus 103 meteorologischen Messstationen betrieben. Das Rückgrat des Messnetzes bildeten 82 eigens für diesen Zweck entwickelte und gebaute APOLLO-Stationen (Autonomous cold POoL LOgger), die Lufttemperatur und -druck mit trägheitsarmen Sensoren in sekündlicher Auflösung messen. Das Netzwerk wurde mit 21 Wetterstationen ergänzt, die zusätzlich Luftfeuchte, Windgeschwindigkeit und Niederschlag in 10-sekündiger Auflösung aufzeichnen und auf kommerziellen Sensoren basieren. Ein besonderes Merkmal von FESST@HH ist, dass die Durchführung der Kampagne während der COVID19-Pandemie nur durch eine große Zahl Freiwilliger ermöglicht wurde, die kurzfristig Messstandorte bereitgestellt und die Betreuung der Instrumente unterstützt haben.</p> <p>Wir präsentieren die neuartigen Messinstrumente und den Datensatz der FESST@HH-Kampagne (DOI: 10.25592/UHHFDM.8966). Ein Fallbeispiel zeigt, dass das dichte Messnetz in der Lage ist sowohl die horizontale Heterogenität des Temperaturfeldes innerhalb eines Cold Pools als auch seine Größe und Ausbreitungsgeschwindigkeit während verschiedener Phasen des Lebenszyklus abzubilden. Darüber hinaus erlauben die Messungen einen neuen Blick auf weitere Quellen sub-mesoskaliger Variabilität wie die nächtliche städtische Wärmeinsel und die Variation turbulenter Temperaturfluktuationen als Ausdruck charakteristischer Standorteigenschaften.</p>

scholarly journals Evaluation of Forecasts of a Convectively Generated Bore Using an Intensively Observed Case Study from PECAN

2018 ◽  
Vol 146 (9) ◽  
pp. 3097-3122 ◽  
Author(s):  
Aaron Johnson ◽  
Xuguang Wang ◽  
Kevin R. Haghi ◽  
David B. Parsons
Keyword(s):  
Cross Sections ◽  
Turbulent Mixing ◽  
Weather Research And Forecasting ◽  
Cold Pool ◽  
Model Errors ◽  
Near Surface ◽  
Cold Pools ◽  
Convection Model

Abstract This paper presents a case study from an intensive observing period (IOP) during the Plains Elevated Convection at Night (PECAN) field experiment that was focused on a bore generated by nocturnal convection. Observations from PECAN IOP 25 on 11 July 2015 are used to evaluate the performance of high-resolution Weather Research and Forecasting Model forecasts, initialized using the Gridpoint Statistical Interpolation (GSI)-based ensemble Kalman filter. The focus is on understanding model errors and sensitivities in order to guide forecast improvements for bores associated with nocturnal convection. Model simulations of the bore amplitude are compared against eight retrieved vertical cross sections through the bore during the IOP. Sensitivities of forecasts to microphysics and planetary boundary layer (PBL) parameterizations are also investigated. Forecasts initialized before the bore pulls away from the convection show a more realistic bore than forecasts initialized later from analyses of the bore itself, in part due to the smoothing of the existing bore in the ensemble mean. Experiments show that the different microphysics schemes impact the quality of the simulations with unrealistically weak cold pools and bores with the Thompson and Morrison microphysics schemes, cold pools too strong with the WDM6 and more accurate with the WSM6 schemes. Most PBL schemes produced a realistic bore response to the cold pool, with the exception of the Mellor–Yamada–Nakanishi–Niino (MYNN) scheme, which creates too much turbulent mixing atop the bore. A new method of objectively estimating the depth of the near-surface stable layer corresponding to a simple two-layer model is also introduced, and the impacts of turbulent mixing on this estimate are discussed.

scholarly journals A numerical study of convection in rainbands of Typhoon Morakot (2009) with extreme rainfall: roles of pressure perturbations with low-level wind maxima

2015 ◽  
Vol 15 (6) ◽  
pp. 8479-8523
Author(s):  
C.-C. Wang ◽  
H.-C. Kuo ◽  
R. H. Johnson ◽  
C.-Y. Lee ◽  
S.-Y. Huang ◽  
...  
Keyword(s):  
Wind Shear ◽  
Vertical Wind Shear ◽  
Extreme Rainfall ◽  
Vertical Wind ◽  
Cold Pool ◽  
Rear Side ◽  
Gradient Force ◽  
Typhoon Morakot ◽  
Low Level ◽  
Pressure Perturbations

Abstract. This paper investigates the formation and evolution of deep convection inside the east–west oriented rainbands associated with a low-level jet (LLJ) in Typhoon Morakot (2009). With typhoon center to the northwest of Taiwan, the westerly LLJ was resulted from the interaction of typhoon circulation with the southwest monsoon flow, which supplied the water vapor for the extreme rainfall (of ~1000 mm) over southwestern Taiwan. The Cloud-Resolving Storm Simulator with 1 km grid spacing was used to simulate the event, and it successfully reproduced the slow-moving rainbands, the embedded cells, and the dynamics of merger and back-building (BB) on 8 August as observed. Our model results suggest that the intense convection interacted strongly with the westerly LLJ that provided reversed vertical wind shear below and above the jet core. Inside mature cells, significant dynamical pressure perturbations (pd') are induced with positive (negative) pd' at the western (eastern) flank of the updraft near the surface and a reversed pattern aloft (>2 km). This configuration produced an upward directed pressure gradient force (PGF) to the rear side and favors new development to the west, which further leads to cell merger as the mature cells slowdown in eastward propagation. The strong updrafts also acted to elevate the jet and enhance the local vertical wind shear at the rear flank. Additional analysis reveals that the upward PGF there is resulted mainly by the shearing effect but also by the extension of upward acceleration at low levels. In the horizontal, the upstream-directed PGF induced by the rear-side positive pd' near the surface is much smaller, but can provide additional convergence for BB development upstream. Finally, the cold-pool mechanism for BB appears to be not important in the Morakot case, as the conditions for strong evaporation in downdrafts do not exist.

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