Revisiting Azimuthally Asymmetric Moist Instability in the Outer Core of Sheared Tropical Cyclones

2019 ◽  
Vol 148 (3) ◽  
pp. 1297-1319
Author(s):  
Qingqing Li ◽  
Yufan Dai
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Tropical Cyclones ◽  
Convective Available Potential Energy ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Outer Core ◽  
Asymmetric Distribution ◽  
Horizontal Advection ◽  
Conditional Instability

Abstract This study revisits the characteristics and physical processes of the azimuthally asymmetric distribution of moist instability in the outer core of vertically sheared tropical cyclones (TCs) using a numerical model. The results indicate that a downshear–upshear contrast in outer-core conditional instability occurs in the weakly sheared TCs, while an enhanced downshear-left–downshear-right difference is found in strongly sheared storms. Specifically, lower (higher) conditional instability arises downshear left (right) in the strongly sheared TCs. Downward transports of low-entropy air by convective and mesoscale downdrafts in principal rainbands reduce the equivalent potential temperature (θe) in the downshear-left boundary layer, contributing to lower convective available potential energy. Positive horizontal advection of both potential temperature and water vapor by the asymmetric outflow leads to a midlevel maximum of θe in the same quadrant. Hence, a positive θe vertical gradient (thus potential stability) is present in the downshear-left outer core. In the downshear-right quadrant, a lack of convective downdrafts, together with surface fluxes, leads to higher θe in the boundary layer. A dry intrusion is found at the middle to upper levels in the downshear-right outer core, and significant negative horizontal advection of water vapor produces low θe near the midtroposphere. A negative vertical gradient of θe (thus potential instability) in the outer core arises below the downshear-right midtroposphere. The presence of azimuthally asymmetric moist instability is expected to play an important role in fostering and maintaining azimuthally asymmetric convective activity in the outer core of TCs.

scholarly journals Effects of Boundary Layer Vertical Mixing on the Evolution of Hurricanes over Land

2017 ◽  
Vol 145 (6) ◽  
pp. 2343-2361 ◽  
Author(s):  
Feimin Zhang ◽  
Zhaoxia Pu ◽  
Chenghai Wang
Keyword(s):  
Boundary Layer ◽  
Positive Impact ◽  
Potential Temperature ◽  
Vertical Mixing ◽  
Vertical Wind Shear ◽  
Vertical Gradient ◽  
Weather Research And Forecasting ◽  
Virtual Potential Temperature ◽  
Dynamic Structures ◽  
Hurricane Boundary Layer

Abstract After a hurricane makes landfall, its evolution is strongly influenced by its interaction with the planetary boundary layer (PBL) over land. In this study, a series of numerical experiments are performed to examine the effects of boundary layer vertical mixing on hurricane simulations over land using a research version of the NCEP Hurricane Weather Research and Forecasting (HWRF) Model with three landfalling hurricane cases. It is found that vertical mixing in the PBL has a strong influence on the simulated hurricane evolution. Specifically, strong vertical mixing has a positive impact on numerical simulations of hurricanes over land, with better track, intensity, synoptic flow, and precipitation simulations. In contrast, weak vertical mixing leads to the strong hurricanes over land. Diagnoses of the thermodynamic and dynamic structures of hurricane vortices further suggest that the strong vertical mixing in the PBL could cause a decrease in the vertical wind shear and an increase in the vertical gradient of virtual potential temperature. As a consequence, these changes destroy the turbulence kinetic energy in the hurricane boundary layer and thus stabilize the hurricane boundary layer and limit its maintenance over land.

scholarly journals Why is the Tropical Cyclone Boundary Layer Not “Well Mixed”?

2016 ◽  
Vol 73 (3) ◽  
pp. 957-973 ◽  
Author(s):  
Jeffrey D. Kepert ◽  
Juliane Schwendike ◽  
Hamish Ramsay
Keyword(s):  
Boundary Layer ◽  
Tropical Cyclone ◽  
Mixed Layer ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Static Stability ◽  
Layer Depth ◽  
Turbulent Dissipation ◽  
Unstable Layer ◽  
Tropical Cyclone Boundary Layer

Abstract Plausible diagnostics for the top of the tropical cyclone boundary layer include (i) the top of the layer of strong frictional inflow and (ii) the top of the “well mixed” layer, that is, the layer over which potential temperature θ is approximately constant. Observations show that these two candidate definitions give markedly different results in practice, with the inflow layer being roughly twice the depth of the layer of nearly constant θ. Here, the authors will present an analysis of the thermodynamics of the tropical cyclone boundary layer derived from an axisymmetric model. The authors show that the marked dry static stability in the upper part of the inflow layer is due largely to diabatic effects. The radial wind varies strongly with height and, therefore, so does radial advection of θ. This process also stabilizes the boundary layer but to a lesser degree than diabatic effects. The authors also show that this differential radial advection contributes to the observed superadiabatic layer adjacent to the ocean surface, where the vertical gradient of the radial wind is reversed, but that the main cause of this unstable layer is heating from turbulent dissipation. The top of the well-mixed layer is thus distinct from the top of the boundary layer in tropical cyclones. The top of the inflow layer is a better proxy for the top of the boundary layer but is not without limitations. These results may have implications for boundary layer parameterizations that diagnose the boundary layer depth from thermodynamic, or partly thermodynamic, criteria.

scholarly journals Nature of the Mesoscale Boundary Layer Height and Water Vapor Variability Observed 14 June 2002 during the IHOP_2002 Campaign

2009 ◽  
Vol 137 (1) ◽  
pp. 414-432 ◽  
Author(s):  
F. Couvreux ◽  
F. Guichard ◽  
P. H. Austin ◽  
F. Chen
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Mesoscale Model ◽  
Surface Flux ◽  
Early Morning ◽  
Surface Fluxes ◽  
Layer Height ◽  
Horizontal Advection ◽  
Boundary Layer Height ◽  
The Impact

Abstract Mesoscale water vapor heterogeneities in the boundary layer are studied within the context of the International H2O Project (IHOP_2002). A significant portion of the water vapor variability in the IHOP_2002 occurs at the mesoscale, with the spatial pattern and the magnitude of the variability changing from day to day. On 14 June 2002, an atypical mesoscale gradient is observed, which is the reverse of the climatological gradient over this area. The factors causing this water vapor variability are investigated using complementary platforms (e.g., aircraft, satellite, and in situ) and models. The impact of surface flux heterogeneities and atmospheric variability are evaluated separately using a 1D boundary layer model, which uses surface fluxes from the High-Resolution Land Data Assimilation System (HRLDAS) and early-morning atmospheric temperature and moisture profiles from a mesoscale model. This methodology, based on the use of robust modeling components, allows the authors to tackle the question of the nature of the observed mesoscale variability. The impact of horizontal advection is inferred from a careful analysis of available observations. By isolating the individual contributions to mesoscale water vapor variability, it is shown that the observed moisture variability cannot be explained by a single process, but rather involves a combination of different factors: the boundary layer height, which is strongly controlled by the surface buoyancy flux, the surface latent heat flux, the early-morning heterogeneity of the atmosphere, horizontal advection, and the radiative impact of clouds.

scholarly journals An Extended Model for the Potential Intensity of Tropical Cyclones

2012 ◽  
Vol 69 (2) ◽  
pp. 641-661 ◽  
Author(s):  
Thomas Frisius ◽  
Daria Schönemann
Keyword(s):  
Steady State ◽  
Wind Speed ◽  
Tropical Cyclone ◽  
Tropical Cyclones ◽  
Convective Instability ◽  
Cloud Model ◽  
Convective Available Potential Energy ◽  
Extended Model ◽  
Flow Imbalance ◽  
Conditional Instability

Abstract Emanuel’s theory of hurricane potential intensity (E-PI) makes use of the assumption that slantwise convective instability vanishes in a steady-state vortex of a tropical cyclone. In the framework of an extended mathematical potential intensity model it is shown that relaxing this assumption and including an eye results in a larger maximum wind speed compared to that of the predictions made by E-PI. Previous studies by Bryan and Rotunno demonstrate that the effect of unbalanced flow considerably contributes to maximum winds in excess of E-PI (“superintensity”). The authors argue that the proposed mechanism induced by convective instability provides another possible explanation for simulated and observed tropical cyclones exceeding E-PI in addition to flow imbalance. Further evidence for the relevance of conditional instability in mature tropical cyclones to superintensity is given by the fact that convective available potential energy arises in numerical simulations of tropical cyclones. This is demonstrated by means of an axisymmetric cloud model that is in qualitative agreement with the analytical model. These simulations reveal a dependence of superintensity on the amount of CAPE outside the eyewall and also reproduce the decrease in superintensity with increased horizontal diffusion as found in previous studies.

scholarly journals The Dryline on 22 May 2002 during IHOP: Ground-Radar and In Situ Data Analyses of the Dryline and Boundary Layer Evolution

2007 ◽  
Vol 135 (7) ◽  
pp. 2473-2505 ◽  
Author(s):  
Michael S. Buban ◽  
Conrad L. Ziegler ◽  
Erik N. Rasmussen ◽  
Yvette P. Richardson
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Doppler Radar ◽  
Potential Temperature ◽  
Vertical Motion ◽  
Temperature Fields ◽  
Lagrangian Analysis ◽  
Thermodynamic Structure ◽  
In Situ Data ◽  
Water Vapor Mixing Ratio

Abstract On the afternoon and evening of 22 May 2002, high-resolution observations of the boundary layer (BL) and a dryline were obtained in the eastern Oklahoma and Texas panhandles during the International H2O Project. Using overdetermined multiple-Doppler radar syntheses in concert with a Lagrangian analysis of water vapor and temperature fields, the 3D kinematic and thermodynamic structure of the dryline and surrounding BL have been analyzed over a nearly 2-h period. The dryline is resolved as a strong (2–4 g kg−1 km−1) gradient of water vapor mixing ratio that resides in a nearly north–south-oriented zone of convergence. Maintained through frontogenesis, the dryline is also located within a gradient of virtual potential temperature, which induces a persistent, solenoidally forced secondary circulation. Initially quasi-stationary, the dryline retrogrades to the west during early evening and displays complicated substructures including small wavelike perturbations that travel from south to north at nearly the speed of the mean BL flow. A second, minor dryline has similar characteristics to the first, but has weaker gradients and circulations. The BL adjacent to the dryline exhibits complicated structures, consisting of combinations of open cells, horizontal convective rolls, and transverse rolls. Strong convergence and vertical motion at the dryline act to lift moisture, and high-based cumulus clouds are observed in the analysis domain. Although the top of the analysis domain is below the lifted condensation level height, vertical extrapolation of the moisture fields generally agrees with cloud locations. Mesoscale vortices that move along the dryline induce a transient eastward dryline motion due to the eastward advection of dry air following misocyclone passage. Refractivity-based moisture and differential reflectivity analyses are used to help interpret the Lagrangian analyses.

scholarly journals A Planetary Boundary Layer Height Climatology Derived from ECMWF Reanalysis Data

2013 ◽  
Vol 26 (17) ◽  
pp. 6575-6590 ◽  
Author(s):  
Axel von Engeln ◽  
João Teixeira
Keyword(s):  
Boundary Layer ◽  
Planetary Boundary Layer ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Atmospheric Temperature ◽  
Reanalysis Data ◽  
Radiosonde Data ◽  
Convective Boundary ◽  
Seasonal And Diurnal Variations ◽  
Ecmwf Reanalysis

Abstract A planetary boundary layer (PBL) height climatology from ECMWF reanalysis data is generated and analyzed. Different methods are first compared to derive PBL heights from atmospheric temperature, pressure, and relative humidity (RH), which mostly make use of profile gradients, for example, in RH, refractivity, and virtual or potential temperature. Three methods based on the vertical gradient of RH, virtual temperature, and potential temperature were selected for the climatology generation. The RH-based method appears to capture the inversion that caps the convective boundary layer very well as a result of its temperature and humidity dependence, while the temperature-based methods appear to capture the PBL better at high latitudes. A validation of the reanalysis fields with collocated radiosonde data shows generally good agreement in terms of mean PBL height and standard deviation for the RH-based method. The generated ECMWF-based PBL height climatology shows many of the expected climatological features, such as a fairly low PBL height near the west coast of continents where stratus clouds are found and PBL growth as the air is advected over warmer waters toward the tropics along the trade winds. Large seasonal and diurnal variations are primarily found over land. The PBL height can exceed 3 km, mostly over desert areas during the day, although large values can also be found in areas such as the ITCZ. The robustness of the statistics was analyzed by using information on the percentage of outliers. Here in particular, the sea-based PBL was found to be very stable.

scholarly journals Contribution of horizontal and vertical advection to the formation of small-scale vertical structures of ozone in the lower and middle stratosphere at Fairbanks, Alaska

2019 ◽  
Author(s):  
Miho Yamamori ◽  
Yasuhiro Murayama ◽  
Kazuo Shibasaki ◽  
Isao Murata ◽  
Kaoru Sato
Keyword(s):  
Dominant Role ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Small Scale ◽  
Mixing Ratio ◽  
Horizontal Advection ◽  
Vertical Advection ◽  
Background Ozone ◽  
Using Data ◽  
Middle Stratosphere

Abstract. The contribution of vertical and horizontal advection to the production of small-scale vertical ozone structures was investigated using data from an ozonesonde observation performed at intervals of 3 h in Fairbanks (64.8N, 147.9W), Alaska. The dominant vertical scales of the ozone mixing ratio were determined to be 2–5 km, which were similar to those of horizontal winds and the temperature of the lower and middle stratosphere, using spectral analysis. Ozone fluctuations due to vertical advection were estimated from the potential temperature fluctuation and vertical gradient of the background ozone mixing ratio. Residual ozone fluctuations are attributed to horizontal advection. Fluctuations due to horizontal advection are dominant, as reported in previous studies. The cross-correlation of the effects of vertical and horizontal advection was also evaluated. The correlation is relatively larger at altitudes of 18–23 km and 32–33 km compared to those at other height regions. In contrast to previous studies, horizontal advection by gravity waves seems to play a dominant role in the production of small-scale ozone structures at altitudes of 32–35 km.

scholarly journals Impacts of Evaporation from Raindrops on Tropical Cyclones. Part I: Evolution and Axisymmetric Structure

2010 ◽  
Vol 67 (1) ◽  
pp. 71-83 ◽  
Author(s):  
Masahiro Sawada ◽  
Toshiki Iwasaki
Keyword(s):  
Boundary Layer ◽  
Angular Momentum ◽  
Tropical Cyclones ◽  
Convective Available Potential Energy ◽  
Evaporative Cooling ◽  
Development Stage ◽  
Cold Pool ◽  
Mature Stage ◽  
Latent Heating ◽  
Axisymmetric Structure

Abstract Cloud-resolving simulations of an ideal tropical cyclone (TC) on an f plane are performed to investigate the effects of evaporative cooling on the evolution and structure of a TC. Evaporative cooling has markedly different impacts on the TC development and structure than melting/sublimation cooling because of the formation of rainbands. Evaporative cooling suppresses the organization of a TC at the early development stage. Evaporative cooling effectively forms convective downdrafts that cool and dry the boundary layer. Stabilizing the TC boundary layer reduces convective available potential energy (CAPE) around the eyewall by about 40% and slows the development. However, at the mature stage evaporative cooling steadily develops the TC for a longer period and enlarges the TC size because of rainbands, which are formed by the cold pool associated with evaporative cooling outside the eyewall. The large amounts of latent heating greatly induce the secondary circulation and transport large absolute angular momentum inward around the midtroposphere, resulting in the steady development of the TC. After a three-day integration, both the area-averaged precipitation and the kinetic energy become greater than when evaporative cooling is excluded.

scholarly journals A Transition Mechanism for the Spontaneous Axisymmetric Intensification of Tropical Cyclones

2013 ◽  
Vol 70 (1) ◽  
pp. 112-129 ◽  
Author(s):  
Yoshiaki Miyamoto ◽  
Tetsuya Takemi
Keyword(s):  
Tropical Cyclones ◽  
Convective Available Potential Energy ◽  
Potential Temperature ◽  
Three Dimensional ◽  
Horizontal Wind ◽  
Low Level ◽  
Transition Mechanism ◽  
Budget Analysis ◽  
Axisymmetric Structure ◽  
Physics Model

Abstract A mechanism for the transition of tropical cyclones (TCs) to the spontaneous rapid intensification (RI) phase is proposed based on numerical results of a three-dimensional full-physics model. The intensification phase of the simulated TC is divided into three subphases according to the rate of intensification: 1) a slowly intensifying phase, 2) an RI phase, and 3) an adjustment phase toward the quasi-steady state. The evolution of a TC vortex is diagnosed by the energy budget analysis and the degree of axisymmetric structure of the TC vortex, and the simulated TC is determined to be axisymmetrized 12 h before the onset of RI. It is found that equivalent potential temperature θe in the lowest layer suddenly increases inside the radius of maximum azimuthally averaged horizontal wind rma after the TC becomes nearly axisymmetric. Forward trajectory analyses revealed that the enhanced convective instability in the TC core region where the eyewall subsequently forms results from the increased inertial stability of the TC core after the axisymmetrization. Since fluid parcels remain longer inside rma, owing to the increased inertial stability, the parcels obtain more enthalpy from the underlying ocean. As a result, low-level θe and hence convective available potential energy (CAPE) increase. Under the condition with increased CAPE, the eyewall is intensified and the secondary circulation is enhanced, leading to the increased convergence of low-level inflow; this process is considered to be the trigger of RI. Once the eyewall forms, the simulated TC starts its RI.

scholarly journals A Lagrangian Objective Analysis Technique for Assimilating In Situ Observations with Multiple-Radar-Derived Airflow

2007 ◽  
Vol 135 (7) ◽  
pp. 2417-2442 ◽  
Author(s):  
Conrad L. Ziegler ◽  
Michael S. Buban ◽  
Erik N. Rasmussen
Keyword(s):  
Boundary Layer ◽  
Water Vapor ◽  
Potential Temperature ◽  
Cloud Base ◽  
Mixing Ratio ◽  
Lagrangian Analysis ◽  
Analysis Technique ◽  
Virtual Potential Temperature ◽  
Water Vapor Mixing Ratio

Abstract A new Lagrangian analysis technique is developed to assimilate in situ boundary layer measurements using multi-Doppler-derived wind fields, providing output fields of water vapor mixing ratio, potential temperature, and virtual potential temperature from which the lifting condensation level (LCL) and relative humidity (RH) fields are derived. The Lagrangian analysis employs a continuity principle to bidirectionally distribute observed values of conservative variables with the 3D, evolving boundary layer airflow, followed by temporal and spatial interpolation to an analysis grid. Cloud is inferred at any grid point whose height z > zLCL or equivalently where RH ≥ 100%. Lagrangian analysis of the cumulus field is placed in the context of gridded analyses of visible satellite imagery and photogrammetric cloud-base area analyses. Brief illustrative examples of boundary layer morphology derived with the Lagrangian analysis are presented based on data collected during the International H2O Project (IHOP): 1) a dryline on 22 May 2002; 2) a cold-frontal–dryline “triple point” intersection on 24 May 2002. The Lagrangian analysis preserves the sharp thermal gradients across the cold front and drylines and reveals the presence of undulations and plumes of water vapor mixing ratio and virtual potential temperature associated with deep penetrative updraft cells and convective roll circulations. Derived cloud fields are consistent with satellite-inferred cloud cover and cloud-base locations.

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