scholarly journals Tropical Cyclone Motion in Response to Land Surface Friction

2006 ◽  
Vol 63 (4) ◽  
pp. 1324-1337 ◽  
Author(s):  
Martin L. M. Wong ◽  
Johnny C. L. Chan
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Vertical Motion ◽  
Moisture Flux ◽  
Internal Boundary ◽  
Asymmetric Flow ◽  
Horizontal Advection ◽  
Surface Moisture ◽  
Tangential Wind ◽  
The Asymmetry

Abstract Numerical experiments are performed with the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) to study the effects of surface-moisture flux and friction over land on the movement of tropical cyclones (TCs). On an f plane, the TCs are initially placed 150 km due east of a north–south-oriented coastline in an atmosphere at rest. It is found that a TC could drift toward land when the roughness length is 0.5 m over land, with an average drift speed of ∼1 m s−1. Friction, but not surface-moisture flux over land, is apparently essential for the movement toward land. The friction-induced asymmetry in the large-scale flow is the primary mechanism responsible for causing the TC drift. The mechanism responsible for the development of the large-scale asymmetric flow over the lower to midtroposphere (∼900–600 hPa) appears to be the creation of asymmetric vorticity by the divergence term in the vorticity equation. Horizontal advection then rotates the asymmetric vorticity to give a northeasterly flow in the TC periphery (∼500–1000 km from the TC center). The flow near the TC center has a more northerly component because of the stronger rotation by the tangential wind of the TC at inner radii. However, the TC does not move with the large-scale asymmetric flow. Potential vorticity budget calculations indicate that while the horizontal advection term is basically due to the effect of advection by the large-scale asymmetric flow, the diabatic heating and vertical advection terms have to be considered in determining the vortex landward drift, because of the strong asymmetry in vertical motion. Two mechanisms could induce the asymmetry in vertical motion and cause a deviation of the TC track from the horizontal asymmetric flow. First, the large-scale asymmetric flow in the upper troposphere differs from that in the lower troposphere, both in magnitude and direction, which results in a vertical shear that could force the asymmetry. A vertical tilt of the vortex axis is also found that is consistent with the direction of shear and also the asymmetry in rainfall and vertical motion. Second, asymmetric boundary layer convergence that results from the internal boundary layer could also force an asymmetry in vertical motion.

scholarly journals On the internal boundary layer related wind stress curl and its role in generating shallow lake circulations

2014 ◽  
Vol 62 (1) ◽  
pp. 16-23 ◽  
Author(s):  
János Józsa
Keyword(s):  
Boundary Layer ◽  
Wind Stress ◽  
Shallow Lake ◽  
Large Scale ◽  
Internal Boundary Layer ◽  
Internal Boundary ◽  
Development Theory ◽  
Wind Stress Curl ◽  
Flow Models ◽  
Boundary Layer Development

Abstract The paper demonstrates that the wind stress curl as an external vorticity source plays an important role in shaping large scale shallow lake circulations. The analysis of purpose-oriented simultaneous wind and current measurements data from the Hungarian part of Lake Neusiedl reasonably fits well the internal boundary layer development theory over the lake surface. A 2-D vorticity formulation of wind-induced flows is used to demonstrate mathematically the IBL-related large scale circulation generation mechanism well reflected in the measured data. Further validation of the findings is carried out by means of simple 2-D numerical flow modelling, in which details on the flow pattern besides the measurement sites could be also revealed. Wind-induced lake circulations linked to IBL development shows a novelty to be implemented in up-to-date numerical flow models.

scholarly journals Area-Averaged Surface Moisture Flux over Fragmented Sea Ice: Floe Size Distribution Effects and the Associated Convection Structure within the Atmospheric Boundary Layer

Atmosphere ◽  
2019 ◽  
Vol 10 (11) ◽  
pp. 654
Author(s):  
Marta Wenta ◽  
Agnieszka Herman
Keyword(s):  
Boundary Layer ◽  
Sea Ice ◽  
Atmospheric Boundary Layer ◽  
Weather Prediction ◽  
Moisture Flux ◽  
Spatial Arrangement ◽  
Weather Research And Forecasting ◽  
Sea Ice Concentration ◽  
Surface Moisture ◽  
Ice Concentration

Sea ice fragmentation results in the transformation of the surface from relatively homogeneous to highly heterogeneous. Atmospheric boundary layer (ABL) rapidly responds to those changes through a range of processes which are poorly understood and not parametrized in numerical weather prediction (NWP) models. The aim of this work is to increase our understanding and develop parametrization of the ABL response to different floe size distributions (FSD). The analysis is based on the results of simulations with the Weather Research and Forecasting model. Results show that FSD determines the distribution and intensity of convection within the ABL through its influence on the atmospheric circulation. Substantial differences between various FSDs are found in the analysis of spatial arrangement and strength of ABL convection. To incorporate those sub-grid effects in the NWP models, a correction factor for the calculation of surface moisture heat flux is developed. It is expressed as a function of floe size, sea ice concentration and wind speed, and enables a correction of the flux computed from area-averaged quantities, as is typically done in NWP models. In general, the presented study sheds some more light on the sea ice–atmosphere interactions and provides the first attempt to parametrize the influence of FSD on the ABL.

scholarly journals Observed Kinematic and Thermodynamic Structure in the Hurricane Boundary Layer during Intensity Change

2019 ◽  
Vol 147 (8) ◽  
pp. 2765-2785 ◽  
Author(s):  
Kyle Ahern ◽  
Mark A. Bourassa ◽  
Robert E. Hart ◽  
Jun A. Zhang ◽  
Robert F. Rogers
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Inner Core ◽  
Intensity Change ◽  
Conditional Stability ◽  
Thermodynamic Structure ◽  
Wind Structure ◽  
Tangential Wind ◽  
Axisymmetric Structure ◽  
Hurricane Boundary Layer

Abstract The axisymmetric structure of the inner-core hurricane boundary layer (BL) during intensification [IN; intensity tendency ≥20 kt (24 h)−1, where 1 kt ≈ 0.5144 m s−1], weakening [WE; intensity tendency <−10 kt (24 h)−1], and steady-state [SS; the remainder] periods are analyzed using composites of GPS dropwindsondes from reconnaissance missions between 1998 and 2015. A total of 3091 dropsondes were composited for analysis below 2.5-km elevation—1086 during IN, 1042 during WE, and 963 during SS. In nonintensifying hurricanes, the low-level tangential wind is greater outside the radius of maximum wind (RMW) than for intensifying hurricanes, implying higher inertial stability (I2) at those radii for nonintensifying hurricanes. Differences in tangential wind structure (and I2) between the groups also imply differences in secondary circulation. The IN radial inflow layer is of nearly equal or greater thickness than nonintensifying groups, and all groups show an inflow maximum just outside the RMW. Nonintensifying hurricanes have stronger inflow outside the eyewall region, likely associated with frictionally forced ascent out of the BL and enhanced subsidence into the BL at radii outside the RMW. Equivalent potential temperatures (θe) and conditional stability are highest inside the RMW of nonintensifying storms, which is potentially related to TC intensity. At greater radii, inflow layer θe is lowest in WE hurricanes, suggesting greater subsidence or more convective downdrafts at those radii compared to IN and SS hurricanes. Comparisons of prior observational and theoretical studies are highlighted, especially those relating BL structure to large-scale vortex structure, convection, and intensity.

Lidar-based Water Vapor, Temperature and Wind Measurements with Turbulence Resolution during the EUREC4A Field Campaign onboard RV Merian

2020 ◽  
Author(s):  
Diego Lange Vega ◽  
Andreas Behrendt ◽  
Florian Späth ◽  
Volker Wulfmeyer
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Sensible Heat Flux ◽  
Vertical Motion ◽  
Radiative Properties ◽  
Cloud Formation ◽  
Horizontal Wind ◽  
Backscatter Coefficient ◽  
Field Campaign ◽  
Microphysical Properties

<p>The EUREC4A (ElUcidating the RolE of Clouds-Circulation Coupling in Climate) field campaign takes place in the lower Atlantic trades, over the ocean east of Barbados from 20 January to 20 February 2020. During this campaign, for the first time, simultaneous measurements of surface turbulence, cloud microphysical properties, cloud radiative properties, convective activity and the large-scale environment in which clouds and convection are embedded (large-scale vertical motion, thermodynamic stratification, surface properties, turbulent and radiative sources or sinks of energy).</p><p>Our new Atmospheric Raman Temperature and Humidity Sounder (ARTHUS) observes temperature and moisture profiles over the ocean with turbulence resolution of up to 10 s and 7.5 m. By this, the thermodynamic properties as well as statistics of their turbulent fluctuations in the oceanic boundary layer can be investigated in detail including relative humidity, buoyancy, CAPE, and CIN. In addition, ARTHUS is also a aerosol Raman lidar and provides profiles of particle extinction and backscatter coefficient independently at 355 nm. Two Doppler lidars – one vertical pointing the second in scanning mode – measure horizontal wind profiles as well as profiles of vertical wind fluctuations, turbulent kinetic energy, and momentum flux. The combination of the three lidars will provide synergetic data products like latent and sensible heat flux profiles. Thus, this combination allows to investigate boundary-layer properties including cloud formation and aerosol-cloud interaction.</p><p>During the EGU General Assembly, we will show our first results from the campaign.</p>

scholarly journals Boundary Layer Updrafts Driven by Airflow over Heated Terrain

2014 ◽  
Vol 71 (4) ◽  
pp. 1425-1442 ◽  
Author(s):  
Daniel J. Kirshbaum ◽  
Chun-Chih Wang
Keyword(s):  
Boundary Layer ◽  
Numerical Simulations ◽  
Large Scale ◽  
Vertical Motion ◽  
Vertical Displacement ◽  
Surface Heating ◽  
Thermal Flow ◽  
Convective Boundary ◽  
Strong Surface ◽  
Linear And Nonlinear

Abstract This study presents linear and nonlinear scalings for boundary layer ascent forced by airflow over heated terrain and compares them to results from corresponding high-resolution numerical simulations. Close agreement between theory and simulation is found over most of the parameter space considered, including variations in background winds, boundary layer stability, mountain height, and diabatic heating rate. As expected, the linear and nonlinear scalings perform best for linear and nonlinear flows, respectively. For a convective boundary layer, the scalings accurately predict vertical motion for all flows considered, including those that extend well into the nonlinear regime. Thus, these scalings may ultimately help to improve the parameterization of subgrid orographic ascent in large-scale models. The vertical velocity scalings are less accurate for mechanically blocked flows in stable boundary layers, for which a simple vertical displacement scaling is superior. Although the scalings do not treat interactions between mechanical and thermal flow responses, these interactions are generally weak except in blocked flows with strong surface heating. Numerical simulations of such cases suggest that a hydrostatically induced pressure decrease in the lee associated with the diabatic surface heating drives stronger flow reversal within the wake and leeside convergence downwind of it, both of which produce strong surface-based updrafts. Thus, nonlinear interactions between mechanical and thermal flow responses may significantly enhance the likelihood of convection initiation over heated mountains.

scholarly journals Boundary Layer Convergence Induced by Strong Winds across a Midlatitude SST Front*

2014 ◽  
Vol 27 (4) ◽  
pp. 1698-1718 ◽  
Author(s):  
Thomas Kilpatrick ◽  
Niklas Schneider ◽  
Bo Qiu
Keyword(s):  
Boundary Layer ◽  
Gravity Wave ◽  
Mesoscale Model ◽  
Vertical Motion ◽  
Internal Gravity Wave ◽  
Front Surface ◽  
Internal Boundary Layer ◽  
Internal Boundary ◽  
Free Atmosphere ◽  
Sst Front

Abstract Recent studies indicate that the influence of midlatitude SST fronts extends through the marine atmospheric boundary layer (MABL) into the free atmosphere, with implications for climate variability. To better understand the mechanisms of this ocean-to-atmosphere influence, SST-induced MABL convergence is explored here with the Weather Research and Forecasting mesoscale model in an idealized, dry, two-dimensional configuration, for winds crossing from cold to warm SST and from warm to cold SST. For strong cross-front winds, O(10 m s−1), changes in the turbulent mixing and MABL depth across the SST front lead to MABL depth-integrated convergence in the cold-to-warm case and depth-integrated divergence in the warm-to-cold case. The turbulent stress divergence term changes over a shorter length scale than the pressure gradient and Coriolis terms, such that the MABL response directly above the SST front is governed by nonrotating, internal boundary layer–like physics, which are consistent with the vertical mixing mechanism. An important consequence is that the increment in the cross-front surface stress diagnoses the vertical motion at the top of the MABL. These physics are at variance with some previously proposed SST frontal MABL models in which pressure adjustments determine the MABL convergence. The SST-induced MABL convergence results in vertical motion that excites a stationary internal gravity wave in the free atmosphere, analogous to a mountain wave. For a 15 m s−1 cross-front wind, the gravity wave forced by an SST increase of 3°C over 200 km is comparable to that forced by an 80-m change in topography.

scholarly journals Effects of Roll Vortices on the Evolution of Hurricane Harvey During Landfall

2021 ◽  
Author(s):  
Xin Li ◽  
Zhaoxia Pu ◽  
Zhiqiu Gao
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Large Scale ◽  
Vertical Flux ◽  
Mesoscale Simulation ◽  
Roll Vortices ◽  
Hurricane Wind ◽  
Tangential Wind ◽  
The Impact ◽  
Negative Flux

AbstractHorizontal boundary layer roll vortices are a series of large-scale turbulent eddies that prevail in a hurricane’s boundary layer. In this paper, a one-way nested sub-kilometer-scale large eddy simulation (LES) based on the Weather Research and Forecasting model (WRF) was used to examine the impact of roll vortices on the evolution of Hurricane Harvey around its landfall from 0000z on 25 to 1800z 27 August 2017. The simulation results imply that the turbulence in the LES can be attributed mainly to roll vortices. With the representation of roll vortices, the LES simulation provided a better simulation of hurricane wind vertical structure and precipitation. In contrast, the mesoscale simulation with the YSU PBL scheme overestimated the precipitation for the hurricane over the ocean.Further analysis indicates that the roll vortices introduced a positive vertical flux and thinner inflow layer, whereas a negative flux maintained the maximum tangential wind at around 400 m above ground. During hurricane landfall, the weak negative flux maintained the higher wind in the LES simulation. The overestimated low-level vertical flux in the mesoscale simulation with the YSU scheme led to overestimated hurricane intensity over the ocean and accelerated the decay of the hurricane during landfall. Rainfall analysis reveals that the roll vortices led to a weak updraft and insufficient water vapor supply in the LES. For the simulation with the YSU scheme, the strong updraft combined with surplus water vapor eventually led to unrealistic heavy rainfall for the hurricane over the ocean.

scholarly journals Convectively Coupled Equatorial Waves Simulated on an Aquaplanet in a Global Nonhydrostatic Experiment

2008 ◽  
Vol 65 (4) ◽  
pp. 1246-1265 ◽  
Author(s):  
Tomoe Nasuno ◽  
Hirofumi Tomita ◽  
Shinichi Iga ◽  
Hiroaki Miura ◽  
Masaki Satoh
Keyword(s):  
Boundary Layer ◽  
Large Scale ◽  
Lower Troposphere ◽  
Moisture Flux ◽  
Moisture Distribution ◽  
Convective System ◽  
Dynamical Structure ◽  
Kelvin Wave ◽  
Free Atmosphere ◽  
Low Level

Abstract Large-scale tropical convective disturbances simulated in a 7-km-mesh aquaplanet experiment are investigated. A 40-day simulation was executed using the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). Two scales of eastward-propagating disturbances were analyzed. One was tightly coupled to a convective system resembling super–cloud clusters (SCCs) with a zonal scale of several thousand kilometers (SCC mode), whereas the other was characterized by a planetary-scale dynamical structure (40 000-km mode). The typical phase velocity was 17 (23) m s−1 for the SCC (40 000 km) mode. The SCC mode resembled convectively coupled Kelvin waves in the real atmosphere around the equator, but was accompanied by a pair of off-equatorial gyres. The 40 000-km mode maintained a Kelvin wave–like zonal structure, even poleward of the equatorial Rossby deformation radius. The equatorial structures in both modes matched neutral eastward-propagating gravity waves in the lower troposphere and unstable (growing) waves in the upper troposphere. In both modes, the meridional mass divergence exceeded the zonal component, not only in the boundary layer, but also in the free atmosphere. The forcing terms indicated that the meridional flow was primarily driven by convection via deformation in pressure fields and vertical circulations. Moisture convergence was one order of magnitude greater than the moisture flux from the sea surface. In the boundary layer, frictional convergence in the (anomalous) low-level easterly phase accounted for the buildup of low-level moisture leading to the active convective phase. The moisture distribution in the free atmosphere suggested that the moisture–convection feedback operated efficiently, especially in the SCC mode.

scholarly journals Observations of Air–Sea Interaction and Intensity Change in Hurricanes

2013 ◽  
Vol 141 (7) ◽  
pp. 2368-2382 ◽  
Author(s):  
Joseph J. Cione ◽  
Evan A. Kalina ◽  
Jun A. Zhang ◽  
Eric W. Uhlhorn
Keyword(s):  
Large Scale ◽  
Vertical Wind Shear ◽  
Moisture Flux ◽  
Intensity Change ◽  
Separate Analysis ◽  
Near Surface ◽  
Surface Moisture ◽  
Prediction Scheme ◽  
Low Levels ◽  
Moisture Conditions

Abstract Recent enhancements to the tropical cyclone-buoy database (TCBD) have incorporated data from the Extended Best Track (EBT) and the Statistical Hurricane Intensity Prediction Scheme (SHIPS) archive for tropical cyclones between 1975 and 2007. This information is used to analyze the relationships between large-scale atmospheric parameters, radial and shear-relative air–sea structure, and intensity change in strengthening and weakening hurricanes. Observations from this research illustrate that the direction of the large-scale vertical wind shear at mid- to low levels can impact atmospheric moisture conditions found near the surface. Drier low-level environments were associated with northerly shear conditions. In a separate analysis comparing strengthening and weakening hurricanes, drier surface conditions were also found for the intensifying sample. Since SST conditions were similar for both groups of storms, it is likely that the atmosphere was primarily responsible for modifying the near-surface thermodynamic environment (and ultimately surface moisture flux conditions) for this particular analysis.

scholarly journals On the Initial Development of Asymmetric Vertical Motion and Horizontal Relative Flow in a Mature Tropical Cyclone Embedded in Environmental Vertical Shear

2013 ◽  
Vol 70 (11) ◽  
pp. 3471-3491 ◽  
Author(s):  
Yamei Xu ◽  
Yuqing Wang
Keyword(s):  
Tropical Cyclone ◽  
Dominant Role ◽  
Vertical Motion ◽  
Vertical Shear ◽  
Horizontal Wind ◽  
Asymmetric Flow ◽  
Initial Development ◽  
Budget Analysis ◽  
Tangential Wind ◽  
Relative Flow

Abstract In this paper, the authors focus on the initial development of asymmetric vertical motion and horizontal relative flow in a mature tropical cyclone (TC) embedded in an environmental vertical shear. The fully compressible, nonhydrostatic TC model was used to perform a series of numerical experiments with a mature TC with different intensities embedded in shear with different magnitudes and different vertical profiles. Results show that the development of both the wavenumber-1 asymmetric vertical motion and horizontal relative flow for a TC embedded in vertical shear is quite sensitive to both the magnitude and the vertical profile of wind shear, as well as the intensity of the TC itself. Diagnostic analysis based on the quasi-balanced potential vorticity inversion indicates that the balanced dynamics can only explain a small portion of the asymmetric vertical motion and relative flow. The unbalanced processes contribute predominantly to the development of the asymmetric flow in the simulations. It is shown that the eyewall of a mature TC plays a role somewhat like a material cylinder embedded in an environmental flow with vertical shear. The interaction between the environmental shear and the eyewall produces vertical gradient of convergence/divergence of horizontal wind around the lateral edge of the eyewall. This forces much stronger asymmetric vertical motion than the balanced processes do and drives significant horizontal relative divergent flow over the storm core, which opposes vertical shear and reduces the vertical tilt of the storm axis. In addition, the budget analysis for the axisymmetric tangential wind demonstrates that the asymmetric flow plays a dominant role in weakening the storm top down.

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