Flux Convergence Research Articles

Instability of a Surface Jet over Rough Topography

Abstract The instability of a surface-trapped jet over rough bottom topography is examined using a linearized quasigeostrophic model. The jet is laterally sheared and thus susceptible to both barotropic and baroclinic instability. The relative magnitude of the two depends on the jet width and on the spectral characteristics and amplitude of the bathymetry. The most unstable eddies in the upper layer are typically smaller over bathymetry than with a flat bottom. Topography also alters momentum flux convergence in the upper layer and causes the perturbations to resemble eddies in a 1.5-layer flow. But as long as the jet is wider than the deformation radius, baroclinic instability is present, yielding deep eddies that are phase-locked to those at the surface. In addition, topography facilitates scattering of energy at depth to other scales. So, instability over rough topography could be an efficient, and largely overlooked, means of transferring mesoscale energy to the dissipative scales. Significance Statement This study investigates the effects of bottom roughness on large-scale ocean currents and their associated eddies. Roughness affects the eddy size and speed and how they exchange energy with the mean flow. Roughness also facilitates energy transfer to smaller scales where it can be dissipated. Thus, instability over rough topography could be an important part of the oceanic energy balance.

Dynamic Downscaling Simulation and Projection of Precipitation Extremes Over China Under a Shared Socioeconomic Pathway Scenario

AbstractIn this study, Weather Research and Forecasting (WRF) ‐based dynamic downscaling simulation was performed over China at a horizontal resolution of 25 km. To reduce the systematic large‐scale biases from lateral boundary conditions, a dynamic blending (DB) technique was introduced in downscaling, and its performance was compared with downscaling without DB (NO_DB) as well as the driving global climate model (GCM, i.e., HadGEM3). In the present‐day simulation for verification, added values were found in DB, relative to the GCM and NO_DB, in simulating the precipitation extremes, especially over southeastern China. Possible causes responsible for this improvement were further analyzed. In the GCM, excessive moisture and atmospheric heating in the upper troposphere in conjunction with abnormally strong deep convection resulted in excessive extreme precipitation over southeastern China, while in NO_DB, insufficient moisture and atmospheric heating in the whole troposphere in conjunction with abnormally weak convection suppressed extreme precipitation there. In comparison, DB showed a closer representation of observations in terms of vertical velocity and vertical profiles of atmospheric moisture and heating, accounting for the improved simulation of extreme precipitation. In the future projection under the SSP5‐8.5 scenario, both the GCM and DB predicted that extreme precipitation would increase in most parts of China; however, DB indicated a larger increase over southeastern China relative to the GCM. Stronger moisture flux convergence over southeastern China in DB accounted for this larger increase, and in addition, the thermodynamic effect associated with more precipitable water dominated the stronger moisture flux convergence.

A storm-centered multivariate modeling of extreme precipitation frequency based on atmospheric water balance

Abstract. Conventional rainfall frequency analysis faces several limitations. These include difficulty incorporating relevant atmospheric variables beyond precipitation and limited ability to depict the frequency of rainfall over large areas that is relevant for flooding. This study proposes a storm-based model of extreme precipitation frequency based on the atmospheric water balance equation. We developed a storm tracking and regional characterization (STARCH) method to identify precipitation systems in space and time from hourly ERA5 precipitation fields over the contiguous United States from 1951 to 2020. Extreme “storm catalogs” were created by selecting annual maximum storms with specific areas and durations over a chosen region. The annual maximum storm precipitation was then modeled via multivariate distributions of atmospheric water balance components using vine copula models. We applied this approach to estimate precipitation average recurrence intervals for storm areas from 5000 to 100 000 km2 and durations from 2 to 72 h in the Mississippi Basin and its five major subbasins. The estimated precipitation distributions show a good fit to the reference data from the original storm catalogs and are close to the estimates from conventional univariate GEV distributions. Our approach explicitly represents the contributions of water balance components in extreme precipitation. Of these, water vapor flux convergence is the main contributor, while precipitable water and a mass residual term can also be important, particularly for short durations and small storm footprints. We also found that ERA5 shows relatively good water balance closure for extreme storms, with a mass residual on average 10 % of precipitation. The approach can incorporate nonstationarities in water balance components and their dependence structures and can benefit from further advancements in reanalysis products and storm tracking techniques.

Lp-CMFD acceleration schemes in multi-energy group 2D Monte Carlo transport

The linear prolongation flux update scheme is extended to both regular CMFD acceleration, as well as partial CMFD acceleration in 2D multi energy group Monte Carlo k-eigenvalue neutron transport problems. The acceleration performance of these CMFD variants were investigated in simple 2D slab geometries, first with a monoenergetic case and then with a three group problem on the same geometry based on the monoenergetic cross sections. Flux convergence was determined via an on-the-fly convergence diagnostics developed by Ueki and Brown. It is found that on top of providing better acceleration in general, the linear prolongation scheme is also able to correct for instabilities in the CMFD scheme. Overall, the lp-pCMFD scheme employing a maximum history length is found to have the best performance across the cases presented.

Two Extratropical Pathways to Forcing Tropical Convective Disturbances

Abstract Observational evidence of two extratropical pathways to forcing tropical convective disturbances is documented through a statistical analysis of satellite-derived OLR and ERA5 reanalysis. The forcing mechanism and the resulting disturbances are found to strongly depend on the structure of the background zonal wind. Although Rossby wave propagation is prohibited in easterlies, modeling studies have shown that extratropical forcing can still excite equatorial waves through resonance between the tropics and extratropics. Here this “remote” forcing pathway is investigated for the first time in the context of convectively coupled Kelvin waves over the tropical Pacific during northern summer. The extratropical forcing is manifested by eddy momentum flux convergence that arises when extratropical eddies propagate into the subtropics and encounter their critical line. This nonlinear forcing has similar wavenumbers and frequencies with Kelvin waves and excites them by projecting onto their meridional eigenstructure in zonal wind, as a form of resonance. This resonance is also evidenced by a momentum budget analysis, which reveals the nonlinear forcing term is essential for maintenance of the waves, while the remaining linear terms are essential for propagation. In contrast, the “local” pathway of extratropical forcing entails the presence of a westerly duct during northern winter that permits Rossby waves to propagate into the equatorial east Pacific, while precluding any sort of resonance with Kelvin waves due to Doppler shifting effects. The intruding disturbances primarily excite tropical “cloud plumes” through quasigeostrophic forcing, while maintaining their extratropical nature. This study demonstrates the multiple roles of the extratropics in forcing in tropical circulations and illuminates how tropical–extratropical interactions and extratropical basic states can provide be a source of predictability at the S2S time scale. Significance Statement This study seeks to understand how circulations in the midlatitudes excite the weather systems in the tropics. Results show that the mechanisms, as well as the types of tropical weather systems excited, are strongly dependent on the mean large-scale wind structure. In particular, when the large-scale wind blows from east to west, a special type of eastward-moving tropical weather system, the Kelvin wave, is excited owing to its resonance with remote eastward-moving weather systems in the extratropics. On the contrary, when the average wind blows from west to east, midlatitude systems are observed to intrude into the lower latitudes and directly force tropical convection, the cloud plumes, while maintaining their extratropical nature. These results speak to how the midlatitudes can excite distinct types of tropical weather systems under different climatological wind regimes. Understanding these tropical weather systems and their interactions with the midlatitudes may ultimately help to improve predictions of weather beyond 2 weeks.

Evaluation of IMERG Over CONUS Complex Terrain Using Environmental Variables

AbstractSatellite‐based precipitation products (SPPs) provide extensive spatial and temporal coverage globally but are challenged in complex terrain. This study aims at understanding uncertainties of SPP estimation due to topographically induced rainfall. Environmental and physical parameters, that is, orographically forced upward motion and horizontal moisture flux convergence, are considered to condition precipitation mechanisms for a detailed uncertainty analysis. The Global Precipitation Measurement (GPM) precipitation product Integrated Multi‐Satellite Retrievals for GPM (IMERG) is compared at its native resolution against a high quality and accurate radar and rain gauge precipitation reference Ground Validation Multi‐Radar Multi‐Sensor (GV‐MRMS). Conditional analysis of IMERG using environmental parameters is conducted over mountainous terrain. The highest GV‐MRMS mean rainfall rate is found to be associated with positive high Q and moderate w values, confirming that vigorous moisture flux convergence is a strong condition for heavy rainfall. IMERG shows over‐ and underestimation for positive w and Q when GV‐MRMS rainfall magnitudes between 0.8 and 13 mm h−1.

Computational simulation of aircraft electrothermal de-icing using an unsteady formulation of phase change and runback water in a unified framework

Accurately predicting de-icing processes is essential to ensure the proper sizing and design of ice protection systems in aircraft icing. A unified framework was developed to simulate an unsteady electrothermal de-icing process, using an unsteady formulation to account for phase change and runback water. Two physically-motivated concepts were newly introduced to the icing model to accurately describe the unsteady electrothermal de-icing (ice accretion/melting) process. A conjugate heat transfer method was utilized to tightly couple the ice and conduction solvers. Sub-iterations were incorporated at every time step to ensure the convergence of temperature and heat flux at the interface. The unsteady de-icing framework consists of a compressible Navier-Stokes-Fourier airflow solver, Eulerian droplet impingement solver, unsteady ice accretion/melting solver, and heat conduction solver, which can handle multilayer composite materials. All solvers were formulated based on partial differential equations and developed in a unified finite volume framework, enabling the use of a single grid system and eliminating unnecessary grid generation for the ice layer. The results showed better agreement with experimental data compared to other results. The unified solver was used to analyze the unsteady electrothermal de-icing process by investigating the ice accretion, ice melting, runback water film, and freezing of water film in unprotected areas. Runback water film due to ice melting may exceed the impingement limits and freeze in unprotected areas, leading to ice ridge formation.

Extreme Precipitation Events on the East Coast of Brazil’s Northeast: Numerical and Diagnostic Analysis

The Northeast of Brazil (NEB) is the region with the highest number of municipal decrees of emergency situation declaration caused by weather events in the period from 2013 to 2022 and with the highest rate of natural disasters per risk area. In the NEB, the city of Recife and its metropolitan region are the biggest localities with populations in risk areas. Focusing on this region, five events of extreme precipitation were chosen for simulations using the WRF model and diagnostics analyses. First, a set of configurations of the model was tested, including 11 microphysics (MPH) schemes, 9 planetary boundary layer (PBL) schemes, 5 cumulus (CUM), and 7 surface layer (SFC) schemes. Then, through diagnostic analysis, the conditional instability, the moisture supply at low levels, and the support of the medium and high levels in storm formation were verified. The model’s configurations were verified by 298 rain gauges with hourly registrations through statistical metrics such as bias, MSE, standard deviation, and Pearson’s correlation, and demonstrated that the MPH schemes of Thompson Aerosol-Aware and NSSL + CCM, ACM2, MYJ for the PBL, KFCuP for CUM, and RUC for SFC were considered the best. All the cases were better with CUM parametrizations turned on. In all cases, diagnostics analyses highlighted the strong moisture flux convergence at the low levels, the presence of wind shear on the middle layer, weak cyclonic vorticity advection at high levels, and CAPE values around 1500 J/kg, in addition to an inverse relationship between wind shear action and CAPE values. This work is part of the national strategy for monitoring, diagnosis, and modeling of information that can minimize or even prevent damage caused by severe precipitation events.

The frictional layer in the observed momentum budget of the trades.

Profiles of eddy momentum flux divergence are calculated as the residual in the momentum budget constructed from airborne circular dropsonde arrays ( 220 km) for 13 days during the EUREC A/ATOMIC field campaign. The observed dynamical forcing averaged over all flights agrees broadly with European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) forecasts. In the direction of the flow, a mean flux divergence (friction) exists over a 1.5-km deep Ekman layer, and a mean flux convergence (acceleration) is present near cloud tops. The friction is countergradient between 1 and 1.5 km, where vertical wind shear exceeds the observed thermal wind. From the frictional profile, a 10-m momentum flux of 0.1 N m is derived, in line with Saildrone turbulence measurements. A momentum flux divergence in the cross-wind direction is pronounced near the surface and acts to veer the wind, opposing the friction-induced cross-isobaric wind turning. Weaker friction and upper-level acceleration of easterly flow are observed when stronger winds and more vigorous convection prevail. Turbulence measurements on board the SAFIRE ATR-42 aircraft and the Uncrewed Aircraft System (UAS) RAAVEN reveal pronounced spatial variability of momentum fluxes, with a non-negligible contribution of mesoscales (5-30 km). The findings highlight the nontrivial impact of turbulence, convection, and mesoscale flows in the presence of diverse cloud fields on the depth and strength of the frictional layer.

Evaluation of ECMWF Lightning Flash Forecast over Indian Subcontinent during MAM 2020

During the pre-monsoon season (March–April–May), the eastern and northeastern parts of India, Himalayan foothills, and southern parts of India experience extensive lightning activity. Mean moisture, surface and upper-level winds, the sheared atmosphere in the lower level, and high positive values of vertically integrated moisture flux convergence (VIMFC) create favorable conditions for deep convective systems to occur, generating lightning. From mid-2018, the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS) operationally introduced lightning flash density on a global scale. This study evaluates the ECMWF lightning forecasts over India during the pre-monsoon season of 2020 using the Indian Institute of Tropical Meteorology (IITM) Lightning Location Network (LLN) observation data. Qualitative and quantitative analysis of the ECMWF lightning forecast has shown that the lightning forecast with a 72-h lead time can capture the spatial and temporal variation of lightning with a 90% skill score.

The Arctic Surface Heating Efficiency of Tropospheric Energy Flux Events

Abstract This paper examines the processes that drive Arctic anomalous surface warming and sea ice loss during winter-season tropospheric energy flux events, synoptic periods of increased tropospheric energy flux convergence (Ftrop), using the NASA MERRA-2 reanalysis. During an event, a poleward anomaly in Ftrop initially increases the sensible and latent energy of the Arctic troposphere; as the warm and moist troposphere loses heat, the anomalous energy source is balanced by a flux upward across the tropopause and a downward net surface flux. A new metric for the Arctic surface heating efficiency (Etrop) is defined, which measures the fraction of the energy source that reaches the surface. Composites of high-, medium-, and low-efficiency events help identify key physical factors, including the vertical structure of Ftrop and Arctic surface preconditioning. In high-efficiency events (Etrop ≥ 0.63), a bottom-heavy poleward Ftrop occurs in the presence of an anomalously warm and unstratified Arctic—a consequence of decreased sea ice—resulting in increased vertical mixing, enhanced near-surface warming and moistening, and further sea ice loss. Smaller Etrop, and thus weaker surface impacts, are found in events with anomalously large initial sea ice extent and more vertically uniform Ftrop. These differences in Etrop are manifested primarily through turbulent heat fluxes rather than downward longwave radiation. The frequency of high-efficiency events has increased from the period 1980–99 to the period 2000–19, contributing to Arctic surface warming and sea ice decline.

Extreme large-scale atmospheric circulation associated with the “21·7” Henan flood

From July 19 to 21, 2021, Henan, a province in northern China (NC), was affected by severe flooding (referred to hereafter as “21·7”) caused by a prolonged record-breaking extreme precipitation (EP) event. Understanding the extremes of the large-scale circulation pattern during “21·7” is essential for predicting EP events and preventing future disasters. In this study, daily atmospheric large-scale circulations over NC in the summers from 1979 to 2021 were investigated using the circulation classification method of an obliquely rotated principal component analysis in T-mode (PCT). The geopotential height at 500 hPa and 925 hPa were applied successively in classification. Among the nine summer circulation patterns at 500 hPa were found, the 3 days of “21·7” belonged to the Type 8 pattern, which had the second highest probability of EP days among all patterns. It was characterized by a southeasterly wind toward North China Plain driven by a dipole geopotential height field, with the West Pacific subtropical high (WPSH) extending far north to 30°N and low to the south near NC. Tropical cyclones (TCs) occurred on 72.5% of EP days, in which larger amounts of precipitation and a longer duration of EP days were found along the mountains in NC, as compared with other patterns. The distribution of EP events under this pattern was mainly influenced by the location of the low at 925 hPa in the dipole. The subtype 8−3 circulation, with lows in the east of Taiwan Island, included “21·7” and accounted for 1.6% of all summer days. Typhoon In-fa, together with the WPSH, gave rise to intense column integrated moisture flux convergence (IMFC) via the southeasterly wind to Henan, which occurred continuously during the 3 days of “21·7”, resulting in the largest (second largest) mean IMFC among 3 consecutive EP days under type 8 (all types) during the past 43 summers in NC. Further analysis revealed that the large-scale dynamic process could not completely explain the record-breaking EP during “21·7”, indicating possible contributions of other dynamic processed related to meso-scale convective storms.

Distinct roles of global cyclonic and anticyclonic eddies in regulating near-inertial internal waves in the ocean interior

In this study, we examine the regulation of wind-powered near-inertial internal waves (NIWs) in the ocean interior by mesoscale eddies, using an eddy-resolving global climate model. There is an enhancement (weakening) of NIW activity within anticyclonic (cyclonic) eddies from the surface boundary layer (SBL) base down to ~4,000 m, associated with a decreased (increased) wave vertical scale. The influence of anticyclonic eddies on NIW activity is more pronounced than that of cyclonic eddies, suggesting that the net effect of mesoscale eddies is to intensify the NIWs in the ocean interior. The enhanced (weakened) NIW activity under the SBL in anticyclonic (cyclonic) eddies is ascribed to both horizontal convergence (divergence) of NIW energy flux under the SBL and increased (decreased) downward NIW energy flux from the SBL base. The former is only confined to a layer within several hundred meters under the SBL, whereas the latter becomes dominant in the deeper ocean.

Flood Seasonality Over the Third Pole Region Modulated by Upper Level Moisture Transport

AbstractThe nexus between atmospheric moisture transport and basin‐scale flood response is still lacking over the Third Pole (TP) region, despite projected increases in extreme rainfall under a changing climate. Based on long‐term daily streamflow observations, we show that peaks‐over‐threshold floods over the Yarlung Zangbo River (YZR) basin show two temporal clusters, that is, July and late August, with the flood magnitudes of the second cluster larger than the first by 20%. These floods are resultant from a mixture of flood‐producing storms with contrasting storm motion and moisture transport pathways. A suite of coupled atmospheric‐hydrological modeling experiments further quantify the impacts of different moisture transport pathways on the space‐time rainfall structure and flood response. A 50% reduction of upper level moisture transport (i.e., through the upslope of the Himalayas) leads to decreased flood peak magnitudes by 30% at the basin outlet for the August 1998 floods over the YZR basin. Upper‐level moisture transport is associated with the notable low pressure over the central‐east India and westward extension of West Pacific Subtropical High that enhances convergence of water vapor fluxes from Bay of Bengal toward TP. The magnitude of impact can potentially alter the seasonal clustering of high‐flow events over TP. Our analyses contribute to improved characterization of flood risk over TP, and advances in flood frequency hydrology.

Circulation Patterns Linked to the Positive Sub-Tropical Indian Ocean Dipole

The positive phase of the subtropical Indian Ocean dipole (SIOD) is one of the climatic modes in the subtropical southern Indian Ocean that influences the austral summer inter-annual rainfall variability in parts of southern Africa. This paper examines austral summer rain-bearing circulation types (CTs) in Africa south of the equator that are related to the positive SIOD and the dynamics through which specific rainfall regions in southern Africa can be influenced by this relationship. Four austral summer rain-bearing CTs were obtained. Among the four CTs, the CT that featured (i) enhanced cyclonic activity in the southwest Indian Ocean; (ii) positive widespread rainfall anomaly in the southwest Indian Ocean; and (iii) low-level convergence of moisture fluxes from the tropical South Atlantic Ocean, tropical Indian Ocean, and the southwest Indian Ocean, over the south-central landmass of Africa, was found to be related to the positive SIOD climatic mode. The relationship also implies that positive SIOD can be expected to increase the amplitude and frequency of occurrence of the aforementioned CT. The linkage between the CT related to the positive SIOD and austral summer homogeneous regions of rainfall anomalies in Africa south of the equator showed that it is the principal CT that is related to the inter-annual rainfall variability of the south-central regions of Africa, where the SIOD is already known to significantly influence its rainfall variability. Hence, through the large-scale patterns of atmospheric circulation associated with the CT, the SIOD can influence the spatial distribution and intensity of rainfall over the preferred landmass through enhanced moisture convergence.

Dynamic and Thermodynamic Factors Involved in Future Changes in Extreme Summertime Precipitation in Japan Projected by Convection-Permitting Regional Climate Model Simulations

Abstract Dynamic and thermodynamic factors involved in future changes in local-scale short-term extreme summertime precipitation on the mesoscale and hourly time scale in Japan were examined using convection-permitting regional climate model simulations under the representative concentration pathway 8.5 scenario. The change in the dynamic component primarily contributes to the total change in vertically integrated moisture flux convergence over the analysis domain that is located off Okinawa Island, whereas the thermodynamic component is dominant over the analysis domain that is located off Kyushu Island. Differences in the amount of the dynamic and thermodynamic components are noticeable in these two domains. These results are explained by the difference in the vertical profiles of the convergence term, and hence the convergence itself between the two specific domains. A mesoscale low pressure system on the seasonal rain front—termed the baiu front—is a key factor underlying the difference in the magnitudes and vertical profiles of convergence between the two specific domains. In the vicinity of the domain off Okinawa Island, a mesoscale low pressure system on the baiu front enhances low-level convergence in the future climate when compared with the present climate. This atmospheric state is attributable to the location of the baiu front itself, which is located relatively southward in the future climate and is affected by the domain off Okinawa Island. In the domain where the dynamic component is dominant such as the domain off Okinawa Island, the total moisture flux convergence follows a super–Clausius–Clapeyron scaling.

The Remote Effect of Binary Typhoon Infa and Cempaka on the “21.7” Heavy Rainfall in Henan Province, China

AbstractIn this study, the effect of binary Typhoons Infa and Cempaka on the “21.7” heavy rainfall event over Henan Province in China is investigated based on numerical simulations. According to the rain gauge data, the temporal‐spatial distribution of precipitation in the targeted area and the large‐scale circulation in the control run show reasonable agreement with the observations. By separately excluding the relative circulation of Typhoon Infa (N_Infa) and Cempaka (N_Cempaka) at initialization, less convergence of water vapor flux within the targeted area is obtained, leading to a reduction in accumulated precipitation and rainfall intensity, especially for the mid‐to‐late raining period. Plenty of moisture mainly comes from the south and east of the rainfall region. In terms of the 60‐hr back trajectories, an overwhelming proportion of moist air parcels entering the targeted area at 850 hPa is transported along the easterly flow, which prevails between Typhoon Infa and the subtropical high. A growing proportion of air parcels advected from the south band highlights the nonnegligible effect of Typhoon Cempaka and the southwesterly flow to water vapor supply at relatively higher levels. Additional test by excluding both typhoons at initialization (N_Infa&Cempaka) suggests the complicated interaction between binary typhoons and the surrounding large‐scale circulation (such as subtropical high) may influence the initiation and development of mesoscale convection in the targeted area. With the enhancement of convergence by southwesterly and easterly flow at the lower layer from N_Infa&Cempaka, larger accumulated precipitation is obtained with the shifted location of extreme rainfall compared to that from N_Infa.

Diurnal Variation Mechanisms of the Zonal Shear Line Inducing Anomalous Heavy Precipitation Over the Tibetan Plateau in Boreal Summer

AbstractThe Tibetan Plateau (TP) is one of the regions with the most remarkable diurnal variations in the global atmosphere. The zonal shear line (ZSL) is the primary synoptic system inducing precipitation over the TP in boreal summer. It is of great significance in studying its diurnal variation. This study objectively identified and composited 11 ZSL cases inducing anomalous heavy precipitation based on the ERA5 hourly reanalysis datasets from June to August during 1980–2019. The diurnal variation characteristics and mechanisms of ZSL’s intensity were explored based on the composited ZSL. Results suggest that the intensity of ZSL has significant diurnal variation, reaching the peak at midnight (about 23 Local Solar Time). The dynamical and thermal structures near the ZSL both show significant diurnal variations. There is a significant heating effect near the ZSL after sunrise, followed by strong divergence (convergence) in the upper (lower) layer and significant increases in water vapor flux convergence and ascending motions. The vorticity is strongest at midnight. Diagnosis by the complete‐form vertical vorticity tendency equation reveals that the thermal effect, dominated by vertically non‐uniform distribution of atmospheric diabatic heating, is the most crucial factor affecting the diurnal variation of ZSL’s intensity. The configuration of solar radiation and sensible heat in the near‐ground layer and latent heat in the middle and upper layers jointly dominate the vertically non‐uniform heating. The horizontally non‐uniform heating only contributes little to the local change of vorticity at 500 hPa. Ascending motion and vorticity advection have only weak effects.

Physical mechanism of vertical gradient of pressure flux and its impact on turbulent flux estimation

• Flux-variance method is an effective way to indirectly obtain pressure flux. • Pressure flux convergence/divergence is partially related to larger-scale eddies. • Ejection and sweep events lead to opposite vertical gradient of pressure flux. • There's a quadratic relation between pressure flux gradient and friction velocity. • Taking pressure transport into account refines estimation of turbulent flux in CBL. Using multi-layer pressure fluctuations and other conventional turbulence observational data over a nearly flat and homogeneous dune underlying surface, the temporal and spatial characteristics of pressure flux and its profile were analyzed to illustrate the physical mechanism of pressure flux convergence/divergence and the impact of pressure transport terms on the estimation of turbulent fluxes in this study. The flux-variance (FV) method is an effective approach to indirectly obtain pressure flux through momentum flux when no fast-response observations of pressure fluctuations are available. The fitted functions are w ′ p ′ ¯ = 0.16 · ( − F Z ( z ) · u ′ w ′ ¯ σ p σ u ) and u ′ p ′ ¯ = 0.55 · ( F X ( z ) · u ′ w ′ ¯ σ p σ w ) , where F Z ( z ) and F X ( z ) are the height normalized functions for vertical and horizontal directions. Under the circumstance of pressure flux divergence (FD), the transport efficiency of pressure and contribution of larger-scale eddies to pressure flux at higher levels are greater than those at lower levels, and flows in the atmospheric surface layer (ASL) are mainly dominated by the sweep events; while the pressure flux convergence (FC) cases are exactly the opposite situations. A good quadratic function relationship was found between the vertical gradient of pressure flux and friction velocity under different atmospheric stabilities: ∂ w ′ p ′ ¯ ∂ z = 0.01 u * ( 16 u * − 1 ) . Turbulence kinetic energy (TKE) and its vertical transport are greater in the FD cases than those in the FC cases. The momentum and sensible heat fluxes in the convective boundary layer (CBL) were overestimated by their common second-order diagnostic equations and Rotta model (pressure redistribution term parameterization) which neglect the diffusion terms. After adding the contribution of pressure transport terms into the diagnostic equations and Rotta model and modifying the original Rotta constants, the overestimations were well corrected and the estimated turbulent fluxes were more consistent with the observations. This work provides a novel direction and idea for the future improvement of the planetary boundary layer (PBL) parameterization schemes in many numerical weather and climate models.

An Analog Comparison between Rapidly and Slowly Intensifying Tropical Cyclones

Abstract To better understand the conditions that favor tropical cyclone (TC) rapid intensification (RI), this study assesses environmental and storm-scale characteristics that differentiate TCs that undergo RI from TCs that undergo slow intensification (SI). This comparison is performed between analog TC pairs that have similar initial intensity, vertical wind shear, and maximum potential intensity. Differences in the characteristics of RI and SI TCs in the North Atlantic and western North Pacific basins are evaluated by compositing and comparing data from the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA5) and the Gridded Satellite (GridSat) dataset. In the period leading up to the start of RI, RI TCs tend to have a stronger and deeper vortex that is more vertically aligned than SI TCs. Additionally, surface latent heat fluxes are significantly larger in RI TCs prior to the intensity change period, compared to SI TCs. The largest surface latent heat flux differences are initially located to the left of shear; subsequently, upshear and right-of-shear differences amplify, resulting in a more symmetric distribution of surface latent heat fluxes in RI TCs. Increasing azimuthal symmetry of surface latent heat fluxes in RI TCs, together with an increasing azimuthal symmetry of horizontal moisture flux convergence, promote the upshear migration of convection in RI TCs. These differences, and their evolution before and during the intensity change period, are hypothesized to support the persistence and invigoration of upshear convection and, thus, a more symmetric latent heating pattern that favors RI.