Monthly Weather Review

doi: 10.1175/mwr-1532mastheadpmid: N/A

Rohith, B.; Gasparin, Florent; Ruggiero, Giovanni; Remy, Elisabeth; Cravatte, Sophie

doi: 10.1175/mwr-d-24-0027.1pmid: N/A

AbstractThis study investigates the ability of a global ocean reanalysis at 1/12° horizontal resolution, GLORYS12, to represent oceanic processes at intraseasonal and higher-frequency scales. GLORYS12, which includes data assimilation of satellite and multi-instrument in situ observations, is compared to a twin-free simulation (with no assimilation) in the tropical Pacific Ocean. Spectral analyses show that data assimilation improves the realism of sea surface height intraseasonal variability in the entire tropical Pacific Ocean, in both amplitude and phase, with an increase in the amplitude of more than 50% for the 20–90-day band and up to 15% for the 2–20-day band. The improvement is largest along the 5°N/S latitudes, where the magnitude of tropical instability waves is maximum, but is limited along the equator where steric height variability is dominated by intraseasonal oceanic Kelvin waves, already well represented in the free simulation. Wavenumber–frequency spectra show that data assimilation constraint improves both the spatial and temporal scales of intraseasonal waves and their timing. Data assimilation impacts the realism of oceanic simulations in two ways. By modifying the background oceanic stratification, it corrects the phase speed of westward-propagating waves. It is also shown that the intraseasonal component of analysis increments (data assimilation corrections applied) is dynamically consistent and exhibits clear intraseasonal propagation. By demonstrating the benefits of data assimilation for intraseasonal processes in the tropical Pacific Ocean, this study highlights the high value of both in situ and satellite observations to constrain ocean models in a wide range of time scales.

Fischer, Michael S.; Reasor, Paul D.; Dunion, Jason P.; Rogers, Robert F.

doi: 10.1175/mwr-d-24-0118.1pmid: N/A

AbstractThe largest tropical cyclone (TC) intensity forecast errors are typically associated with episodes of rapid intensification (RI). Here, we explore whether TCs that undergo RI are associated with different vortex and convective characteristics compared to TCs that are either slowly intensifying (SI) or nonintensifying (NI). Because characteristics of TC structure are strongly linked to intensity, a normalization technique is employed to examine how the anomalous TC structure, relative to the average structure for a given intensity, is related to TC intensity change. TC structure is assessed using a recently developed database of airborne Doppler radar analyses collected by NOAA’s WP-3D aircraft over the last three decades, primarily in the North Atlantic basin. We find that RI episodes are associated with significantly taller and narrower primary circulations than both SI and NI episodes. RI episodes also tend to occur in storms with anomalously deep overturning circulations and larger azimuthally averaged ascent in the upper troposphere than SI and NI episodes. Additionally, the inner core of RI TCs exhibits a significantly greater areal coverage of convective bursts, which are defined as locations with relatively vigorous ascent in the mid–upper troposphere. Ultimately, TC intensity change is governed by multiscale processes, and RI is found to occur preferentially in TCs that have both favorable synoptic-scale environmental conditions and favorable vortex structures for intensification. Consequently, observations of the anomalous TC structure may serve as useful predictors for TC intensity change and RI.Significance StatementCurrent forecasts of tropical cyclone (TC) intensity have the largest errors when storms experience periods of rapid intensification (RI). This study explores whether TCs that undergo RI have unique vortex and convective characteristics compared to TCs that either slowly intensify or those that fail to intensify altogether. It is found that TCs which undergo RI tend to have taller and narrower vortex structures than non-RI storms. TCs that undergo RI also display stronger ascent in the upper troposphere than non-RI storms. These characteristics may serve as useful predictors for TC intensity change and forecasting whether RI will occur.

Lopez, Andres; Kirshbaum, Daniel J.; Lareau, Neil

doi: 10.1175/mwr-d-24-0125.1pmid: N/A

AbstractThe north–south-oriented Sierras de Córdoba (SDC) ridge in central Argentina is noted for initiating thunderstorms that may grow into intense mesoscale convective systems (MCSs). It also initiates more isolated, shorter-lived cells under weaker synoptic forcing. These cells are less impactful than MCSs but may be difficult to predict in convective-scale numerical weather prediction (NWP) due to their strong sensitivities to subgrid and partially resolved processes. To study the mechanisms and predictability of such cells, convection-permitting ensemble simulations were conducted of an isolated, diurnally forced SDC thunderstorm during Cloud, Aerosol, and Complex Terrain Interactions (CACTI)/Remote Sensing of Electrification, Lightning, and Mesoscale/Microscale Processes with Adaptive Ground Observations (RELAMPAGO). The rich observational data facilitated detailed ensemble verification, where dry biases in the surface energy balance and soil moisture were identified. These biases promoted rapid removal of convective inhibition and an early onset of precipitating cells over the SDC that were shallower and weaker than the observed cell. Correction, and then overcorrection, of the soil moisture bias in two successive ensembles was required to rectify the surface energy balance and improve the representation of the SDC cell. Nevertheless, substantial ensemble variability in convective precipitation was found, with some members producing more widespread convection than observed and others producing no deep convection at all. This variability was largely explained by a combination of thermodynamic and dynamic mechanisms, dominated by a positive sensitivity of convective precipitation to preconvective moist instability over the ridge. Secondary sensitivities were found to low-level upward mass flux and midlevel cross-barrier winds, the latter of which caused gravity waves with elevated downdrafts that tended to suppress incipient clouds.

Munsell, Erin B.; Braun, Scott A.; Greenwald, Thomas; Bennartz, Ralf; Blackwell, William J.

doi: 10.1175/mwr-d-23-0191.1pmid: N/A

AbstractThis study examines the three-dimensional thermodynamic structure of a rapidly intensifying tropical cyclone (TC) through the utilization of synthetic retrievals based off of the specifications of NASA’s Time-Resolved Observations of Precipitation Structure and Storm Intensity with a Constellation of Smallsats (TROPICS) mission. Proxy TROPICS vertical profiles of temperature and water vapor mixing ratio generated from the Hurricane Nature Run (HNR1) are utilized to analyze the TC structure over a 10-day period that includes the HNR1 TC’s rapid intensification (RI) from a tropical storm to a major hurricane. Analyses are performed to assess how accurately TROPICS may be able to determine thermodynamic profiles both within the storm and in the environment by validating against the HNR1 model data. It is found that the TROPICS retrievals compare favorably with the HNR1 data at most heights and times with errors consistently less than the proposed mission requirements (2 K for temperature; 25% for humidity). In addition, the retrievals show the ability to qualitatively track extensive dry air that is present in the vicinity of the TC. Although a substantial dry bias is present within the storm region of the TC (between 0 and 200 km from the surface center) in the 350–550-hPa layer in the TROPICS retrievals, this bias is reduced when the retrievals associated with precipitating grid points are removed from the analyses. However, despite this filtering, a significant bias remains, which suggests that the TROPICS retrievals will likely lose accuracy in regions of stronger scattering.

Wu, Pin-Ying; Kawabata, Takuya; Duc, Le

doi: 10.1175/mwr-d-24-0067.1pmid: N/A

AbstractEnsemble simulations involve perturbations of error sources in numerical models to represent uncertainties. The rank of the perturbation matrix is expected to be equal to the ensemble size; thus, each ensemble member may have an independent perturbation. However, ensembles without independent perturbations for each member have been commonly adopted, and their impacts remain unknown. This study explores how a lack of perturbation rank affects the quality of an ensemble and its estimates. Focusing on mesoscale weather forecasts, lateral boundary conditions (LBCs) were considered since they are a primary source of uncertainty in regional models. Two sets of 1000-member ensemble simulations of Typhoon Hagibis (2019) were performed with different ranks of LBC perturbations (LBPs). A lack of perturbation rank caused members to cluster based on the given LBC because the LBCs strongly constrained the simulated states. This clustering is a sign of poor orthogonality, that is, a deficiency in the effective ensemble size. Clustering also implies distortion in the ensemble error space. A resulting spurious probability distribution can occur even when the ensemble mean and spread are valid. From the perspective of physical space, spurious probability distributions are caused by the clustering of typhoon locations and synoptic-scale environmental conditions owing to the lack of an LBP rank. Additionally, using positive and negative perturbation pairs was one cause of the perturbation rank defect. Our findings highlight the importance of the perturbation rank, particularly for high-order estimates of probability distributions with large ensembles.Significance StatementEnsemble methods address uncertainty in weather forecasts by presenting possible scenarios through simulations with small differences or perturbations in numerical model components that affect forecasts. This study examines how a lack of independent perturbations in the model’s lateral boundary conditions affects the quality of a large ensemble with 1000 scenarios of typhoon forecasts. The results demonstrated that a lack of independent perturbations damaged the ensemble scenario variety. This damage led to spurious estimates of ensemble probability distributions. The findings suggest that a large ensemble requires independent perturbations corresponding to its size to adequately provide advanced probability estimates.

Knupp, Kevin; Wingo, Stephanie M.; Goudeau, Barrett; Pangle, Preston

doi: 10.1175/mwr-d-24-0007.1pmid: N/A

AbstractObservations obtained from scanning radars, two profiling systems, surface, and radiosonde instruments are utilized to describe the weakening stages of a long-lived (>4 h), shallow bore that evolved within a low-shear environment on 23 August 2013 over northern Alabama. RHI scans from a scanning X-band radar provided details on the bore shape and surrounding mesoscale perturbations in airflow, while a Doppler lidar afforded high-resolution vertical motion profiles at a location 24 km away. During passage over both profiling sites, updrafts (∼2 m s−1) were sampled over the lowest several hundred meters AGL, and aerosol backscatter showed upward displacements of 300–400 m near the 1.2 km AGL level. The surface pressure rise of ∼0.3 hPa at multiple locations over the observational network corroborates the shallowness of both the bore and the surface-based inversion. Several other unique features were documented, including bore movement in the direction of the weak low-level flow; a gust front structure with a well-defined feeder flow trailing the bore gust front, both confined to the lowest 500 m AGL; the presence of waves preceding and following the bore disturbance, with a 12–21-min period within the 0.5–2.5-km layer above the bore at both profiler locations; and variations in the horizontal flow in the form of a weak jet near 1.4 km AGL, with winds in the jet aligned in the same direction as bore movement, and located more than 10 km ahead of the bore gust front.Significance StatementThis study documents a shallow bore utilizing a suite of ground-based remote sensing instruments. A scanning X-band radar, important in this case, defined the shape and the horizontal/vertical extent of perturbation flows associated with the bore. The bore remained identifiable as a radar fine line for a period of around 4.5 h, despite evolving within a low-shear environment. One implication of this work is that weak/shallow bores may be more common than previously realized.

Chen, Yueh-Li; Wu, Chun-Chieh

doi: 10.1175/mwr-d-24-0004.1pmid: N/A

AbstractThe objectives of this study are to clarify the role of initial outer-core structure in intensity development and eye formation of tropical cyclones (TCs) and to examine the inner-core evolution in both the troposphere and lower stratosphere during intensification. A set of TCs with modified outer-core wind profiles of Typhoon Soudelor (2015) is set up. There are systematic variations of intensity, radius of maximum wind (RMW), and size, wherein larger initial vortices exhibit lower peak intensity, intensification rate, and RMW contraction rate and tend to maintain their larger size. While TCs in most of the experiments form their eyes through “clearing formation” processes, the largest experiment forms an eye through “banding formation” with more asymmetric eyewall convection. As compared to the largest TC, the smallest TC possesses low-level convergence and midlevel latent heating more concentrating inside the RMW with higher inertial stability and a stronger, deeper, and higher-altitude upper-level warm core. For the smallest TC, more subsidence warming is found in the lower stratosphere above the eyewall associated with more vigorous overshooting. The warmer air can be efficiently advected into the upper-level center since 1) the overshooting convection aggregates at a smaller radius and 2) the overshooting convection more frequently occurs at the upwind side of environmental flow owing to its higher angular velocity and faster axisymmetrization. It can be concluded that, for the smaller TCs, the more dominant role of the stratosphere in transporting much higher potential temperature downward appears to be the key leading to their higher intensification rate.

Zhang, Yunji; Stensrud, David J.; Comer, C. Lyn; Stouffer, Braedon C.

doi: 10.1175/mwr-d-24-0154.1pmid: N/A

AbstractThe structure of the planetary boundary layer (PBL) is important for the initiation, development, and organization of convection. High-spatiotemporal-resolution networks that directly observe the PBL structure are currently unavailable. Recent studies discovered that differential reflectivity (ZDR) observations from dual-polarization Doppler weather radars in clear-air conditions can be used to characterize the top of the daytime PBL. Compared with other observational platforms that observe the PBL, these ZDR-derived PBL depth observations have high temporal resolution and relatively dense and uniform distributions over the CONUS. Therefore, assimilating these observations could potentially improve the estimation and forecast skill of thermodynamic structures in the lower troposphere. This study examines the impact of assimilating ZDR-derived PBL depth observations on the forecasts of the torrential rainfall and flash flood event in eastern Kentucky on 27–28 July 2022 using a strongly coupled land–atmosphere data assimilation system. The model configuration in the experiments mimics the operational HRRR. Results show that assimilating ZDR-derived PBL depth observations leads to considerable changes in temperature and moisture in the lower troposphere. Soil conditions, including soil moisture and associated surface heat fluxes, are also modified. The assimilation of ZDR-derived PBL depth observations contributes to a better match between model-diagnosed PBL depth with the observations. Subsequently, rainfall forecasts are statistically significantly improved using both gridwise and neighborhood metrics, especially for the most extreme rainfall. Sensitivity experiments also show that the assimilation frequency and the observation errors assigned to ZDR-derived PBL depth observations influence the performance of the rainfall forecasts, which deserve future study.Significance StatementThis is the first study that presents a comprehensive investigation on the impact of assimilating planetary boundary layer (PBL) depth observations estimated from dual-polarization NEXRAD radars on the convection-allowing analyses and forecasts of severe weather events. The torrential rainfall and flash flood event that occurred in eastern Kentucky on 27–28 July 2022 is utilized as a case study. Assimilating radar-derived PBL depth observations leads to substantial changes in lower-tropospheric thermodynamic conditions, instability, surface heat fluxes, and soil states. As a result, rainfall forecasts of this event were significantly improved. Since the dual-polarization radar observations used to estimate PBL depth are available over most of the CONUS every 5–10 min, this study demonstrates the great value of these novel observations in data assimilation and numerical weather prediction practices.

Wood, Michaela J.; Van Den Broeke, Matthew S.

doi: 10.1175/mwr-d-24-0080.1pmid: N/A

AbstractLandfalling tropical cyclones (TCs) contain highly sheared environments that are conducive for supercell thunderstorm development. These TC supercells can produce tornadoes, often with little warning. In this study, dual-polarization radar signatures of tornadic and nontornadic TC supercells are examined in the context of known extratropical supercell radar signatures. Prior studies have only presented dual-polarization characteristics of TC supercells using a case study approach. Therefore, this paper aims to create a more comprehensive picture with a larger sample of cases, and an attempt is made to distinguish differences between tornadic and nontornadic TC supercells that may hold operational promise. The environments and characteristic structures of these supercells are notably different from prior conceptual models of supercells developed. Differential reflectivity ZDR columns are shallower in TC supercells when compared to their extratropical counterparts. The ZDR columns are also less common in TC cases. The ZDR arc is more pronounced in TC supercells, with maximum and mean ZDR values within the arcs being larger. Separation angle between the specific differential phase KDP foot and ZDR arc is larger in TC supercells than in extratropical supercells. Tornadic TC supercells had significantly stronger low-level mesocyclones than nontornadic TC supercells as measured by normalized rotation (NROT). The observed differences may help operational meteorologists use these signatures more effectively in warning decisions and motive further research into the evolution of dual-polarization signatures in tornadic and nontornadic TC supercells.Significance StatementSupercell thunderstorms are supported in the highly sheared environments of tropical cyclones. Recurring dual-polarization radar signatures can provide insight into the vertical motion and size-sorting processes occurring in supercells which might be beneficial for operational meteorologists. In this study, dual-polarization signatures are compared between tornadic and nontornadic tropical cyclone supercells and examined against their extratropical counterparts. Various differences were discovered between these signatures in tropical cyclone versus extratropical supercells, which could be attributed to differences in their thermodynamic and kinematic environments. Statistically significant differences were not found between dual-polarization signatures in tornadic and nontornadic tropical cyclone supercells.