Strong Stable Layer Research Articles

Airstream Association of Large Boundary Layer Rolls during Extratropical Transition of Post-Tropical Cyclone Sandy (2012)

Better understanding of roll vortices that often occur in the tropical cyclone (TC) boundary layer is required to improve forecasts of TC intensification and the granularity of damaging surface winds. It is especially important to characterize rolls over a wide variety of TCs, their environments, and TC development phases. Boundary layer rolls have been observed in TCs since 1998, but only recently in a TC during its extratropical transition phase. The work reported herein is the first to analyze how boundary layer rolls are distributed among the extratropical features of a transitioning TC. To this end, routine and special operational observations recorded during landfalling Post-tropical Cyclone Sandy (2012) were leveraged, including radar, surface, rawinsonde, and aircraft reconnaissance observations. Large rolls occurred in cold airstreams, both in the cold conveyor belt within the northwestern storm quadrant and in the secluding airstream within the northeastern quadrant, but roll presence was much diminished within the intervening warm sector. The large size of the rolls and their confinement to cold airstreams is attributed to an optimum inflow layer depth, which is deep enough below a strong stable layer to accommodate deep and strong positive radial wind shear to promote roll growth, yet not so deep as to limit radial wind shear magnitude, as occurred in the warm sector.

Analysis of Back-Building Convection in Simulations with a Strong Low-Level Stable Layer

Abstract In a mesoscale convective system (MCS), convection that redevelops over (i.e., back-builds), and/or repeatedly passes over (i.e., trains) a region for an extended period of time can contribute to extreme rainfall and flash flooding. Past studies have indicated that both mesoscale ascent and lifting of the inflow layer by a cold pool or bore are important when this back-building/training convection is displaced from the leading line [sometimes called rearward off-boundary development (ROD)]. However, Plains Elevated Convection At Night (PECAN) field campaign observations suggest that the stability of the nocturnal boundary layer is highly variable and some MCSs with ROD have only a weak surface cold pool. Numerical simulations presented in this study suggest that in an environment with strong boundary layer stability, ROD can be supported by mechanisms other than those mentioned above. Simulations were initialized using a sounding from ahead of a PECAN MCS with a strong stable layer and ROD, and the three-dimensional simulation produced an MCS similar to that observed despite the homogeneous initial conditions. Some of the findings presented herein challenge existing understanding of nocturnal MCSs, and especially how downdrafts interact with a stable boundary layer. Notably, downdrafts can reach the surface, and different regions of the MCS may have different propagation mechanisms and different relevant inflow layers. Unlike previous studies of ROD, parcel lifting may be supported by an intrusion (an elevated layer of downdraft air) modified by the three-dimensional vertical wind shear.

Kinematic and Moisture Characteristics of a Nonprecipitating Cold Front Observed during IHOP. Part I: Across-Front Structures

Abstract A wide array of ground-based and airborne instrumentation is used to examine the kinematic and moisture characteristics of a nonprecipitating cold front observed in west-central Kansas on 10 June 2002 during the International H2O Project (IHOP). This study, the first of two parts, is focused on describing structures in the across-front dimension. Coarsely resolved observations from the operational network and dropsondes deployed over a 200-km distance centered on the front are combined with higher-resolution observations from in situ sensors, Doppler radars, a microwave radiometer, and a differential absorption lidar that were collected across a ∼40-km swath that straddled a ∼100-km segment of the front. The northeast–southwest-oriented cold front moved toward the southeast at ∼8–10 m s−1 during the morning hours, but its motion slowed to less than 1 m s−1 in the afternoon. In the early afternoon, the cold front separated cool air with a northerly component flow of 2–4 m s−1 from a 10-km-wide band of hot, dry air with 5 m s−1 winds out of the south-southwest. The average updraft at the frontal interface was ∼0.5 m s−1 and slightly tilted back toward the cool air. A dryline was located to the southeast of the front, separating the hot, dry air mass from a warm, moist air mass composed of 10 m s−1 southerly winds. Later in the afternoon, the warm, moister air moved farther to the northwest, approaching the cold front. The dryline was still well observed in the southwestern part of the observational domain while it vanished almost completely in the northeastern part. Low-level convergence (∼1 × 10−3 s−1), vertical vorticity (∼0.5 × 10−3 s−1), and vertical velocity (∼1 m s−1) increased. The strong stable layer located at ∼2.0–2.5 km MSL weakened in the course of the afternoon, providing a basis for the development of isolated thunderstorms. The applicability of gravity current theory to the cold front was studied. There was evidence of certain gravity current characteristics, such as Froude numbers between 0.7 and 1.4, a pronounced feeder flow toward the leading edge, and a rotor circulation. Other characteristics, such as a sharp change in pressure and lobe and cleft structures, remain uncertain due to the temporally and spatially variable nature of the phenomenon and the coarse resolution of the measurements.

Formation and Maintenance Mechanisms of the Stable Layer over the Po Valley during MAP IOP-8

Abstract During intensive observation period 8 (IOP-8) of the Mesoscale Alpine Program, a strong stable layer formed over Italy’s Po Valley and the northern Ligurian Sea. This stable layer has been shown in previous research to be important for the formation of convection over the Ligurian Sea and the lack thereof over the Po Valley and southern slopes of the Alps. The purpose of this study is to investigate the mechanisms that acted to form and maintain the stable layer during IOP-8. This aim is accomplished through inspection of observed data as well as numerical simulations and sensitivity experiments. Observations and reanalysis data show that starting on 17 October 1999, a relatively cool, stable air mass was advected around the eastern side of the Alps into the lower atmosphere of the Po Valley. Both observational data and model output show this air mass as being blocked as it encountered the western Alps, thus resulting in an accumulation of cool, stable air at low levels in the Po Valley during the ensuing 60 h. When southerly flow approached northern Italy beginning on 20 October 1999, both the western Alps and the northern Alps appeared to help retain the low-level, cool, stable air over the Po Valley. A trajectory and sounding analysis shows that warmer, less stable air originating from over the southern Mediterranean Sea was advected atop the low-lying stable layer within the Po Valley. It is hypothesized that this differential advection, as well as blocking by the western and northern flanks of the Alps, were responsible for the longevity of the stable layer. A series of numerical simulations and sensitivity experiments were performed to test the above hypotheses. These tests support the hypotheses. Other mechanisms were also considered, including blocking of solar radiation by clouds, friction, and evaporative cooling. These simulations revealed that all three processes were critical for the longevity of the stable layer and point to the importance of accurate model representation of subgrid-scale processes.

Formation Mechanisms for Convection over the Ligurian Sea during MAP IOP-8

Abstract The dynamical impacts of an unusually strong stable layer that developed over the Po Valley and northern Ligurian Sea during Mesoscale Alpine Program (MAP) intensive observation period 8 (IOP-8) on the formation of convection over the Ligurian Sea are explored. Based on numerically simulated equivalent potential temperature, wind vectors, and by a trajectory analysis of parcels both beneath and above the stable layer, it is shown that the stable layer behaved as a material surface or “effective mountain” to the airstreams impinging on it from the south. Additional analyses show that the leading edge of the stable layer was collocated with maxima in upward motion and a strong positive moisture flux. Hence, it was further argued and demonstrated through inspection of soundings upstream of the cold dome and trajectory analyses that lifting by the stable layer enhanced convective activities over the Ligurian Sea. Finally, processes contributing to the maintenance of the stable layer during IOP-8 were explored. It was found that the differential advection of a warm, less stable air mass on top of a cooler, more stable air mass helped maintain the stable layer. The Ligurian Apennines made a secondary contribution to the stagnation of the cool air in the Po Valley by partially blocking this air mass from exiting the valley to the south.

Atmospheric boundary layer during ARMEX-2002 at stationary positions - comparative study

A study has been carried out to explore the salient features of the Atmospheric Boundary Layer (ABL) over the eastern Arabian Sea regions during the summer monsoon season of 2002. For this purpose the aerological observations and surface meteorological observations collected onboard ORV Sagar Kanya during the Phase-I of ARMEX-2002 for the two stationary positions (30 June-10 July and 22 July – 5 August 2002) have been considered. Utilizing these observations the vertical structure of the Atmospheric Boundary Layer and the temporal variation of different surface parameters has been investigated. The results showed prominent diurnal variation in different surface meteorological parameters. The study also revealed that there is no influence of the offshore trough on the vertical structure of the Atmospheric Boundary Layer. The ABL showed multilayered structure with shallow sub cloud layer, deep cloud layer, weak stable layer and the ABL extending up to higher levels in the presence of the cyclonic circulation (2-4 August 2002). Whereas the ABL is associated with deep sub cloud layer, shallow cloud layer, strong stable layer (inversion layer) and shallow ABL depth under the influence of weak offshore trough observed along the west coast of India.

ドップラーレーダーとACARSを用いた低層内部重力波の事例研究

An observational analysis of the low-level internal gravity waves over the Kanto Plain in Japan on 15 January 1998 was performed. The data from the Doppler radar for Airport Weather (DRAW), and aircraft soundings of wind and temperature (ACARS), provided unique observations with high spatial and temporal resolutions, and were used to analyze detailed wave structures and environmental conditions, especially wave duct structures. According to the Doppler data analysis, the horizontal wavelength was approximately 6.5 km, the ground relative phase speed of the waves was approximately 4 m/s, and the direction of propagation was toward 50° (NE). The wave region spread 80 km-100 km horizontally. Doppler data analysis revealed that the waves maintained their coherent structures, lasted for more than one hour, and propagated horizontally for long distances as an isolated wave packet. The profiles from ACARS revealed the existence of a strong stable layer with vertical wind shear. Above the stable layer, there was a near-neutral layer with a critical level, around which the Richardson number was close to 0.25. These results indicate that the waves were trapped in the duct and propagated horizontally without a large amount of energy loss in the absence of a forcing mechanism. The observed horizontal wavelength, profile of horizontal velocity amplitudes, and surface pressure perturbations are in agreement with our estimations derived from a linear theory on neutral modes. Power spectrum and bandpass filtering analyses conducted on surface pressure show that the frequency and the phase of pressure perturbations are consistent with theoretical relations with internal gravity waves, suggesting that they were caused by the passage of the internal gravity waves.

Buoyancy reversal and cloud-top entrainment instability

In 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system. We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy-reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh-Bénard convection in the lower layer, but only slightly distorts the interface. For D > 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 < D < 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 < D < 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short-wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.

Buoyancy reversal and cloud‐top entrainment instability

AbstractIn 1980, Randall suggested that cloud top entrainment instability, a runaway positive feedback between mixing, evaporative cooling and entrainment, may cause the transition from the subtropical stratocumulus layers, frequently observed over the cold water of the eastern oceans, to tradewind cumulus. Predicting how the mean position of this transition, and the associated large change in albedo, would vary with, and feed back on, a change in climate is important for any credible model of the climate system.We have used numerical and laboratory simulations, as well as field observations, to study entrainment in buoyancy‐reversing systems. We consider a system in which an upper fluid stably overlies a second lower fluid; but some mixtures are less buoyant than either fluid. the laboratory results show that buoyancy reversal does not cause runaway ‘explosive entrainment’ unless D (the ratio of the density difference between the most cooled mixed parcel and the lower fluid to the density difference across the unmixed layers, a measure of CEI potential) exceeds 1.3, which is far greater than the range of D observed in stratocumulus. For D below this threshold, buoyancy reversal leads to weak circulations rather reminiscent of Rayleigh‐Bénard convection in the lower layer, but only slightly distorts the interface. For D &gt; 1.3 an entrainment tongue of upper fluid penetrates a sizable fraction of the depth of the lower layer and entrainment is explosively rapid. Numerical experiments suggest that even when 0 &lt; D &lt; 1.3, entrainment can break up idealized subtropical marine stratocumulus within about an hour if radiation and surface fluxes are removed, but comparison of numerical simulations with the laboratory experiments suggests that the numerically simulated entrainment rate may be substantially too large in this regime owing to the inability to accurately produce and resolve the entraining eddies. Our observations of stratocumulus from FIRE typically had a strong stable layer above the cloud top inversions, leading to some ambiguity in defining D, but clearly show that stable stratocumulus layers often have 0 &lt; D &lt; 0.2. We suggest that sudden breakup of stratocumulus due to runaway entrainment is unlikely, but evaporatively enhanced entrainment may aid the transition from stratocumulus to tradewind cumulus in conjunction with some other mechanism, such as short‐wave absorption by liquid water, that can help dynamically decouple the cloud layer from the surface layer.

The Kinematics of Orographic Airflow During Sierra Storms

Abstract Two case studies of the kinematics of the airflow over the Sierra barrier are presented. The observations consisted of rawinsondes and single Doppler RHI and velocity azimuth display (VAD) analysis of PPI scans. The RHI scans were made orthogonal to the nearly two-dimensional Sierra barrier. The cloud in the first case study contained a strong stable layer at 0°C while the second storm was highly unstable. The radar bright band and soundings near the radar indicated that an ∼250 m thick 0°C isothermal layer was present in response to the diabatic process of melting. When the bright band was impinging upon the barrier, the associated 0°C isothermal layer was inferred to expand until it finally reached the ground. At that point it was ∼1 km in depth. Direct thermodynamic soundings am presented for a similar situation which agrees with the modified soundings inferred for this study. The resulting effects on the airflow and precipitation are discussed. The unstable case contained a line of deep conve...

Evolution of the Dynamics and Thermodynamics of a Mesoscale Convective System: A Case Study

Abstract A well-organized severe storm complex on 30 May 1976 developed in the southeastern part of the mesonetwork of the National Severe Storms Laboratory (NSSL) in central Oklahoma and merged, in its mature stage, with a band of thunderstorms along a stationary front. Rawinsonde observations at nine stations within the mesonetwork, supplemented by satellite time lapse images, have been analyzed to depict the kinematic and thermodynamic structure of the storm and its near environment. An objective analysis technique is used to describe the evolving storm structure. Among the significant features the analysis revealed, the low-level horizontal convergence was confined to the southeast prior to the appearance of the first cloud in the satellite images, and a well-defined mixed layer, capped by a strong stable layer, developed with northwest to southeast horizontal temperature and moisture, gradients. The height of the mixed layer increased with time in the dry northwest side, while it decreased in the sou...

A wind tunnel for studying the effects of thermal stratification in the atmosphere

A new wind tunnel designed to study the effects of thermal stratification on flow and diffusion in the atmospheric boundary layer has been constructed. The wind tunnel features three independent temperature systems that control the ambient air temperature (from 12 to 87° C), the temperature profile (gradient up to ∂T ∂z = 25°C m −1 ), and the surface temperature (eight individual floor panels, from 7 to 112°C for each) in the test section. The wind tunnel is also equipped with a velocity profile generating cart, and surface roughness, wind orientation, variable area source, and sampling grid floor panels. Combined, these features can generate a wide range of thermal stratification and other conditions. The effects of neutral, strongly unstable and strongly stable stabilities on the flow were examined. Visualization by smoke tracer showed that in the stable case, the turbulence is damped near the ground giving a laminar-like, wavy streamline, while for the unstable case, large convective eddy motion is observed. The instantaneous w-component turbulent velocity near the ground for the stable case was drastically reduced from the neutral and unstable cases. Simulating a sea breeze configuration, a strong stable layer developed over the sea, and at the shoreline, a mixed (unstable) layer began to grow beneath the stable layer. The observation of a downward flow in the lower layer near the shore was supported by a similar flow pattern in the streamfunction calculation.

Analysis of the Convectively Modified GATE Boundary Layer Usingin situand Acoustic Sounder Data

Abstract A boundary-layer categorization scheme based on GATE acoustic sounder data stratifies surface data and tethered balloon data from the NOAA research ship Oceanographer. The results indicate a clear increase in the sensible heat flux across the sea surface during disturbed conditions (the gravity current and storm wake of the disturbance) but no conclusive differences of latent heat and momentum fluxes. The tethered balloon profiles show a near disappearance of the mixed layer within the gravity current and, in the storm wake, a very shallow and cool mixed layer capped by a strong stable layer relative to the undisturbed category. We calculate the vertical motion due to buoyancy-driven entrainment for a range on entrainment parameters after exploiting the categorized tethered balloon profiles to obtain mean gradients at the top of the mixed layer. Because we and others have observed that the shallow mixed-layer depth remains nearly constant with time in the storm wake, this calculated entrainment-i...