Rotating Shallow-Water Models with Moist Convection

2018 ◽  
Author(s):  
Vladimir Zeitlin
Keyword(s):  
Baroclinic Instability ◽  
Potential Temperature ◽  
Moist Convection ◽  
Mathematical Properties ◽  
Vertical Fluxes ◽  
Equivalent Potential ◽  
Shallow Water Models ◽  
Wave Emission ◽  
Rotating Shallow Water ◽  
Convective Fluxes

It is shown how the standard RSW can be ’augmented’ to include phase transitions of water. This chapter explains how to incorporate extra (convective) vertical fluxes in the model. By using Lagrangian conservation of equivalent potential temperature condensation of the water vapour, which is otherwise a passive tracer, is included in the model and linked to convective fluxes. Simple relaxational parameterisation of condensation permits the closure of the system, and surface evaporation can be easily included. Physical and mathematical properties of thus obtained model are explained, and illustrated on the example of wave scattering on the moisture front. The model is applied to ’moist’ baroclinic instability of jets and vortices. Condensation is shown to produce a transient increase of the growth rate. Special attention is paid to the moist instabilities of hurricane-like vortices, which are shown to enhance intensification of the hurricane, increase gravity wave emission, and generate convection-coupled waves.

scholarly journals Isentropic Analysis of a Simulated Hurricane

2016 ◽  
Vol 73 (5) ◽  
pp. 1857-1870 ◽  
Author(s):  
Agnieszka A. Mrowiec ◽  
Olivier M. Pauluis ◽  
Fuqing Zhang
Keyword(s):  
Mass Transport ◽  
Potential Temperature ◽  
Wrf Model ◽  
Secondary Circulation ◽  
Equivalent Potential Temperature ◽  
Moist Convection ◽  
Shallow Convection ◽  
High Entropy ◽  
Equivalent Potential ◽  
Wide Range

Abstract Hurricanes, like many other atmospheric flows, are associated with turbulent motions over a wide range of scales. Here the authors adapt a new technique based on the isentropic analysis of convective motions to study the thermodynamic structure of the overturning circulation in hurricane simulations. This approach separates the vertical mass transport in terms of the equivalent potential temperature of air parcels. In doing so, one separates the rising air parcels at high entropy from the subsiding air at low entropy. This technique filters out oscillatory motions associated with gravity waves and separates convective overturning from the secondary circulation. This approach is applied here to study the flow of an idealized hurricane simulation with the Weather Research and Forecasting (WRF) Model. The isentropic circulation for a hurricane exhibits similar characteristics to that of moist convection, with a maximum mass transport near the surface associated with a shallow convection and entrainment. There are also important differences. For instance, ascent in the eyewall can be readily identified in the isentropic analysis as an upward mass flux of air with unusually high equivalent potential temperature. The isentropic circulation is further compared here to the Eulerian secondary circulation of the simulated hurricane to show that the mass transport in the isentropic circulation is much larger than the one in secondary circulation. This difference can be directly attributed to the mass transport by convection in the outer rainband and confirms that, even for a strongly organized flow like a hurricane, most of the atmospheric overturning is tied to the smaller scales.

scholarly journals Understanding Instabilities of Tropical Cyclones and Their Evolution with a Moist Convective Rotating Shallow-Water Model

2016 ◽  
Vol 73 (2) ◽  
pp. 505-523 ◽  
Author(s):  
Noé Lahaye ◽  
Vladimir Zeitlin
Keyword(s):  
Shallow Water ◽  
Water Model ◽  
Moist Convection ◽  
Shallow Water Model ◽  
Maximum Wind ◽  
Maximal Growth Rate ◽  
Wave Emission ◽  
Rotating Shallow Water ◽  
Selection Of

Abstract Instabilities of hurricane-like vortices are studied with the help of a rotating shallow-water model, including the effects of moist convection. Linear stability analysis demonstrates that dominant unstable modes are mixed Rossby–inertia–gravity waves. It is shown that, depending on fine details of the vorticity profile, a wavenumber selection of the instability may operate or not, leading in some cases to an unstable mode with a distinctively maximal growth rate and in other cases to an ensemble of unstable modes with close growth rates. Numerical simulations are performed in order to investigate nonlinear saturation of the instability and to understand the dynamical role of moisture. In agreement with previous studies, the authors confirm axisymmetrization of vorticity in the course of the development of the instability, which induces changes of intensity of the hurricane. In “dry” simulations, winds are intensified only inside the radius of maximum wind, while the maximum value of the wind decreases. “Moist precipitating” simulations (with and without evaporation) exhibit a net increase of winds, also at the radius of maximum wind, as compared to the dry simulations. Dynamical effects of moisture on the reorganization of the vortex and on the efficiency of inertia–gravity wave emission are quantified and shown to be considerable. Periodic bursts in the emission of waves related to the development of the unstable modes inside the vortex are evidenced, as well as the appearance of convectively coupled waves in the moist precipitating simulations with evaporation.

Getting Rid of Fast Waves: Slow Dynamics

2018 ◽  
Author(s):  
Vladimir Zeitlin
Keyword(s):  
Rossby Waves ◽  
Baroclinic Instability ◽  
Nonlinear Effects ◽  
Kelvin Wave ◽  
Beta Plane ◽  
Equatorial Wave ◽  
Flow Over Topography ◽  
Wave Guides ◽  
Rotating Shallow Water ◽  
Fast Waves

After analysis of general properties of horizontal motion in primitive equations and introduction of principal parameters, the key notion of geostrophic equilibrium is introduced. Quasi-geostrophic reductions of one- and two-layer rotating shallow-water models are obtained by a direct filtering of fast inertia–gravity waves through a choice of the time scale of motions of interest, and by asymptotic expansions in Rossby number. Properties of quasi-geostrophic models are established. It is shown that in the beta-plane approximations the models describe Rossby waves. The first idea of the classical baroclinic instability is given, and its relation to Rossby waves is explained. Modifications of quasi-geostrophic dynamics in the presence of coastal, topographic, and equatorial wave-guides are analysed. Emission of mountain Rossby waves by a flow over topography is demonstrated. The phenomena of Kelvin wave breaking, and of soliton formation by long equatorial and topographic Rossby waves due to nonlinear effects are explained.

What are the Best Thermodynamic Quantity and Function to Define a Front in Gridded Model Output?

2019 ◽  
Vol 100 (5) ◽  
pp. 873-895 ◽  
Author(s):  
Carl M. Thomas ◽  
David M. Schultz
Keyword(s):  
Real Time ◽  
Potential Temperature ◽  
Thermodynamic Quantity ◽  
Kinematics And Dynamics ◽  
Horizontal Gradient ◽  
Model Output ◽  
Equivalent Potential Temperature ◽  
Thermal Front ◽  
Equivalent Potential ◽  
Definition Of

AbstractFronts can be computed from gridded datasets such as numerical model output and reanalyses, resulting in automated surface frontal charts and climatologies. Defining automated fronts requires quantities (e.g., potential temperature, equivalent potential temperature, wind shifts) and kinematic functions (e.g., gradient, thermal front parameter, and frontogenesis). Which are the most appropriate to use in different applications remains an open question. This question is investigated using two quantities (potential temperature and equivalent potential temperature) and three functions (magnitude of the horizontal gradient, thermal front parameter, and frontogenesis) from both the context of real-time surface analysis and climatologies from 38 years of reanalyses. The strengths of potential temperature to identify fronts are that it represents the thermal gradients and its direct association with the kinematics and dynamics of fronts. Although climatologies using potential temperature show features associated with extratropical cyclones in the storm tracks, climatologies using equivalent potential temperature include moisture gradients within air masses, most notably at low latitudes that are unrelated to the traditional definition of a front, but may be representative of a broader definition of an airmass boundary. These results help to explain previously published frontal climatologies featuring maxima of fronts in the subtropics and tropics. The best function depends upon the purpose of the analysis, but Petterssen frontogenesis is attractive, both for real-time analysis and long-term climatologies, in part because of its link to the kinematics and dynamics of fronts. Finally, this study challenges the conventional definition of a front as an airmass boundary and suggests that a new, dynamically based definition would be useful for some applications.

scholarly journals Improved moist-convective rotating shallow water model and its application to instabilities of hurricane-like vortices

2018 ◽  
Author(s):  
LMD
Keyword(s):  
Tropical Cyclone ◽  
Shallow Water ◽  
Water Vapour ◽  
Large Scale ◽  
Precipitable Water ◽  
Water Model ◽  
Moist Convection ◽  
Shallow Water Model ◽  
Atmospheric Motions ◽  
Rotating Shallow Water

We show how the two-layer moist-convective rotating shallow water model (mcRSW), which proved to be a simple and robust tool for studying effects of moist convection on large-scale atmospheric motions, can be improved by including, in addition to the water vapour, precipitable water, and the effects of vaporisation, entrainment, and precipitation. Thus improved mcRSW becomes cloud-resolving. It is applied, as an illustration, to model the development of instabilities of tropical cyclone-like vortices.

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 Radar Kinematic Information as a Surrogate for Isentropes in Stratiform Precipitation Systems

2017 ◽  
Vol 145 (9) ◽  
pp. 3763-3774 ◽  
Author(s):  
Bart Geerts ◽  
Jordan I. Christian
Keyword(s):  
Wind Shear ◽  
Doppler Radar ◽  
Potential Temperature ◽  
Vertical Gradient ◽  
Horizontal Wind ◽  
Model Output ◽  
Vertical Resolution ◽  
Equivalent Potential ◽  
Stratiform Precipitation ◽  
Kinematic Information

Abstract This study illustrates that dual-Doppler-derived wind shear (vertical gradient of the horizontal wind) in stratiform, nonturbulent flow is structured in long, thin striations. The reason this has not been documented before is that scanning ground-based radars have inadequate vertical resolution, deteriorating with range. Here data from an airborne radar with a fine, range-independent vertical resolution are used. A comparison of the radar-derived wind shear with model output of isentropes in vertical transects in the comma head of two frontal disturbances suggests that the wind shear layers describe material surfaces. Model output itself further confirms the alignment of isentropes with wind shear in vertical transects. Thus, Doppler-radar-derived wind shear (a kinematic conserved variable) may serve as a suitable proxy for thermodynamic conserved variables such as equivalent potential temperature in stratiform precipitation. Furthermore, the presence of shear striations in vertical transects can be used as a marker for nonturbulent flow, and their persistence as an indicator of limited dispersion in such flow.

Surface Characteristics of Observed Cold Pools

2008 ◽  
Vol 136 (12) ◽  
pp. 4839-4849 ◽  
Author(s):  
Nicholas A. Engerer ◽  
David J. Stensrud ◽  
Michael C. Coniglio
Keyword(s):  
Life Cycle ◽  
Potential Temperature ◽  
Surface Characteristics ◽  
Life Cycle Stage ◽  
Cold Pool ◽  
Convective System ◽  
Cold Pools ◽  
Equivalent Potential ◽  
Life Cycle Stages ◽  
The Mean

Abstract Cold pools are a key element in the organization of precipitating convective systems, yet knowledge of their typical surface characteristics is largely anecdotal. To help to alleviate this situation, cold pools from 39 mesoscale convective system (MCS) events are sampled using Oklahoma Mesonet surface observations. In total, 1389 time series of surface observations are used to determine typical rises in surface pressure and decreases in temperature, potential temperature, and equivalent potential temperature associated with the cold pool, and the maximum wind speeds in the cold pool. The data are separated into one of four convective system life cycle stages: first storms, MCS initiation, mature MCS, and MCS dissipation. Results indicate that the mean surface pressure rises associated with cold pools increase from 3.2 hPa for the first storms’ life cycle stage to 4.5 hPa for the mature MCS stage before dropping to 3.3 hPa for the dissipation stage. In contrast, the mean temperature (potential temperature) deficits associated with cold pools decrease from 9.5 (9.8) to 5.4 K (5.6 K) from the first storms to the dissipation stage, with a decrease of approximately 1 K associated with each advance in the life cycle stage. However, the daytime and early evening observations show mean temperature deficits over 11 K. A comparison of these observed cold pool characteristics with results from idealized numerical simulations of MCSs suggests that observed cold pools likely are stronger than those found in model simulations, particularly when ice processes are neglected in the microphysics parameterization. The mean deficits in equivalent potential temperature also decrease with the MCS life cycle stage, starting at 21.6 K for first storms and dropping to 13.9 K for dissipation. Mean wind gusts are above 15 m s−1 for all life cycle stages. These results should help numerical modelers to determine whether the cold pools in high-resolution models are in reasonable agreement with the observed characteristics found herein. Thunderstorm simulations and forecasts with thin model layers near the surface are also needed to obtain better representations of cold pool surface characteristics that can be compared with observations.

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