ON THE RAPID INTENSIFICATION OF HURRICANE WILMA (2005)

Previous studies have focused mostly on the roles of environmental factors in the rapid intensification (RI) of tropical cyclones (TCs) due to the lack of high-resolution data in the inner-core regions. In this study, we examine the RI issue by analyzing 72-h cloud-permitting model predictions of Hurricane Wilma (2005) with the Weather and Research Forecast (WRF) model at the finest grid sizes of 1-2 km. The 72-h predictions cover Hurricane Wilma¡¦s initial 18-h spin up, an 18-h RI and the subsequent 36-h weakening stage. The model prediction uses the initial and lateral boundary conditions, including a bogus vortex, that are identical to the Geophysical Fluid Dynamics Laboratory's then-operational data, except for the time-independent sea surface temperature (SST) field. The model predicts an RI rate of more than 4 hPa h-1 for an 18-h period, with the minimum central pressure of less than 889 hPa. It was found that an upper-level warm core forms in the same layer as the upper outflow, in coincidence with the onset of RI. The warm core results from the subsidence of stratospheric air associated with the detrainment of convective bursts (CBs). The upper divergent outflow appears to play an important role in protecting the warm core from ventilation by environmental flows. Results also show the development of more CBs preceding RI, but most subsidence warming radiates away by internal gravity waves and storm-relative flows. In contrast, many fewer CBs occur during RI, but more subsidence warming contributes to the balanced upper-level cyclonic circulation in the warm core (as intense as 20,,aC) region. Furthermore, considerable CB activity can still take place in the outer eyewall as the storm weakens during its eyewall replacement. Sensitivity simulations reveal that the upper-level warm core and CB activity depend critically on warm SST. We conclude that significant CB activity in the inner-core regions is an important ingredient in generating an upper-level warm core that is hydrostatically more efficient to the RI of TCs, given all the other favorable environmental conditions. The formation of a divergent upper-level outflow that prevents the warm core from ventilation is examined through asymmetric contraction processes associated with new rainbands forming inside the eyewall. The relative vorticity, generated in the downshear region and then advected cyclonically downstream, can induce convergence in the boundary layer. With the aid of high moisture content, the convergence can trigger deep convection and contribute to the formation of the new rainbands. Finally, the importance of a small eye size is demonstrated using three widely accepted approximations: angular momentum conservation, solid body rotation and gradient wind balance. Results show that the storm intensifies much faster for a given contraction speed if the eye size is small

Orographic and convective gravity waves above the Alps and Andes mountains during GPS radio occultation events – a case study

The significant distortions introduced in the measured atmospheric gravity wavelengths by soundings other than in vertical and horizontal directions, are discussed as a function of elevation angle of the sounding path and the gravity waves aspect ratio. Under- or overestimation of real vertical wavelengths during the measurement process depends basically on the value of these two parameters. The consequences of these distortions on the calculation of the energy and vertical flux of horizontal momentum are analyzed and discussed in the context of two experimental limb satellite setups: GPS-LEO radio occultations and TIMED/SABER measurements. Possible discrepancies previously found between the momentum flux calculated from satellite temperature profiles, on site and from model simulations, may, to a certain degree, be attributed to these distortions. A recalculation of previous momentum flux climatologies based on these considerations seems to be a difficult goal.Fil: Hierro, Rodrigo Federico. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Steiner, Andrea K.. Universidad de Graz; AustriaFil: de la Torre, Alejandro. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; ArgentinaFil: Alexander, Pedro Manfredo. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Física de Buenos Aires. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Física de Buenos Aires; ArgentinaFil: Llamedo Soria, Pablo Martin. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Austral. Facultad de Ingeniería. Departamento de Ciencias Básicas; ArgentinaFil: Cremades, Pablo Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina. Universidad Nacional de Cuyo. Facultad de Ciencias Exactas y Naturales; Argentin

Large‐Amplitude Mountain Waves in the Mesosphere Accompanying Weak Cross‐Mountain Flow During DEEPWAVE Research Flight RF22

Mountain wave (MW) propagation and dynamics extending into the upper mesosphere accompanying weak forcing are examined using in situ and remote‐sensing measurements aboard the National Science Foundation/National Center for Atmospheric Research Gulfstream V (GV) research aircraft and the German Aerospace Center Falcon. The measurements were obtained during Falcon flights FF9 and FF10 and GV Research Flight RF22 of the Deep Propagating Gravity Wave Experiment (DEEPWAVE) performed over Mount Cook, New Zealand, on 12 and 13 July 2014. In situ measurements revealed both trapped lee waves having zonal wavelengths of λₓ ~ 12 km and less, and larger‐scale, vertically propagating MWs primarily at λₓ ~ 20–60 km and ~100–300 km extending from west to ~400 km east of Mount Cook. GV Rayleigh lidar measurements from 25‐ to 60‐km altitudes showed that the weak forcing and zonal winds that increased from ~12 m/s at 12 km to ~40 and 130 m/s at 30 and 55 km, respectively, enabled largely linear MW propagation and strong amplitude growth with altitude into the mesosphere. GV Na lidar and airglow imager measurements revealed an extensive MW response from ~70 to 87 km with large amplitudes and vertical displacements at λₓ ~ 40–300 km but with both decreasing with altitude approaching a critical level near 90 km. These MWs exhibited large‐scale MW breaking and among the largest sustained momentum fluxes observed in the mesosphere. UK Met Office Unified Model simulations of the RF22 MW event captured many aspects of the observed MW field and revealed that despite the dominant large‐scale MW responses in the stratosphere, the major momentum fluxes accompanied smaller‐scale waves

Report of the proceedings of the Colloquium and Workshop on Multiscale Coupled Modeling

The Colloquium and Workshop on Multiscale Coupled Modeling was held for the purpose of addressing modeling issues of importance to planning for the Cooperative Multiscale Experiment (CME). The colloquium presentations attempted to assess the current ability of numerical models to accurately simulate the development and evolution of mesoscale cloud and precipitation systems and their cycling of water substance, energy, and trace species. The primary purpose of the workshop was to make specific recommendations for the improvement of mesoscale models prior to the CME, their coupling with cloud, cumulus ensemble, hydrology, air chemistry models, and the observational requirements to initialize and verify these models

Simulation of all-scale atmospheric dynamics on unstructured meshes

The advance of massively parallel computing in the nineteen nineties and beyond encouraged finer grid intervals in numerical weather-prediction models. This has improved resolution of weather systems and enhanced the accuracy of forecasts, while setting the trend for development of unified all-scale atmospheric models. This paper first outlines the historical background to a wide range of numerical methods advanced in the process. Next, the trend is illustrated with a technical review of a versatile nonoscillatory forward-in-time finite-volume (NFTFV) approach, proven effective in simulations of atmospheric flows from small-scale dynamics to global circulations and climate. The outlined approach exploits the synergy of two specific ingredients: the MPDATA methods for the simulation of fluid flows based on the sign-preserving properties of upstream differencing; and the flexible finite-volume median-dual unstructured-mesh discretisation of the spatial differential operators comprising PDEs of atmospheric dynamics. The paper consolidates the concepts leading to a family of generalised nonhydrostatic NFTFV flow solvers that include soundproof PDEs of incompressible Boussinesq, anelastic and pseudo-incompressible systems, common in large-eddy simulation of small- and meso-scale dynamics, as well as all-scale compressible Euler equations. Such a framework naturally extends predictive skills of large-eddy simulation to the global atmosphere, providing a bottom-up alternative to the reverse approach pursued in the weather-prediction models. Theoretical considerations are substantiated by calculations attesting to the versatility and efficacy of the NFTFV approach. Some prospective developments are also discussed

Mathematical Theory and Modelling in Atmosphere-Ocean Science

Mathematical theory and modelling in atmosphere-ocean science combines a broad range of advanced mathematical and numerical techniques and research directions. This includes the asymptotic analysis of multiscale systems, the deterministic and stochastic modelling of sub-grid-scale processes, and the numerical analysis of nonlinear PDEs over a broad range of spatial and temporal scales. This workshop brought together applied mathematicians and experts in the disciplinary fields of meteorology and oceanography for a wide-ranging exchange of ideas and results in this area with the aim of fostering fundamental interdisciplinary work in this important science area

Simulating meteorological profiles to study noise propagation from freeways

Forecasts of noise pollution from a highway line segment noise source are obtained from a sound propagation model utilizing effective sound speed profiles derived from a Numerical Weather Prediction (NWP) limited area forecast with 1 km horizontal resolution and near-ground vertical resolution finer than 20 m. Methods for temporal along with horizontal and vertical spatial nesting are demonstrated within the NWP model for maintaining forecast feasibility. It is shown that vertical nesting can improve the prediction of finer structures in near-ground temperature and velocity profiles, such as morning temperature inversions and low level jet-like features. Accurate representation of these features is shown to be important for modeling sound refraction phenomena and for enabling accurate noise assessment. Comparisons are made using the parabolic equation model for predictions with profiles derived from NWP simulations and from field experiment observations during mornings on November 7 and 8, 2006 in Phoenix, Arizona. The challenges faced in simulating accurate meteorological profiles at high resolution for sound propagation applications are highlighted and areas for possible improvement are discussed

Orographic and convective gravity waves above the Alps and Andes Mountains during GPS radio occultation events – a case study

Gravity waves (GWs) and convective systems play a fundamental role in atmospheric circulation, weather, and climate. Two usual main sources of GWs are orographic effects triggering mountain waves and convective activity. In addition, GW generation by fronts and geostrophic adjustment must also be considered. The utility of Global Positioning System (GPS) radio occultation (RO) observations for the detection of convective systems is tested. A collocation database between RO events and convective systems over subtropical to midlatitude mountain regions close to the Alps and Andes is built. From the observation of large-amplitude GW structures in the absence of jets and fronts, subsets of RO profiles are sampled. A representative case study among those considered at each region is selected and analyzed. The case studies are investigated using mesoscale Weather Research and Forecasting (WRF) simulations, ERA-Interim reanalysis data, and measured RO temperature profiles. The absence of fronts or jets during both case studies reveals similar relevant GW features (main parameters, generation, and propagation). Orographic and convective activity generates the observed GWs. Mountain waves above the Alps reach higher altitudes than close to the Andes. In the Andes case, a critical layer prevents the propagation of GW packets up to stratospheric heights. The case studies are selected also because they illustrate how the observational window for GW observations through RO profiles admits a misleading interpretation of structures at different altitude ranges. From recent results, the distortion introduced in the measured atmospheric vertical wavelengths by one of the RO events is discussed as an illustration. In the analysis, both the elevation angle of the sounding path (line of tangent points) and the gravity wave aspect ratio estimated from the simulations and the line of sight are taken into account. In both case studies, a considerable distortion, over- and underestimation of the vertical wavelengths measured by RO, may be expected

Impacts of convective treatment on tropical rainfall variability in realistic and idealized simulations

The prediction of precipitation in the tropics is a challenge for numerical weather prediction (NWP), meaning very low practical predictability there. However, previous studies indicated that intrinsic predictability in the tropics is up to a few weeks and thus longer than in the extratropics. Equatorial waves (EWs) from the linear shallow-water theory are considered the source of this long predictability. Most weather and climate models still struggle to accurately capture EWs, which is often attributed to parameterized convection. With advanced computing power, model development is moving toward high-resolution models with explicit convection. To evaluate the value of these high-resolution models, this thesis aims to provide important insights into the behavior of tropical precipitation due to the treatment of deep and shallow convection using the ICOsahedral Nonhydrostatic (ICON) model. First, we examine the sensitivity of EWs to model configuration using realistic ICON simulations with varying horizontal grid spacings (80-2.5 km) and with different convectivetreatments between parameterized versus explicit deep and shallow convection. To robustly identify wave signals, we use two objective methods, one filtering rainfall using a fast Fourier transform and the other projecting two-dimensional wind and geopotential onto theoretical wave patterns. The results demonstrate that large-scale EWs are surprisingly consistent in terms of phase speed and wave amplitude with little sensitivity to model resolution, convective treatment and wave identification method. Rainfall signals of westward inertio-gravity waves (WIGs), however, show a large difference between parameterized and explicit convection with the latter showing marked rainfall signals but with no corresponding wind patterns. A composite analysis to link rainfall and wind fields of waves reveals that the identified signals in rainfall appear to be associated with mesoscale convective systems, the spatiotemporal scales of which overlap with those of WIGs, and thus are isolated as waves through space-time filtering. Secondly, we analyze idealized ICON simulations in a tropical aquachannel configuration with zonally symmetric sea surface temperatures and with rigid walls at 30°N/S. The aquachannel simulations vary in the representation of deep and shallow convection but with the same horizontal grid spacing of 13 km. All aquachannel simulations have maximum rainfall at the equator, showing an intertropical convergence zone (ITCZ) there, but the rainfall amount increases by 35% with explicit deep convection. To physically understand this difference, we adapt a diagnostic based on a conceptual model by Emanuel (2019), assuming boundary-layer quasi-equilibrium (BLQE), the weak temperature gradient approximation, and mass and energy conservation. BLQE implies that moist entropy is in balance between surface enthalpy fluxes, which import high moist entropy to the BL, and convective downdrafts, which transport low moist entropy from the free troposphere into the BL. The results reveal that the rainfall differences are primarily associated with surface enthalpy fluxes through BLQE, while precipitation efficiency is surprisingly constant in the ITCZ. Further detailed analysis demonstrates that mean surface wind speed, which is closely related to the large-scale circulation, contributes most to the differences in surface enthalpy fluxes. Thus, the treatment of deep convection alters mean rainfall through tight links between surface winds, associated surface fluxes and convective mass flux. Lastly, variability associated with EWs is examined in the aquachannel simulations by using the same wave identification methods used for the realistic simulations. All simulations show prominent signals of Kelvin waves (KWs) with large variations among them. Parameterized deep convection produces various eastward propagation with speeds of 5–27 m/s, while explicit deep convection exhibits a dominance of KWs with a zonal wavenumber of one and with a propagation speed of 24 m/s. Furthermore, explicit deep convection causes more pronounced structures of zonal wind and temperature in the lower stratosphere and a stronger link of wind-induced surface enthalpy flux exchange to the development of convection. Meanwhile, the treatment of shallow convection plays a role for temperature variation below 2.5 km. However, BL warming is in phase with maximum rainfall associated with KWs, which is opposite to observations. Parameterized deep convection generates a feature sharing similarities with the Madden Julian Oscillation, which is not found in the other aquachannel simulations. The novelty of this thesis lies in understanding the behavior of tropical rainfall in both realistic and idealized simulations by using diagnostics adapted for systematic comparisons between different simulations, mainly due to different convective treatments. This allows us to obtain valuable insights into the sensitivity of tropical rainfall and its variability to model configuration, ultimately paving the way for developing more accurate weather and climate predictions in the tropics