scholarly journals Planetary boundary layer evolution over the Amazon rainforest in episodes of deep moist convection at the Amazon Tall Tower Observatory

2020 ◽  
Vol 20 (1) ◽  
pp. 15-27 ◽  
Author(s):  
Maurício I. Oliveira ◽  
Otávio C. Acevedo ◽  
Matthias Sörgel ◽  
Ernani L. Nascimento ◽  
Antonio O. Manzi ◽  
...  
Keyword(s):  
Heat Flux ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Moist Convection ◽  
Early Afternoon ◽  
Virtual Potential Temperature ◽  
Gust Fronts ◽  
Deep Moist Convection ◽  
Tall Tower ◽  
Gust Front

Abstract. In this study, high-frequency, multilevel measurements, performed from late October to mid-November of 2015 at a 80 m tall tower of the Amazon Tall Tower Observatory (ATTO) project in the central state of Amazonas, Brazil, were used to diagnose the evolution of thermodynamic and kinematic variables as well as scalar fluxes during the passage of outflows generated by deep moist convection (DMC). Outflow associated with DMC activity over or near the tall tower was identified through the analysis of storm echoes in base reflectivity data from an S-band weather radar at Manaus, combined with the detection of gust fronts and cold pools utilizing tower data. Four outflow events were selected, three of which took place during the early evening transition or nighttime hours and one during the early afternoon. Results show that the magnitude of the drop in virtual potential temperature and changes in wind velocity during outflow passages vary according to the type, organization, and life cycle of the convective storm. The nocturnal events had well-defined gust fronts with moderate decreases in virtual potential temperature and increases in wind speed. The early afternoon event lacked a sharp gust front and only a gradual drop in virtual potential temperature was observed, probably because of weak or undeveloped outflow. Sensible heat flux (H) increased at the time of the gust front arrival, which was possibly due to the sinking of colder air. This was followed by a prolonged period of negative H, associated with enhanced nocturnal negative H in the wake of the storms. In turn, increased latent heat flux (LE) was observed following the gust front, owing to drier air coming from the outflow; however, malfunctioning of the moisture sensors during rain precluded a better assessment of this variable. Substantial enhancements of turbulent kinetic energy (TKE) were observed during and after the gust front passage, with values comparable to those measured in grass fire experiments, evidencing the highly turbulent character of convective outflows. The early afternoon event displayed slight decreases in the aforementioned quantities in the passage of the outflow. Finally, a conceptual model of the time evolution of H in nocturnal convective outflows observed at the tower site is presented.

scholarly journals Planetary boundary layer evolution over the Amazon rain forest in episodes of deep moist convection at ATTO

2019 ◽  
Author(s):  
Maurício I. Oliveira ◽  
Otávio C. Acevedo ◽  
Matthias Sörgel ◽  
Ernani L. Nascimento ◽  
Antonio O. Manzi ◽  
...  
Keyword(s):  
Heat Flux ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Moist Convection ◽  
Early Afternoon ◽  
Virtual Potential Temperature ◽  
Gust Fronts ◽  
Deep Moist Convection ◽  
Tall Tower ◽  
Gust Front

Abstract. In this study, high-frequency, multi-level measurements performed from late October to mid-November of 2015 at a 80-m tall tower of the Amazon Tall Tower Observatory (ATTO) project in central Amazonas State, Brazil, were used to diagnose the evolution of thermodynamic and kinematic variables as well as scalar fluxes during the passage of outflows generated by deep moist convection (DMC). Outflow associated with DMC activity over or near the tall tower was identified through the analysis of storm echoes in base reflectivity data from S-band weather radar at Manaus, combined with the detection of gust fronts and cold pools utilizing tower data. Four outflow events were selected, three of which took place during the early evening transition or nighttime hours and one during the early afternoon. Results show that the magnitude of the drop in virtual potential temperature and changes in wind velocity during outflow passages vary according to the type, organization, and life cycle of the convective storm. Overall, the nocturnal events highlighted the passage of well-defined gust fronts with moderate decrease in virtual potential temperature and increase in wind speed. The early afternoon event lacked a sharp gust front and only a gradual drop in virtual potential temperature was observed, probably because of weak or undeveloped outflow. Sensible heat flux (H) experienced an increase at the time of gust front arrival, which was possibly due to sinking of colder air. This was followed by a prolonged period of negative H, associated with enhanced nocturnal negative H in the storms' wake. In turn, increased latent heat flux (LE) was observed following the gust front, owing to drier air coming from the outflow; however, malfunctioning of the moisture sensors during rain precluded a better assessment of this variable. Substantial enhancements of Turbulent Kinetic Energy (TKE) were observed during and after gust front passage, with values comparable to those measured in grass fire experiments, evidencing the highly turbulent character of convective outflows. The early afternoon event displayed slight decreases in the aforementioned quantities in the passage of the outflow. Finally, a conceptual model of the time evolution of H in nocturnal convective outflows observed at the tower site is presented.

scholarly journals Countergradient heat flux observations during the evening transition period

2014 ◽  
Vol 14 (17) ◽  
pp. 9077-9085 ◽  
Author(s):  
E. Blay-Carreras ◽  
E. R. Pardyjak ◽  
D. Pino ◽  
D. C. Alexander ◽  
F. Lohou ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Surface Layer ◽  
Transition Period ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Buoyancy Flux ◽  
Horizontal Shear ◽  
Evening Transition ◽  
Virtual Potential Temperature

Abstract. Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several intensive observational periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~ 30–80 min. The existence of the delay and its duration can be explained by considering the convective timescale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turnover timescale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover timescale.

A Comparison between Moist and Dry Tropical Cyclones: The Low Effectiveness of Surface Sensible Heat Flux in Storm Intensification

2021 ◽  
Author(s):  
Zhanhong Ma ◽  
Jianfang Fei
Keyword(s):  
Heat Flux ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Equivalent Potential Temperature ◽  
Sensible Heat ◽  
Budget Analysis ◽  
Equivalent Potential ◽  
Surface Sensible Heat Flux

AbstractRecent numerical modeling studies demonstrate that dry tropical cyclones can be stably sustained via supply of surface sensible heat flux. This raises questions of whether surface sensible heat flux (SHX) and latent heat flux (LHX) have the same effect on the intensity evolution of tropical cyclones. An estimation of equivalent potential temperature budget in the boundary layer shows that LHX leads to larger increase in equivalent potential temperature than SHX even when they possess the same magnitude. By formulating these two kinds of surface heat fluxes with the same mathematical framework, the simulated intensifications of moist and dry tropical cyclones are compared, with the former driven exclusively by LHX and the latter by SHX. Results show significantly larger intensification rates for the tropical cyclone driven by LHX than that by SHX, revealing low effectiveness of SHX in the intensification of tropical cyclones. The diabatic heating in the moist tropical cyclone occurs accompanying the convection, while it is merely pronounced near the surface in the dry tropical cyclone and is decoupled from the dry convection. A new surface pressure tendency equation is proposed, without incorporating implicit pressure tendency term on the right-hand side. The budget analysis indicates that the SHX is less effective than LHX in lowering surface central pressure and therefore in tropical cyclone intensification. A series of sensitivity experiments suggest that the threshold of energy input required for spinning up a tropical cyclone is lower in the form of LHX than that of SHX.

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>

scholarly journals Potential and Limitations in Estimating Sensible-Heat-Flux Profiles from Consecutive Temperature Profiles Using Remotely-Piloted Aircraft Systems

2019 ◽  
Vol 174 (1) ◽  
pp. 145-177 ◽  
Author(s):  
Line Båserud ◽  
Joachim Reuder ◽  
Marius O. Jonassen ◽  
Timothy A. Bonin ◽  
Phillip B. Chilson ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Mesoscale Model ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Time Of Day ◽  
Sensible Heat ◽  
Tethered Balloon ◽  
Flux Divergence ◽  
Remotely Piloted Aircraft

Abstract Profiles of the sensible heat flux are key to understanding atmospheric-boundary-layer (ABL) structure and development. Based on temperature profiling by a remotely-piloted aircraft system (RPAS), the Small Unmanned Meteorological Observer (SUMO) platform, during the Boundary Layer Late Afternoon and Sunset Turbulence (BLLAST) field campaign, 108 heat-flux profiles are estimated using a simplified version of the prognostic equation for potential temperature $$\theta $$θ that relates the tendency in $$\theta $$θ to the flux divergence over the time span between two consecutive flights. We validate for the first time RPAS-based heat-flux profiles against a network of 12 ground-based eddy-covariance stations (2–60 m above ground), in addition to a comparison with fluxes from a manned aircraft and a tethered balloon, enabling the detailed investigation of the potential and limitations related to this technique for obtaining fluxes from RPAS platforms. We find that appropriate treatment of horizontal advection is crucial for obtaining realistic flux values, and present correction methods specific to the state of the ABL. Advection from a mesoscale model is also tested as another correction method. The SUMO heat-flux estimates with appropriate corrections compare well with the reference measurements, with differences in the performance depending on the time of day, since the evening period shows the best results (94$$\%$$% within the spread of ground stations), and the afternoon period shows the poorest results (63$$\%$$% within the spread). The diurnal cycle of the heat flux is captured by the SUMO platform for several days, with the flux values from the manned aircraft and tethered balloon coinciding well with those from the SUMO platform.

scholarly journals Countergradient heat flux observations during the evening transition period

2014 ◽  
Vol 14 (6) ◽  
pp. 7711-7737 ◽  
Author(s):  
E. Blay-Carreras ◽  
E. R. Pardyjak ◽  
D. Pino ◽  
D. C. Alexander ◽  
F. Lohou ◽  
...  
Keyword(s):  
Boundary Layer ◽  
Heat Flux ◽  
Surface Layer ◽  
Time Scale ◽  
Transition Period ◽  
Potential Temperature ◽  
Buoyancy Flux ◽  
Horizontal Shear ◽  
Evening Transition ◽  
Virtual Potential Temperature

Abstract. Gradient-based turbulence models generally assume that the buoyancy flux ceases to introduce heat into the surface layer of the atmospheric boundary layer in temporal consonance with the gradient of the local virtual potential temperature. Here, we hypothesize that during the evening transition a delay exists between the instant when the buoyancy flux goes to zero and the time when the local gradient of the virtual potential temperature indicates a sign change. This phenomenon is studied using a range of data collected over several Intensive Observational Periods (IOPs) during the Boundary Layer Late Afternoon and Sunset Turbulence field campaign conducted in Lannemezan, France. The focus is mainly on the lower part of the surface layer using a tower instrumented with high-speed temperature and velocity sensors. The results from this work confirm and quantify a flux-gradient delay. Specifically, the observed values of the delay are ~30–80 min. The existence of the delay and its duration can be explained by considering the convective time scale and the competition of forces associated with the classical Rayleigh–Bénard problem. This combined theory predicts that the last eddy formed while the sensible heat flux changes sign during the evening transition should produce a delay. It appears that this last eddy is decelerated through the action of turbulent momentum and thermal diffusivities, and that the delay is related to the convective turn – over time – scale. Observations indicate that as horizontal shear becomes more important, the delay time apparently increases to values greater than the convective turnover time-scale.

scholarly journals Comments on “Unusually Long Duration, Multiple-Doppler Radar Observations of a Front in a Convective Boundary Layer”

2007 ◽  
Vol 135 (12) ◽  
pp. 4237-4239 ◽  
Author(s):  
David M. Schultz
Keyword(s):  
Temperature Gradient ◽  
Doppler Radar ◽  
Cold Front ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Convective Boundary ◽  
Long Duration ◽  
Virtual Potential Temperature ◽  
Surface Sensible Heat Flux

Abstract Stonitsch and Markowski perform multiple-Doppler radar analyses of a cold front over Oklahoma and Kansas. Despite their interesting results, their explanations include a number of misconceptions about cold fronts. These misconceptions include the proper interpretation of the frontogenesis function, the role of entrainment versus differential surface sensible heat flux toward weakening the virtual potential temperature gradient across a cold front, a separation of the wind shift from the virtual potential temperature gradient, and the factors that affect the motion of the cold front.

Urban Modification of Thunderstorms: An Observational Storm Climatology and Model Case Study for the Indianapolis Urban Region

2011 ◽  
Vol 50 (5) ◽  
pp. 1129-1144 ◽  
Author(s):  
Dev Niyogi ◽  
Patrick Pyle ◽  
Ming Lei ◽  
S. Pal Arya ◽  
Chandra M. Kishtawal ◽  
...  
Keyword(s):  
Heat Flux ◽  
Urban Area ◽  
Layer Structure ◽  
Potential Temperature ◽  
Sensible Heat Flux ◽  
Urban Land Use ◽  
Urban Region ◽  
Study Results ◽  
Radar Imagery

AbstractA radar-based climatology of 91 unique summertime (May 2000–August 2009) thunderstorm cases was examined over the Indianapolis, Indiana, urban area. The study hypothesis is that urban regions alter the intensity and composition/structure of approaching thunderstorms because of land surface heterogeneity. Storm characteristics were studied over the Indianapolis region and four peripheral rural counties approximately 120 km away from the urban center. Using radar imagery, the time of event, changes in storm structure (splitting, initiation, intensification, and dissipation), synoptic setting, orientation, and motion were studied. It was found that more than 60% of storms changed structure over the Indianapolis area as compared with only 25% over the rural regions. Furthermore, daytime convection was most likely to be affected, with 71% of storms changing structure as compared with only 42% at night. Analysis of radar imagery indicated that storms split closer to the upwind urban region and merge again downwind. Thus, a larger portion of small storms (50–200 km2) and large storms (>1500 km2) were found downwind of the urban region, whereas midsized storms (200–1500 km) dominated the upwind region. A case study of a typical storm on 13 June 2005 was examined using available observations and the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5), version 3.7.2. Two simulations were performed with and without the urban land use/Indianapolis region in the fourth domain (1.33-km resolution). The storm of interest could not be simulated without the urban area. Results indicate that removing the Indianapolis urban region caused distinct differences in the regional convergence and convection as well as in simulated base reflectivity, surface energy balance (through sensible heat flux, latent heat flux, and virtual potential temperature changes), and boundary layer structure. Study results indicate that the urban area has a strong climatological influence on regional thunderstorms.

scholarly journals Mesoscale Convective Systems and Their Synoptic-Scale Environment in Finland

2015 ◽  
Vol 30 (1) ◽  
pp. 182-196 ◽  
Author(s):  
Ari-Juhani Punkka ◽  
Marja Bister
Keyword(s):  
Potential Temperature ◽  
Mesoscale Convective Systems ◽  
Lapse Rate ◽  
Moist Convection ◽  
Mixing Ratio ◽  
Low Level ◽  
Convective Systems ◽  
Mesoscale Convective ◽  
Wind Events ◽  
Deep Moist Convection

Abstract The environments within which high-latitude intense and nonintense mesoscale convective systems (iMCSs and niMCSs) and smaller thunderstorm clusters (sub-MCSs) develop were studied using proximity soundings. MCS statistics covering 8 years were created by analyzing composite radar imagery. One-third of all systems were intense in Finland and the frequency of MCSs was highest in July. On average, MCSs had a duration of 10.8 h and traveled toward the northeast. Many of the linear MCSs had a southwest–northeast line orientation. Interestingly, a few MCSs were observed to travel toward the west, which is a geographically specific feature of the MCS characteristics. The midlevel lapse rate failed to distinguish the environments of the different event types from each other. However, in MCSs, CAPE and the low-level mixing ratio were higher, the deep-layer-mean wind was stronger, and the lifting condensation level (LCL) was lower than in sub-MCSs. CAPE, low-level mixing ratio, and LCL height were the best discriminators between iMCSs and niMCSs. The mean wind over deep layers distinguished the severe wind–producing events from the nonsevere events better than did the vertical equivalent potential temperature difference or the wind shear in shallow layers. No evidence was found to support the hypothesis that dry air at low- and midlevels would increase the likelihood of severe convective winds. Instead, abundant low- and midlevel moisture was present during both iMCS cases and significant wind events. These results emphasize the pronounced role of low- and midlevel moisture on the longevity and intensity of deep moist convection in low-CAPE environments.

Sign in / Sign up

Export Citation Format

Share Document