scholarly journals Sensitivity to the Representation of Microphysical Processes in Numerical Simulations during Tropical Storm Formation

2016 ◽  
Vol 144 (10) ◽  
pp. 3611-3630 ◽  
Author(s):  
Andrew B. Penny ◽  
Patrick A. Harr ◽  
James D. Doyle
Keyword(s):  
Heating Rate ◽  
Diabatic Heating ◽  
Deep Convection ◽  
Initial Period ◽  
Tropical Cyclogenesis ◽  
Heating Rates ◽  
Low Level ◽  
Convective Scale ◽  
Microphysical Processes ◽  
Heating Profiles

An analysis of in situ observations from the nondeveloping tropical disturbance named TCS025 revealed that a combination of unfavorable system-scale and environmental factors limited further development. In this study, a multiphysics ensemble of high-resolution simulations of TCS025 are analyzed and compared. A simulation that overdeveloped the TCS025 disturbance is compared with one that correctly simulated nondevelopment and reveals that convection was stronger and diabatic heating rates were larger in the developing simulation. This led to continued spinup of the low-level circulation primarily through vorticity stretching. In contrast, convection was much weaker in the nondeveloping simulation, and after an initial period of deep convection, average vorticity tendencies from stretching became weakly negative, which allowed for the frictional spindown of the low-level circulation. Convective-scale differences identified early in the simulations appear to have resulted from the explicit representation of graupel in the developing simulation. The net impacts resulting from these differences in convection are manifest in the average diabatic heating profiles that are important for determining the developmental outcome. Additional simulations are conducted whereby the diabatic heating rates are artificially adjusted. Relatively small changes in the diabatic heating rate led to significantly different outcomes with respect to storm development, and the degree of overdevelopment is largely dictated by the diabatic heating rate. These findings suggest the correct representation of convective processes and associated diabatic heating are necessary to adequately forecast tropical cyclogenesis, especially for systems near a threshold of development like TCS025.

scholarly journals Mesoscale Processes during the Genesis of Hurricane Karl (2010)

2019 ◽  
Vol 76 (8) ◽  
pp. 2235-2255 ◽  
Author(s):  
Michael M. Bell ◽  
Michael T. Montgomery
Keyword(s):  
Structural Changes ◽  
Doppler Radar ◽  
Deep Convection ◽  
Tropical Cyclogenesis ◽  
Primary Role ◽  
Low Level ◽  
Stratiform Precipitation ◽  
Environmental Air ◽  
Mesoscale Processes

Abstract Observations from the Pre-Depression Investigation of Cloud Systems in the Tropics (PREDICT), Genesis and Rapid Intensification Processes (GRIP), and Intensity Forecast Experiment (IFEX) field campaigns are analyzed to investigate the mesoscale processes leading to the tropical cyclogenesis of Hurricane Karl (2010). Research aircraft missions provided Doppler radar, in situ flight level, and dropsonde data documenting the structural changes of the predepression disturbance. Following the pre-Karl wave pouch, variational analyses at the meso-β and meso-α scales suggest that the convective cycle in Karl alternately built the low- and midlevel circulations leading to genesis episodically rather than through a sustained lowering of the convective mass flux from increased stabilization. Convective bursts that erupt in the vorticity-rich environment of the recirculating pouch region enhance the low-level meso-β- and meso-α-scale circulation through vortex stretching. As the convection wanes, the resulting stratiform precipitation strengthens the midlevel circulation through convergence associated with ice microphysical processes, protecting the disturbance from the intrusion of dry environmental air. Once the column saturation fraction returns to a critical value, a subsequent convective burst below the midlevel circulation further enhances the low-level circulation, and the convective cycle repeats. The analyses suggest that the onset of deep convection and associated low-level spinup were closely related to the coupling of the vorticity and moisture fields at low and midlevels. Our interpretation of the observational analysis presented in this study reaffirms a primary role of deep convection in the genesis process and provides a hypothesis for the supporting role of stratiform precipitation and the midlevel vortex.

scholarly journals On the Creation and Evolution of Small-Scale Low-Level Vorticity Anomalies during Tropical Cyclogenesis

2019 ◽  
Vol 76 (8) ◽  
pp. 2335-2355 ◽  
Author(s):  
Warren P. Smith ◽  
Melville E. Nicholls
Keyword(s):  
Environmental Parameters ◽  
Cold Pool ◽  
Tropical Cyclogenesis ◽  
Small Scale ◽  
Low Level ◽  
Cold Pools ◽  
Convective Scale ◽  
Vortical Hot Towers ◽  
Formation And Evolution ◽  
Vortex Merger

Abstract Recent numerical modeling and observational studies indicate the importance of vortical hot towers (VHTs) in the transformation of a tropical disturbance to a tropical depression. It has recently been recognized that convective-scale downdraft outflows that form within VHTs also preferentially develop positive vertical vorticity around their edges, which is considerably larger in magnitude than ambient values. During a numerical simulation of tropical cyclogenesis it is found that particularly strong low-level convectively induced vorticity anomalies (LCVAs) occasionally form as convection acts on the enhanced vorticity at the edges of cold pools. These features cycle about the larger-scale circulation and are associated with a coincident pressure depression and low-level wind intensification. The LCVAs studied are considerably deeper than the vorticity produced at the edges of VHT cold pool outflows, and their evolution is associated with persistent convection and vortex merger events that act to sustain them. Herein, we highlight the formation and evolution of two representative LCVAs and discuss the environmental parameters that eventually become favorable for one LCVA to reach the center of a larger-scale circulation as tropical cyclogenesis occurs.

scholarly journals A Simple Model of Climatological Rainfall and Vertical Motion Patterns over the Tropical Oceans

2009 ◽  
Vol 22 (23) ◽  
pp. 6477-6497 ◽  
Author(s):  
Larissa E. Back ◽  
Christopher S. Bretherton
Keyword(s):  
Boundary Layer ◽  
Simple Model ◽  
Diabatic Heating ◽  
Deep Convection ◽  
Vertical Motion ◽  
Central Pacific ◽  
Mode Amplitude ◽  
Tropical Oceans ◽  
Conditional Instability ◽  
Heating Profiles

Abstract A simple model is developed that predicts climatological rainfall, vertical motion, and diabatic heating profiles over the tropical oceans given the sea surface temperature (SST), using statistical relationships deduced from the 40-yr ECMWF Re-Analysis (ERA-40). The model allows for two modes of variability in the vertical motion profiles: a shallow mode responsible for all “boundary layer” convergence between 850 hPa and the surface, and a deep mode with no boundary layer convergence. The model is based on the argument expressed in the authors’ companion paper that boundary layer convergence can be usefully viewed as a forcing on deep convection, not just a result thereof. The shallow mode is either specified from satellite observations or modeled using a simple mixed-layer model that has SST as well as 850-hPa geopotential height, winds, and temperature as boundary conditions. The deep-mode amplitude is empirically shown to be proportional to a simple measure of conditional instability in convecting regions, and is determined by the constraint that radiative cooling must balance adiabatic warming in subsidence regions. This two-mode model is tested against a reanalysis-derived dry static energy budget and in a reanalysis-independent framework based on satellite-derived surface convergence and using SST as a proxy for conditional instability. It can predict the observed annual mean and seasonal cycle of rainfall, vertical motion, and diabatic heating profiles across the tropical oceans with significantly more skill than optimized predictions using a thresholded linear relationship with SST. In most warm-ocean regions, significant rainfall only occurs in regions of monthly-mean boundary layer convergence. In such regions, deep-mode amplitude and rainfall increase linearly with SST, with an additional rainfall contribution from the shallow mode directly tied to boundary layer convergence. This second contribution is significant mainly in the east and central Pacific ITCZ, where it is responsible for that region’s “bottom-heavy” vertical-velocity, diabatic heating, and cloud profiles.

scholarly journals Early Evolution of the 23–26 September 2012 U.K. Floods: Tropical Storm Nadine and Diabatic Heating due to Cloud Microphysics

2017 ◽  
Vol 145 (2) ◽  
pp. 543-563 ◽  
Author(s):  
Sam Hardy ◽  
David M. Schultz ◽  
Geraint Vaughan
Keyword(s):  
United Kingdom ◽  
Diabatic Heating ◽  
Deep Convection ◽  
Cloud Microphysics ◽  
Tropical Storm ◽  
Heating Rates ◽  
River Flooding ◽  
Heating And Cooling ◽  
The United Kingdom ◽  
Pv Generation

Major river flooding affected the United Kingdom in late September 2012 as a slow-moving extratropical cyclone brought over 100 mm of rain to a large swath of northern England and north Wales, with local accumulations approaching 200 mm. The cyclone developed on 20–22 September following the interaction between an equatorward-moving potential vorticity (PV) streamer and Tropical Storm Nadine, near the Azores. A plume of tropical moisture was drawn poleward ahead of the PV streamer over a low-level baroclinic zone, allowing deep convection to develop. Convectively driven latent heat release reduced upper-tropospheric PV near the streamer, causing it to fracture and cut off from the reservoir of high PV over the United Kingdom. Simulations using the Weather Research and Forecasting Model with 4-km horizontal grid spacing in which microphysical heating and cooling tendencies are set to zero, alongside calculations of instantaneous diabatic heating rates and PV tendencies along trajectories, reveal that deposition heating contributed strongly to the fracturing of the PV streamer into a discrete anomaly by directly reducing upper-tropospheric PV to the streamer’s east. Condensation heating contributed to lower-tropospheric PV generation along the cold front as the cyclone developed, while cooling due to sublimation, evaporation, and melting modified the PV much less strongly. The results of this case study show that the collocation of strong deposition heating with positive absolute vorticity in the upper troposphere can lead to substantial PV modification and a very different cyclone evolution to that when deposition heating is suppressed.

scholarly journals The Next-Generation Goddard Convective–Stratiform Heating Algorithm: New Tropical and Warm-Season Retrievals for GPM

2018 ◽  
Vol 31 (15) ◽  
pp. 5997-6026 ◽  
Author(s):  
Stephen E. Lang ◽  
Wei-Kuo Tao
Keyword(s):  
Warm Season ◽  
Tropical Rainfall Measuring Mission ◽  
Shallow Convection ◽  
Heating Rates ◽  
Low Level ◽  
Precipitation Measurement ◽  
Cloud Resolving Model ◽  
Upper Level ◽  
Heating Profiles ◽  
Global Precipitation

The Goddard convective–stratiform heating (CSH) algorithm, used to estimate cloud heating in support of the Tropical Rainfall Measuring Mission (TRMM), is upgraded in support of the Global Precipitation Measurement (GPM) mission. The algorithm’s lookup tables (LUTs) are revised using new and additional cloud-resolving model (CRM) simulations from the Goddard Cumulus Ensemble (GCE) model, producing smoother heating patterns that span a wider range of intensities because of the increased sampling and finer GPM product grid. Low-level stratiform cooling rates are reduced in the land LUTs for a given rain intensity because of the rain evaporation correction in the new four-class ice (4ICE) scheme. Additional criteria, namely, echo-top heights and low-level reflectivity gradients, are tested for the selection of heating profiles. Those resulting LUTs show greater and more precise variation in their depth of heating as well as a tendency for stronger cooling and heating rates when low-level dB Z values decrease toward the surface. Comparisons versus TRMM for a 3-month period show much more low-level heating in the GPM retrievals because of increased detection of shallow convection, while upper-level heating patterns remain similar. The use of echo tops and low-level reflectivity gradients greatly reduces midlevel heating from ~2 to 5 km in the mean GPM heating profile, resulting in a more top-heavy profile like TRMM versus a more bottom-heavy profile with much more midlevel heating. Integrated latent heating rates are much better balanced versus surface rainfall for the GPM retrievals using the additional selection criteria with an overall bias of +4.3%.

scholarly journals Genesis of Hurricane Julia (2010) within an African Easterly Wave: Low-Level Vortices and Upper-Level Warming

2013 ◽  
Vol 70 (12) ◽  
pp. 3799-3817 ◽  
Author(s):  
Stefan F. Cecelski ◽  
Da-Lin Zhang
Keyword(s):  
Model Simulation ◽  
Deep Convection ◽  
Mesoscale Convective System ◽  
Tropical Cyclogenesis ◽  
Convective System ◽  
Easterly Waves ◽  
Low Level ◽  
Mesoscale Convective ◽  
Upper Level ◽  
Scale Surface

Abstract While a robust theoretical framework for tropical cyclogenesis (TCG) within African easterly waves (AEWs) has recently been developed, little work explores the development of low-level meso-β-scale vortices (LLVs) and a meso-α-scale surface low in relation to deep convection and upper-tropospheric warming. In this study, the development of an LLV into Hurricane Julia (2010) is shown through a high-resolution model simulation with the finest grid size of 1 km. The results presented expand upon the connections between LLVs and the AEW presented in previous studies while demonstrating the importance of upper-tropospheric warming for TCG. It is found that the significant intensification phase of Hurricane Julia is triggered by the pronounced upper-tropospheric warming associated with organized deep convection. The warming is able to intensify and expand during TCG owing to formation of a storm-scale outflow beyond the Rossby radius of deformation. Results confirm previous ideas by demonstrating that the intersection of the AEW's trough axis and critical latitude is a preferred location for TCG, while supplementing such work by illustrating the importance of upper-tropospheric warming and meso-α-scale surface pressure falls during TCG. It is shown that the meso-β-scale surface low enhances boundary layer convergence and aids in the bottom-up vorticity development of the meso-β-scale LLV. The upper-level warming is attributed to heating within convective bursts at earlier TCG stages while compensating subsidence warming becomes more prevalent once a mesoscale convective system develops.

Total Heating Characteristics of the ISCCP Tropical and Subtropical Cloud Regimes

2013 ◽  
Vol 26 (18) ◽  
pp. 7097-7116 ◽  
Author(s):  
Justin P. Stachnik ◽  
Courtney Schumacher ◽  
Paul E. Ciesielski
Keyword(s):  
Large Scale ◽  
Marine Boundary Layer ◽  
Deep Convection ◽  
Future Research ◽  
Low Level ◽  
Convective Systems ◽  
Stratocumulus Clouds ◽  
Tropospheric Heating ◽  
Heating Profiles ◽  
Cloud Regimes

Abstract Composite profiles of the apparent heat source Q1 and moisture sink Q2 are calculated for the International Satellite Cloud Climatology Project (ISCCP) cloud regimes (or “weather states”) using sounding observations from 10 field campaigns comprising both tropical and subtropical domains. Distinct heating profiles were determined for each ISCCP cloud regime, ranging from strong, upper-tropospheric heating for mesoscale convective systems (WS1) to integrated cooling for populations typically associated with marine stratus and stratocumulus clouds (WS5, WS6, and WS7). Despite being primarily associated with thin cirrus, the corresponding regime (WS4) has heating maxima in the lower and midtroposphere due to the presence of underlying clouds. Regime-averaged Q2 profiles showed similar transitions with strong drying observed for deep convection and low-level moistening for marine boundary layer clouds. The derived profiles were generally similar over land and ocean with the notable exception of the fair-weather cumulus regime (WS8). Additional midlevel moistening was identified for several weather states over land, suggesting enhanced detrainment and more frequent congestus clouds compared to oceanic domains. A control simulation using the Community Atmosphere Model, version 4 (CAM4), was similar to the large-scale patterns of diabatic heating at low levels produced by the ISCCP composites. Differences were more pronounced at middle and upper levels and are largely attributed to the uncertainty in the heating profiles for the cumulus regime (WS8). Low-level heating anomalies were calculated for each phase of the Madden–Julian oscillation (MJO) and they precede upper-tropospheric heating from deep convection by 3–4 phases. Implications for future research using ISCCP heating reconstructions are also discussed.

Genesis of Hurricane Julia (2010) within an African Easterly Wave: Sensitivity Analyses of WRF-LETKF Ensemble Forecasts

2014 ◽  
Vol 71 (9) ◽  
pp. 3180-3201 ◽  
Author(s):  
Stefan F. Cecelski ◽  
Da-Lin Zhang
Keyword(s):  
Deep Convection ◽  
Empirical Orthogonal Functions ◽  
Sensitivity Analyses ◽  
Tropical Cyclogenesis ◽  
Tropospheric Temperature ◽  
Orthogonal Functions ◽  
Ensemble Forecasts ◽  
Low Level ◽  
Modes Of Variability ◽  
Two Parameters

Abstract In this study, the predictability of tropical cyclogenesis (TCG) is explored by conducting ensemble sensitivity analyses on the TCG of Hurricane Julia (2010). Using empirical orthogonal functions (EOFs), the dominant patterns of ensemble disagreements are revealed for various meteorological parameters such as mean sea level pressure (MSLP) and upper-tropospheric temperature. Using the principal components of the EOF patterns, ensemble sensitivities are generated to elucidate which mechanisms drive the parametric ensemble differences. The dominant pattern of MSLP ensemble spread is associated with the intensity of the pre–tropical depression (pre-TD), explaining nearly half of the total variance at each respective time. Similar modes of variance are found for the low-level absolute vorticity, though the patterns explain substantially less variance. Additionally, the largest modes of variability associated with upper-level temperature anomalies closely resemble the patterns of MSLP variance, suggesting interconnectedness between the two parameters. Sensitivity analyses at both the pre-TD and TCG stages reveal that the MSLP disturbance is strongly correlated to upper-tropospheric temperature and, to a lesser degree, surface latent heat flux anomalies. Further sensitivity analyses uncover a statistically significant correlation between upper-tropospheric temperature and convective anomalies, consistent with the notion that deep convection is important for augmenting the upper-tropospheric warmth during TCG. Overall, the ensemble forecast differences for the TCG of Julia are strongly related to the processes responsible for MSLP falls and low-level cyclonic vorticity growth, including the growth of upper-tropospheric warming and persistent deep convection.

scholarly journals Asymmetric and axisymmetric dynamics of tropical cyclones

2013 ◽  
Vol 13 (24) ◽  
pp. 12299-12341 ◽  
Author(s):  
J. Persing ◽  
M. T. Montgomery ◽  
J. C. McWilliams ◽  
R. K. Smith
Keyword(s):  
Tropical Cyclone ◽  
Heating Rate ◽  
Tropical Cyclones ◽  
Local Heating ◽  
Deep Convection ◽  
Small Scale ◽  
Heating Rates ◽  
Surface Drag ◽  
Mixing Processes ◽  
D Model

Abstract. We present the results of idealized numerical experiments to examine the difference between tropical cyclone evolution in three-dimensional (3-D) and axisymmetric (AX) model configurations. We focus on the prototype problem for intensification, which considers the evolution of an initially unsaturated AX vortex in gradient-wind balance on an f plane. Consistent with findings of previous work, the mature intensity in the 3-D model is reduced relative to that in the AX model. In contrast with previous interpretations invoking barotropic instability and related horizontal mixing processes as a mechanism detrimental to the spin-up process, the results indicate that 3-D eddy processes associated with vortical plume structures can assist the intensification process by contributing to a radial contraction of the maximum tangential velocity and to a vertical extension of tangential winds through the depth of the troposphere. These plumes contribute significantly also to the azimuthally averaged heating rate and the corresponding azimuthal-mean overturning circulation. The comparisons show that the resolved 3-D eddy momentum fluxes above the boundary layer exhibit counter-gradient characteristics during a key spin-up period, and more generally are not solely diffusive. The effects of these eddies are thus not properly represented by the subgrid-scale parameterizations in the AX configuration. The resolved eddy fluxes act to support the contraction and intensification of the maximum tangential winds. The comparisons indicate fundamental differences between convective organization in the 3-D and AX configurations for meteorologically relevant forecast timescales. While the radial and vertical gradients of the system-scale angular rotation provide a hostile environment for deep convection in the 3-D model, with a corresponding tendency to strain the convective elements in the tangential direction, deep convection in the AX model does not suffer this tendency. Also, since during the 3-D intensification process the convection has not yet organized into annular rings, the azimuthally averaged heating rate and radial gradient thereof is considerably less than that in the AX model. This lack of organization results broadly in a slower intensification rate in the 3-D model and leads ultimately to a weaker mature vortex after 12 days of model integration. While azimuthal mean heating rates in the 3-D model are weaker than those in the AX model, local heating rates in the 3-D model exceed those in the AX model and at times the vortex in the 3-D model intensifies more rapidly than AX. Analyses of the 3-D model output do not support a recent hypothesis concerning the key role of small-scale vertical mixing processes in the upper-tropospheric outflow in controlling the intensification process. In the 3-D model, surface drag plays a particularly important role in the intensification process for the prototype intensification problem on meteorologically relevant timescales by helping foster the organization of convection in azimuth. There is a radical difference in the behaviour of the 3-D and AX simulations when the surface drag is reduced or increased from realistic values. Borrowing from ideas developed in a recent paper, we give a partial explanation for this difference in behaviour. Our results provide new qualitative and quantitative insight into the differences between the asymmetric and symmetric dynamics of tropical cyclones and would appear to have important consequences for the formulation of a fluid dynamical theory of tropical cyclone intensification and mature intensity. In particular, the results point to some fundamental limitations of strict axisymmetric theory and modelling for representing the azimuthally averaged behaviour of tropical cyclones in three dimensions.

scholarly journals Airborne Doppler Radar and Sounding Analysis of an Oceanic Cold Front

2008 ◽  
Vol 136 (4) ◽  
pp. 1475-1491 ◽  
Author(s):  
Roger M. Wakimoto ◽  
Hanne V. Murphey
Keyword(s):  
Diabatic Heating ◽  
Doppler Radar ◽  
Frontal Zone ◽  
Cold Front ◽  
Deep Convection ◽  
Dominant Term ◽  
Low Level ◽  
Frontal Zones ◽  
Low Level Jet ◽  
Upper Level

Abstract An analysis of a cold front over the eastern Atlantic Ocean based on airborne Doppler wind syntheses and dropsonde data is presented. The focus and unique aspect of this study is a segment of the front that was near the center of the cyclone. The dual-Doppler wind synthesis of the frontal zone combined with an average dropsonde spacing of ∼30 km covers a total distance of >450 km in the cross-frontal direction. The finescale resolution and areal coverage of the dataset are believed to be unprecedented. The cold front was characterized by a distinct wind shift and a strong horizontal temperature gradient. The latter was most intense aloft and not at the surface, in contrast to the classical paradigm of surface cold fronts. The shear of the alongfront component of the wind was relatively uniform as a function of height within the frontal zone. This observation is contrary to studies suggesting that frontal zones decrease in intensity above the surface. The surface convergence within the frontal zone was weak. This may have been related to the closeness of the analysis region to the surface low pressure. The prefrontal low-level jet and the upper-level polar jet were both shown to be supergeostrophic based on the analysis of the geopotential height field. It is believed that a major contributing factor to the former was the isallobaric wind from the large pressure tendencies associated with the moving cyclone. A dry pocket accompanied by descending air was noted out ahead of the low-level jet. This pocket produced a region of potential instability that could have supported deep convection, although none was observed on this day. The vertical structure of the front revealed couplets of potential vorticity that appeared to be the result of diabatic heat sources from condensation. The diabatic effect in the frontogenesis equation was the dominant term, exceeding the combined effects of the confluence and tilting terms. As a result, an alternating pattern of frontogenesis–frontolysis developed along the flanks of the maxima of diabatic heating. This study highlights the importance of taking diabatic heating into account even in the absence of deep convection.

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