scholarly journals Cold Smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (20) ◽  
pp. 11513-11520 ◽  
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. The first observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler lidar and additional in situ measurements, we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional lidar and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

scholarly journals Cold smoke: smoke-induced density currents cause unexpected smoke transport near large wildfires

2015 ◽  
Vol 15 (13) ◽  
pp. 17945-17966
Author(s):  
N. P. Lareau ◽  
C. B. Clements
Keyword(s):  
Boundary Layer ◽  
Propagation Speed ◽  
Current Speed ◽  
Solar Heating ◽  
Density Current ◽  
Doppler Lidar ◽  
Density Currents ◽  
Smoke Transport ◽  
Smoke Layer

Abstract. First observations of smoke-induced density currents originating from large wildfires are presented. Using a novel mobile Doppler LiDAR and additional in situ measurements we document a deep (~ 2 km) smoke-filled density current that propagates more than 25 km at speeds up to 4.5 m s−1 near a large forest fire in northern California. Based on these observations we show that the dynamics governing the spread of the smoke layer result from differential solar heating between the smoke-filled and smoke-free portions of the atmospheric boundary layer. A calculation of the theoretical density current speed agrees well with the observed propagation speed. Additional LiDAR and photographic documentation of other smoke-filled density currents demonstrate that these previously unknown phenomena are relatively common near large wildfires and can cause severe and unexpected smoke inundation of populated areas.

Mobile Integrated Profiler System (MIPS) Observations of Low-Level Convergent Boundaries during IHOP

2006 ◽  
Vol 134 (1) ◽  
pp. 92-112 ◽  
Author(s):  
Haldun Karan ◽  
Kevin Knupp
Keyword(s):  
Boundary Layer ◽  
Cold Front ◽  
The Other ◽  
Density Current ◽  
Wind Profiler ◽  
Density Currents ◽  
General Increase ◽  
Convective Boundary ◽  
Convective Initiation ◽  
Slow Moving

Abstract Characteristics of convergent boundary zones (CBZs) sampled by the Mobile Integrated Profiling System (MIPS) during the 2002 International H2O Project (IHOP_2002) are presented. The MIPS sensors (915-MHz wind profiler, 12-channel microwave profiling radiometer, ceilometer, and surface instrumentation) provide very fine temporal kinematic and thermodynamic profiles of the atmospheric boundary layer and CBZ properties, including enhanced 915-MHz backscatter within the CBZ updraft (equivalent to the radar fine line), a general increase in integrated water vapor within the updrafts of the CBZ, an increase in the convective boundary layer (CBL) depth, and changes in ceilometer backscatter that are typically coincident with arrival of cooler, moister air (the case for density current CBZ). Three contrasting CBZs are analyzed. Convective initiation was associated with a slow-moving dryline as it passed over the MIPS on 19 June. Updrafts up to 6 m s−1 were measured, and the CBL attained its greatest depth within the CBZ. The CBZ in the other two cases were quite similar to density currents. The retrograding dryline of 18 June produced an enhancement in preexisting convection within 30 km of the MIPS. On 24 May, a shallow cold front, about 800 m deep, was sampled.

scholarly journals A Method for Estimating the Turbulent Kinetic Energy Dissipation Rate from a Vertically Pointing Doppler Lidar, and Independent Evaluation from Balloon-Borne In Situ Measurements

2010 ◽  
Vol 27 (10) ◽  
pp. 1652-1664 ◽  
Author(s):  
Ewan J. O’Connor ◽  
Anthony J. Illingworth ◽  
Ian M. Brooks ◽  
Christopher D. Westbrook ◽  
Robin J. Hogan ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Convective Boundary Layer ◽  
Sonic Anemometer ◽  
Horizontal Wind ◽  
Inertial Subrange ◽  
Doppler Velocity ◽  
Doppler Lidar ◽  
Convective Boundary ◽  
Dissipation Rates

Abstract A method of estimating dissipation rates from a vertically pointing Doppler lidar with high temporal and spatial resolution has been evaluated by comparison with independent measurements derived from a balloon-borne sonic anemometer. This method utilizes the variance of the mean Doppler velocity from a number of sequential samples and requires an estimate of the horizontal wind speed. The noise contribution to the variance can be estimated from the observed signal-to-noise ratio and removed where appropriate. The relative size of the noise variance to the observed variance provides a measure of the confidence in the retrieval. Comparison with in situ dissipation rates derived from the balloon-borne sonic anemometer reveal that this particular Doppler lidar is capable of retrieving dissipation rates over a range of at least three orders of magnitude. This method is most suitable for retrieval of dissipation rates within the convective well-mixed boundary layer where the scales of motion that the Doppler lidar probes remain well within the inertial subrange. Caution must be applied when estimating dissipation rates in more quiescent conditions. For the particular Doppler lidar described here, the selection of suitably short integration times will permit this method to be applicable in such situations but at the expense of accuracy in the Doppler velocity estimates. The two case studies presented here suggest that, with profiles every 4 s, reliable estimates of ε can be derived to within at least an order of magnitude throughout almost all of the lowest 2 km and, in the convective boundary layer, to within 50%. Increasing the integration time for individual profiles to 30 s can improve the accuracy substantially but potentially confines retrievals to within the convective boundary layer. Therefore, optimization of certain instrument parameters may be required for specific implementations.

scholarly journals Observations of Shoaling Density Current Regime Changes in Internal Wave Interactions

2020 ◽  
Vol 50 (6) ◽  
pp. 1733-1751
Author(s):  
Aviv Solodoch ◽  
Jeroen M. Molemaker ◽  
Kaushik Srinivasan ◽  
Maristella Berta ◽  
Louis Marie ◽  
...  
Keyword(s):  
Propagation Speed ◽  
Finite Amplitude ◽  
State Density ◽  
Critical Regime ◽  
Density Current ◽  
Coastal Current ◽  
Density Currents ◽  
Ambient Conditions ◽  
Plume Front ◽  
Small Width

AbstractWe present in situ and remote observations of a Mississippi plume front in the Louisiana Bight. The plume propagated freely across the bight, rather than as a coastal current. The observed cross-front circulation pattern is typical of density currents, as are the small width (≈100 m) of the plume front and the presence of surface frontal convergence. A comparison of observations with stratified density current theory is conducted. Additionally, subcritical to supercritical transitions of frontal propagation speed relative to internal gravity wave (IGW) speed are demonstrated to occur. That is in part due to IGW speed reduction with decrease in seabed depth during the frontal propagation toward the shore. Theoretical steady-state density current propagation speed is in good agreement with the observations in the critical and supercritical regimes but not in the inherently unsteady subcritical regime. The latter may be due to interaction of IGW with the front, an effect previously demonstrated only in laboratory and numerical experiments. In the critical regime, finite-amplitude IGWs form and remain locked to the front. A critical to supercritical transition eventually occurs as the ambient conditions change during frontal propagation, after which IGWs are not supported at the front. The subcritical (critical) to critical (supercritical) transition is related to Froude number ahead (under) the front, consistently with theory. Finally, we find that the front-locked IGW (critical) regime is itself dependent on significant nonlinear speed enhancement of the IGW by their growth to finite amplitude at the front.

Density currents or density wedges: boundary-layer influence and control methods

1987 ◽  
Vol 177 ◽  
pp. 187-206 ◽  
Author(s):  
Gerhard H. Jirka ◽  
Masamitsu Arita
Keyword(s):  
Boundary Layer ◽  
State Density ◽  
Density Current ◽  
Local Form ◽  
Density Currents ◽  
Viscous Effects ◽  
Long Distance ◽  
Ambient Flow ◽  
Wake Zone ◽  
And Control

Density currents and density wedges are two observed manifestations of interactions between an ambient flow and a horizontal buoyant intrusion. In a density current the buoyant pressure force is primarily balanced by the local form drag of the current head which has a blunt shape and abrupt depth change. In a density wedge a distributed interfacial drag is the primary balancing force, leading to a stretched-out shape and long-distance intrusions. A perturbation analysis of the approach flow to the inclined front of a density current shows that slight momentum changes caused by viscous effects in the ambient flow determine which of these two flow types is established. In a uniform ambient channel flow, any momentum deficit relative to the inviscid case will lead to a local flattening of the front and ultimate breakdown into a density wedge. On the other hand, a momentum surplus will support a steady-state density current. Several exploratory experiments on control of the ambient boundary layer through local non-uniformities were performed with the objective of achieving stable density-current forms with limited intrusion lengths. These methods include a small step, a barrier and suction and are applied for intrusions at either the bottom or surface of an ambient water flow. In all cases, good agreement is found with the force balances predicted by Benjamin's (1968) theory and its extension by Britter & Simpson (1978) which accounts for entrainment in the wake zone of the head.

scholarly journals Near-Surface Density Currents Observed in the Southeast Pacific Stratocumulus-Topped Marine Boundary Layer*

2015 ◽  
Vol 143 (9) ◽  
pp. 3532-3555 ◽  
Author(s):  
Matt C. Wilbanks ◽  
Sandra E. Yuter ◽  
Simon P. de Szoeke ◽  
W. Alan Brewer ◽  
Matthew A. Miller ◽  
...  
Keyword(s):  
Marine Boundary Layer ◽  
Propagation Speed ◽  
Sensor Data ◽  
Density Current ◽  
Density Currents ◽  
Near Surface ◽  
Cold Pools ◽  
Southeast Pacific ◽  
The Mean ◽  
Rain Rates

Abstract Density currents (i.e., cold pools or outflows) beneath marine stratocumulus clouds are characterized using 30 days of ship-based observations obtained during the 2008 Variability of American Monsoon Systems (VAMOS) Ocean–Cloud–Atmosphere–Land Study Regional Experiment (VOCALS-REx) in the southeast Pacific. An air density increase criterion applied to the Improved Meteorological (IMET) sensor data identified 71 density current front, core (peak density), and tail (dissipating) zones. The similarity in speeds of the mean density current propagation speed (1.8 m s−1) and the mean cloud-level advection relative to the surface layer wind (1.9 m s−1) allowed drizzle cells to deposit elongated density currents in their wakes. Scanning Doppler lidar captured prefrontal updrafts with a mean intensity of 0.91 m s−1 and an average vertical extent of 800 m. Updrafts were often surmounted by low-lying shelf clouds not connected to the overlying stratocumulus cloud. The observed density currents were 5–10 times thinner and weaker than typical continental thunderstorm cold pools. Nearly 90% of density currents were identified when C-band radar estimated areal average rain rates exceeded 1 mm day−1 over a 30-km diameter. Rather than peaking when rain rates were highest overnight, density current occurrence peaks between 0600 and 0800 local solar time when enhanced local drizzle co-occurred with shallow subcloud dry and stable layers. The dry layers may have contributed to density current formation by enhancing subcloud evaporation of drizzle. Density currents preferentially occurred in a large region of predominantly open cells but also occurred in regions of closed cells.

Simulated Density Currents beneath Embedded Stratified Layers

2012 ◽  
Vol 69 (7) ◽  
pp. 2192-2200 ◽  
Author(s):  
Robert B. Seigel ◽  
Susan C. van den Heever
Keyword(s):  
Propagation Speed ◽  
Cold Pool ◽  
Density Current ◽  
Density Currents ◽  
Stable Layer ◽  
Environmental Regimes ◽  
Weather Events ◽  
Horizontal Pressure ◽  
Numerical Model Experiments ◽  
Neutrally Stratified

Abstract The goal of this research is to investigate the impacts of a stably stratified layer embedded within a neutrally stratified environment on the behavior of density currents in an effort to extend the environmental regimes examined by Liu and Moncrieff. Such environments frequently support severe weather events. To accomplish this goal, nonhydrostatic numerical model experiments are performed in which the strength and height of the embedded stably stratified layer within a neutrally stratified environment are varied. The 1-km-deep stable layer base is varied between 1, 2, and 3 km AGL. Additionally, the strength of the stable layer is systematically varied between Brunt–Väisälä frequencies of 0.006, 0.012, and 0.018 s−1, following the methodology of Liu and Moncrieff. The model and grid setup are also similar to that of Liu and Moncrieff, utilizing the Arakawa C grid, leapfrog advection, a Robert–Asselin filter, and grid spacing of 100 and 50 m in the horizontal and vertical directions, respectively. Results show that the height of the density current decreases and the propagation speed increases with stronger and lower stable layers, provided that the stable layer is sufficiently thin so as to not act as a gravity wave ducting layer. As the strength of the stable layer increases and the height of this layer decreases, the horizontal pressure gradient driving the density current increases, resulting in faster propagation speeds. Such results have implications for cold pool propagation into more stable environments.

scholarly journals Measurements of Boundary Layer Profiles in an Urban Environment

2006 ◽  
Vol 45 (6) ◽  
pp. 821-837 ◽  
Author(s):  
Rod Frehlich ◽  
Yannick Meillier ◽  
Michael L. Jensen ◽  
Ben Balsley ◽  
Robert Sharman
Keyword(s):  
Boundary Layer ◽  
Urban Environment ◽  
Small Scale ◽  
Doppler Lidar ◽  
Stable Boundary Layers ◽  
Low Wind Speed ◽  
Velocity Statistics ◽  
In Situ Sensors ◽  
Scale Turbulence

Abstract Boundary layer profiles of mean temperature, velocity, and small-scale turbulence from in situ sensors, Doppler lidar, sodar, and rawinsondes are intercompared for an urban environment. A new Doppler lidar algorithm is presented to produce high-resolution profiles of small-scale velocity statistics. The lidar-derived profiles are robust and accurate even for challenging conditions such as stable boundary layers with a low-level jet, low turbulence, and low wind speed. Similar results are expected for other locations and convective conditions.

scholarly journals Measurements of Boundary Layer Profiles with In Situ Sensors and Doppler Lidar

2008 ◽  
Vol 25 (8) ◽  
pp. 1328-1340 ◽  
Author(s):  
Rod Frehlich ◽  
Yannick Meillier ◽  
Michael L. Jensen
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Energy Dissipation Rate ◽  
High Rate ◽  
Doppler Lidar ◽  
Stratified Turbulence ◽  
In Situ Sensors ◽  
The Impact ◽  
Turbulence Parameters

Abstract A new in situ measurement system and lidar processing algorithms were developed for improved measurements of boundary layer profiles. The first comparisons of simultaneous Doppler lidar–derived profiles of the key turbulence statistics of the two orthogonal horizontal velocity components (longitudinal and transverse) are presented. The instrument requirements for accurate observations of stably stratified turbulence were determined. A region of stably stratified low turbulence with constant gradients of temperature and velocity was observed above the nocturnal boundary layer using high-rate sensors. The important turbulence parameters were estimated, and turbulence spectra were consistent with new theoretical descriptions of stratified turbulence. The impact of removing the larger-scale velocity features in Doppler lidar estimates of turbulent velocity variance and length scales was investigated. The Doppler lidar–derived estimates of energy dissipation rate ε were found to be insensitive to spatial filtering of the large-scale atmospheric processes. The in situ and lidar-derived profiles were compared for the stable boundary layer in a suburban environment.

scholarly journals The Rapid Deployments to Wildfires Experiment (RaDFIRE): Observations from the Fire Zone

2018 ◽  
Vol 99 (12) ◽  
pp. 2539-2559 ◽  
Author(s):  
Craig B. Clements ◽  
Neil P. Lareau ◽  
David E. Kingsmill ◽  
Carrie L. Bowers ◽  
Chris P. Camacho ◽  
...  
Keyword(s):  
Doppler Radar ◽  
Western United States ◽  
Doppler Lidar ◽  
Density Currents ◽  
Field Campaign ◽  
Plume Dynamics ◽  
Meteorological Field ◽  
In Situ Observations ◽  
Airborne Doppler Radar

AbstractThe Rapid Deployments to Wildfires Experiment (RaDFIRE) was a meteorological field campaign aimed at observing fire–atmosphere interactions during active wildfires. Using a rapidly deployable scanning Doppler lidar, airborne Doppler radar, and a suite of other instruments, the field campaign sampled 21 wildfires from 2013 to 2016 in the western United States. Observations include rotating convective plumes, plume interactions with stable layers and multilayered smoke detrainment, convective plume entrainment processes, smoke-induced density currents, and aircraft in situ observations of developing pyrocumulus. Collectively, these RaDFIRE observations highlight the range of meteorological phenomena associated with wildfires, especially plume dynamics, and will provide a valuable dataset for the modeling community.

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