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>

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>

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.

Bow Echo Sensitivity to Ambient Moisture and Cold Pool Strength

2006 ◽  
Vol 134 (3) ◽  
pp. 950-964 ◽  
Author(s):  
Richard P. James ◽  
Paul M. Markowski ◽  
J. Michael Fritsch
Keyword(s):  
Convective Available Potential Energy ◽  
Three Dimensional ◽  
Cold Pool ◽  
Convective System ◽  
Cold Air ◽  
Convective Systems ◽  
Cold Pools ◽  
Bow Echo ◽  
Dry Conditions ◽  
Water Vapor Mixing Ratio

Abstract Bow echo development within quasi-linear convective systems is investigated using a storm-scale numerical model. A strong sensitivity to the ambient water vapor mixing ratio is demonstrated. Relatively dry conditions at low and midlevels favor intense cold-air production and strong cold pool development, leading to upshear-tilted, “slab-like” convection for various magnitudes of convective available potential energy (CAPE) and low-level shear. High relative humidity in the environment tends to reduce the rate of production of cold air, leading to weak cold pools and downshear-tilted convective systems, with primarily cell-scale three-dimensionality in the convective region. At intermediate moisture contents, long-lived, coherent bowing segments are generated within the convective line. In general, the scale of the coherent three-dimensional structures increases with increasing cold pool strength. The bowing lines are characterized in their developing and mature stages by segments of the convective line measuring 15–40 km in length over which the cold pool is much stronger than at other locations along the line. The growth of bow echo structures within a linear convective system appears to depend critically on the local strengthening of the cold pool to the extent that the convection becomes locally upshear tilted. A positive feedback process is thereby initiated, allowing the intensification of the bow echo. If the environment favors an excessively strong cold pool, however, the entire line becomes uniformly upshear tilted relatively quickly, and the along-line heterogeneity of the bowing line is lost.

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 On the Creation and Evolution of Small-Scale Low-Level Vorticity Anomalies during Tropical Cyclogenesis

2019 ◽  
Vol 76 (8) ◽  
pp. 2335-2355 ◽  
Author(s):  
Warren P. Smith ◽  
Melville E. Nicholls
Keyword(s):  
Environmental Parameters ◽  
Cold Pool ◽  
Tropical Cyclogenesis ◽  
Small Scale ◽  
Low Level ◽  
Cold Pools ◽  
Convective Scale ◽  
Vortical Hot Towers ◽  
Formation And Evolution ◽  
Vortex Merger

Abstract Recent numerical modeling and observational studies indicate the importance of vortical hot towers (VHTs) in the transformation of a tropical disturbance to a tropical depression. It has recently been recognized that convective-scale downdraft outflows that form within VHTs also preferentially develop positive vertical vorticity around their edges, which is considerably larger in magnitude than ambient values. During a numerical simulation of tropical cyclogenesis it is found that particularly strong low-level convectively induced vorticity anomalies (LCVAs) occasionally form as convection acts on the enhanced vorticity at the edges of cold pools. These features cycle about the larger-scale circulation and are associated with a coincident pressure depression and low-level wind intensification. The LCVAs studied are considerably deeper than the vorticity produced at the edges of VHT cold pool outflows, and their evolution is associated with persistent convection and vortex merger events that act to sustain them. Herein, we highlight the formation and evolution of two representative LCVAs and discuss the environmental parameters that eventually become favorable for one LCVA to reach the center of a larger-scale circulation as tropical cyclogenesis occurs.

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 Synoptic-Scale Flow and Valley Cold Pool Evolution in the Western United States

2009 ◽  
Vol 24 (6) ◽  
pp. 1625-1643 ◽  
Author(s):  
Heather Dawn Reeves ◽  
David J. Stensrud
Keyword(s):  
United States ◽  
Large Scale ◽  
Numerical Models ◽  
Western United States ◽  
Cold Pool ◽  
Synoptic Scale ◽  
Temperature Changes ◽  
Cold Pools ◽  
Upper Level ◽  
Forecast Problem

Abstract Valley cold pools (VCPs), which are trapped, cold layers of air at the bottoms of basins or valleys, pose a significant problem for forecasters because they can lead to several forms of difficult-to-forecast and hazardous weather such as fog, freezing rain, or poor air quality. Numerical models have historically failed to routinely provide accurate guidance on the formation and demise of VCPs, making the forecast problem more challenging. In some case studies of persistent wintertime VCPs, there is a connection between the movement of upper-level waves and the timing of VCP formation and decay. Herein, a 3-yr climatology of persistent wintertime VCPs for five valleys and basins in the western United States is performed to see how often VCP formation and decay coincides with synoptic-scale (∼200–2000 km) wave motions. Valley cold pools are found to form most frequently as an upper-level ridge approaches the western United States and in response to strong midlevel warming. The VCPs usually last as long as the ridge is over the area and usually only end when a trough, and its associated midlevel cooling, move over the western United States. In fact, VCP strength appears to be almost entirely dictated by midlevel temperature changes, which suggests large-scale forcing is dominant for this type of VCP most of the time.

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