Abstract The atmospheric planetary boundary layer (PBL) plays a key role in controlling surface heating by solar radiation to influence atmospheric instability, convective initiation, and the location and intensity of precipitation. In this study, the PBL-type-dependent background error covariances (BECs) are first constructed separately for the convective, near-neutral, stable, and all-type boundary layers. The convective boundary layer exhibits the longest vertical length scales and the largest standard deviations of both temperature and relative humidity error among the four BECs. The PBL-type-dependent BECs are then employed in the assimilation of GNSS radio occultation (RO) refractivity observations from FY-3D/FY-3E, MetOp-B/MetOp-C, COSMIC-2, SPIRE, and GeoOptics, together with brightness temperature observations at channels 9 (6.25 μm) and 10 (7.1 μm) from the Advanced Geostationary Radiation Imager (AGRI) onboard FY-4A, aiming at improving quantitative precipitation forecasts (QPFs) of squall-line cases over eastern China. Compared with the data assimilation experiment using the traditional all-type BEC, the PBL-type-dependent data assimilation experiment produced a moister lower atmosphere and a higher PBL, and a slightly shallower 500-hPa trough and a farther northward cold front, resulting in a more northward precipitation forecast consistent with observations. Higher QPF threat scores, higher probabilities of detection, and lower false alarm rates confirm the benefit of using PBL-type-dependent BECs in the assimilation of GNSS RO and FY-4A AGRI data. Additional ensemble forecasts further demonstrate the robustness of these improvements, with consistent increases in threat scores across multiple cases and forecast lead times, indicating reduced model spin-up and more balanced initial conditions with PBL-type-dependent BECs.

Abstract This study investigates the effects of idealized and realistic terrain on tornado characteristics and behavior. It uses a novel simulation approach, nesting a high-fidelity, ultrafine-resolution, tornado-scale, engineering large-eddy simulation (LES) within a Cloud Model 1 (CM1) simulation of a tornadic supercell. We analyze the effects of terrain on the tornado’s central pressure, horizontal and vertical velocities, vortex shape, and path. Seven idealized terrain configurations are used including 1) a control run with flat ground, 2) and 3) an idealized hill with steep and gradual slopes having the height of 25.4 m, 4) and 5) an idealized escarpment with steep and gradual slopes having the height of 25.4 m, and 6) and 7) an idealized hill having heights of either 50 or 65 m. Furthermore, a real-world, complex terrain configuration of the same height is analyzed as the eighth case. Results suggest that the presence of terrain relief increases the central pressure deficit, the peak wind speed, and the width of the high wind speed region in the tornado swath, enhancing tornado intensity and causing path deviation. Specifically, the horizontal and vertical velocities at 10 m above ground level (AGL) are stronger with terrain and the location of the maximum pressure deficit occurs along the uphill segment for all idealized cases except the steep hill. The precise location of the maximum wind velocities and pressure deficits varies with the terrain shape and slope. The real terrain simulation is similar to the idealized terrain simulations to a certain extent; however, the vertical velocities are lower and the strongest winds occur over a smaller region, demonstrating the complexity of the tornado–terrain relationship. Significance Statement This high-resolution numerical study investigates the effects that idealized hills and escarpments have on tornadoes and offers a comparison with a real-world, complex terrain configuration. This study is particularly novel for three reasons: 1) The nested simulation approach with an ultrafine inner grid (spacing of 0.01 m) facilitates a high-fidelity vortex simulation which reflects the variability of a real-world tornado in time and space, at a reasonable computation cost; 2) the ultrafine-scale LESs resolve turbulent features to this spatial scale and facilitate better characterization of tornadic winds; and 3) an experiment having real-world, complex terrain is successfully executed for the first time in tornado research (to the authors’ knowledge). Results suggest that terrain generally causes tornadoes to become stronger and wider than they otherwise would have been if the ground were flat. However, another important result is that the effects of the real-world, complex terrain on the tornado are not the same as those from simplified terrains.

Abstract This work highlights the significant influence of cold wakes on sea spray processes during tropical cyclones. The characteristics of the processes involved in sea spray were investigated with a series of numerical experiments in the presence of a cold wake as well as the feedback on the evolution of Typhoon Mawar (2023). The results indicate that in regions where the cold wake has a significant influence, the decrease in sea surface temperature leads to negative values of sea spray sensible heat flux, particularly near the eyewall where the wind speed is high. The sea surface cooling decreases the near-surface air temperature and consequently increases the relative humidity, although this process exhibits a time lag of ∼24 h. Thus, the droplet evaporation is suppressed, causing the sea spray latent heat flux to decrease, which is primarily observed in the outer regions of the typhoon. The influence of the cold wake on the sea spray causes cooling and drying in the right-rear quadrant of the typhoon. This effect arises mainly because of stronger horizontal advection from the storm being intensified by sea spray, which negatively contributes to the potential temperature and moisture budgets. Furthermore, the stronger cold wake cools the air and suppresses surface evaporation. Through this feedback mechanism, the cold wake modifies the storm structure by influencing the magnitude and distribution of the sea spray heat flux.

Abstract The Ensemble of Data Assimilations (EDA) method is used to evaluate the expected impact from the proposed EUMETSAT Polar System (EPS)-Sterna constellation of small satellites carrying passive microwave (MW) sounding instruments. The instrument comprises frequencies in the 50 and 183 GHz bands, window channels at 89 and 165.5 GHz, and a new set of channels around 325 GHz. This paper focuses on the 50 and 183 GHz temperature and humidity-sounding channels and evaluates different numbers of satellites (3–8) in the constellation. The EPS-Sterna observations are simulated from ECMWF high-resolution analyses, and EDA experiments are performed in which these simulated observations are added to a Baseline observing system of existing real observations. Reductions in the spread of the EDA due to adding observations indicate an improved short-range forecast. The EDA analysis reveals that all EPS-Sterna configurations considered produce a significant positive impact for a range of atmospheric variables. The benefit increases with the number of satellites, and the six-satellite scenario, proposed for nominal operational use, gives just under twice the impact of a three-satellite scenario. Impacts are largest in the southern hemisphere and smallest in the tropics, consistent with results from changes to real MW sounding observation numbers. The six-satellite EPS-Sterna constellation shows an impact of between one and two full, multi-sensor Metop platforms. Further experiments highlight the importance of instrument noise performance, with a significant loss of impact when the assumed instrument noise for all channels is degraded, and a strong improvement if the assumed noise in the temperature-sounding channels is reduced.

Abstract Tropopause polar vortices (TPVs) are sub-synoptic, long-lived, closed circulation features located in the upper-troposphere and lower-stratosphere. Most previous studies focused only on cyclonic TPVs (CTPVs), especially due to their direct influence on Arctic cyclones, cold air outbreaks, and severe convection in middle latitudes. CTPV longevity is unique to polar regions due to the relatively limited role of latent heating and the more dominant role of longwave radiative cooling. CTPVs are also important to global circulations, as recent studies have shown that CTPVs can exit the Arctic, causing the initiation of Rossby wave packets on the polar jet. Rossby wave initiation and subsequent Rossby wave breaks (RWBs) and blocking patterns cause high-impact weather, from droughts and floods to heatwaves and cold air outbreaks. However, current models have low skill when forecasting the impacts of blocking patterns in the sub-seasonal to seasonal timeframe. This study aims to determine if anticyclonic TPVs (ATPVs) have the same unique longevity as CTPVs. ATPVs’ lifecycles are also examined to determine their possible connection to RWBs and the maintenance of blocking patterns. It is hypothesized that ATPVs form from poleward RWBs and are important potential vorticity anomalies aiding in the maintenance of blocks. ERA5 data are used to create ATPV and block tracks. Results show that the most common ATPV genesis regions also have high RWB and blocking frequencies. Blocks with multiple ATPVs in close spatial proximity have longer lifetimes than those without.

Abstract Idealized simulations described in Part I investigated the importance of near-surface [low level (LL); 0–500 m] line-parallel shear, varied in both direction and magnitude, for modulation of quasi-linear convective system (QLCS) updraft and vortex attributes. Here, in Part II, we explore dynamical processes contributing to these attributes. Results indicate that LL shear strongly impacts the strength and streamwise nature of LL inflow vorticity, while free lower-tropospheric line-normal shear modulates storm propagation and LL storm-relative flow (largely through modifications to ground-relative flow). These combine to impact the storm-relative helicity (SRH) of QLCS inflow, with higher SRH for near-storm environments with increased LL hodograph curvature and length (i.e., enhanced LL line-parallel shear). Increased influx of streamwise vorticity fosters the stronger and wider rotating updrafts identified previously, which are driven by intense midlevel dynamic accelerations. When coincident with near-surface circulation, these strongly forced updrafts can support intense, long-lived mesovortices. These results demonstrate that changes to the orientation and magnitude of the ambient LL shear vector, as well as the vertical distribution of shear, can have considerable influence on the underlying dynamics governing the propagation, updrafts, and rotation within squall lines—even for a fixed line-normal environmental shear value. Altogether, this study (along with Part I) provides crucial context for understanding near-storm environmental impacts on QLCS morphology and rotation. Significance Statement Research related to linearly organized storms has largely focused on vertical changes in winds (i.e., shear) oriented normal to squall lines, but squall lines that produce considerable rotation often have a component of low-level shear oriented parallel to storms. This study, Part II of a larger work, assesses how these shear components influence the updrafts within squall lines. Line-normal shear and ambient flow impact how storms propagate, while line-parallel shear impacts their near-surface rotation. These factors combine to influence the overall strength and rotation within updrafts of quasi-linear convective systems (QLCSs) and their ability to support near-surface vortices that often precede squall-line tornadoes. These physical relationships can help improve our understanding and prediction of QLCS evolution and associated hazards.

Abstract Assimilating polarimetric radar data within a double-moment microphysics scheme requires that both hydrometeor mixing ratios and number concentrations be updated simultaneously to effectively utilize the radar information. This study directly assimilates polarimetric radar data in a fraternal twin observing system simulation experiment (OSSE) using both mixing ratios and number concentrations as analysis variables within a variational approach. A newly developed set of parameterized forward operators for polarimetric radar data, incorporating a new continuous melting model, is employed. To address challenges in minimizing the cost function, a power transformation function is applied to the analysis variables of mixing ratios and number concentrations. This approach alleviates issues arising from the very large dynamic range of number concentrations and the highly nonlinear relationship between the model’s hydrometeors and radar variables. Results from several groups of sensitivity experiments show that updating number concentrations using an appropriate power transformation function together with mixing ratios of hydrometeors reduces the analysis errors of radar variables and improves the analysis of polarimetric radar signatures. Updating number concentrations proves to be quite sensitive when assimilating differential reflectivity, while the additional assimilation of specific differential phase yields smaller analysis errors for reflectivity and mixing ratios of water vapor and rainwater compared to differential reflectivity assimilation alone. Experiments with smaller observation errors provide better analyses of the radar variables but also increase model variable analysis errors. Among the threshold values tested for reflectivity and polarimetric variables, assimilating polarimetric variables at grids where reflectivity exceeds 15 dB Z provides the best qualitative analysis.

Abstract The processes that dictate hail melt and hail terminal velocity are complex, poorly understood, and important to the characteristics of the hail that ultimately reaches the ground. Multiple parameterizations describing hail melt and terminal velocity have been developed since the 1960s. However, there has never been a systematic comparison of these parameterizations and how they respond to different environments. The current study combines unique in-cloud hail size distribution data with a melt-only, one-dimensional version of the HAILCAST hail trajectory model, termed HAILMELT, to explore how different hail melt (HMP) and terminal velocity (TVP) parameterizations simulate falling hail in the midlatitudes and tropics. We also explore how sensitive the falling hail is to temperature, moisture, and in-cloud vertical velocity changes. For hail smaller than 40 mm, simulations are frequently at least as sensitive, and often more sensitive, to which parameterization is used than to changes in the environmental conditions. In tropical oceanic environments, results suggest that hail initially smaller than 10 mm requires unrealistically cool or dry conditions to reach the surface. However, initially larger hailstones in tropical environments are shown to be capable of reaching the surface. These results indicate that there are likely larger uncertainties than previously understood in our hail simulations of the current and future climate. Additionally, they suggest all the hail identified by satellite-based hail climatologies in the tropics entirely melts before reaching the surface in some cases. Significance Statement This study is the first systematic comparison of different hail melt and hail fall speed relationships in the midlatitudes and tropics. It explores the environmental sensitivity of these relationships for hail smaller than 40 mm. The amount that hailstones melt is often more sensitive to which relationship is being used than to changes in environmental conditions. This sensitivity likely adds additional uncertainty to simulations that predict the size of hail. We also show that the initial in-cloud hailstone sizes have a larger impact on determining if hail will reach the surface in the tropics than the environmental conditions.

Abstract Accurate forecasts of the development and evolution of deep, moist convection in convection-allowing models (CAMs) remain a challenge in part owing to the difficulties inherent in modeling the microphysical and internal structures of convection, which can affect storm mode, intensity, and longevity. We hypothesize that underused Weather Surveillance Radar-1988 Doppler (WSR-88D) and GOES-16 observations can improve forecasts of deep convection in CAM ensembles. Since the upgrade to the national network of WSR-88D radars was completed in 2013, polarimetric radar data offer a wealth of information about the shape, size, and type of hydrometeors present in precipitation. Several distinct polarimetric signatures within deep convection have been identified, such as the differential reflectivity (ZDR) column, that can aid significantly in characterizing internal storm structures and improve CAM representation of convection. In addition, GOES-16 infrared all-sky brightness temperatures (BTs) provide complimentary information on cloud structures and cover that radars cannot directly measure. The CAM ensemble in this study uses the Advanced Research version of the Weather Research and Forecasting Model with the High-Resolution Rapid Refresh configuration at 3-km horizontal grid spacing and with 40 ensemble members. Observations are assimilated using an Ensemble Kalman Filter, where the radar and satellite observations are assimilated jointly and separately, and results are compared in four proof-of-concept experiments. Results indicate that the assimilation of BT observations improves forecasts of a severe convective event, which are further improved with the assimilation of ZDR observations. While BT assimilation alone improves the convective forecast, ZDR observations provide additional improvements to updraft helicity tracks, precipitation, and hail forecasts. Significance Statement There are several challenges associated with predictions of severe weather in numerical weather models, including the time and location of thunderstorm initiation as well as the hazards associated with severe thunderstorms, including hail, flash flooding, and tornadoes. This study explores using a combination of underused satellite and radar observations to better define the atmospheric state used to start the weather prediction models, with the hope that this will lead to better forecasts before and during the thunderstorms. Results show that underused observations from routinely available Doppler weather radars and a geostationary satellite, all of which are currently available, can work synergistically to improve forecasts of thunderstorms as well as their hazards.

Abstract Recent work has shown that dual-polarization WSR-88D radars can be used to accurately estimate both convective boundary layer depth h and entrainment zone depth Δ h . We build on this work by developing an algorithm to estimate Δ h from WSR-88D data and apply it to a large dataset of WSR-88D observations for the calendar year 2022. Given the existence of a national network of WSR-88Ds, this produces the most spatiotemporally comprehensive dataset of Δ h to date. We find that there are regional and seasonal variations in Δ h with median values near 300 m in January and increasing to 600 m in summer. The values of Δ h are largest in the southeastern CONUS and smallest in the northwestern CONUS. Yearly mean Δ h / h is a maximum at 1000 local time, with values above 0.6, decreasing to ∼0.4 in the afternoon. We also evaluate and extend an existing Δ h parameterization based on lidar data and provide improved fits for days with large Δ h . Significance Statement The boundary layer is the lowest layer of the atmosphere and experiences significant changes in depth throughout the day. The entrainment zone exists at the top of the boundary layer, separating it from the rest of the atmosphere above. Processes within the entrainment zone play a role in boundary layer growth and many weather phenomena. Unfortunately, observations of the depth of the entrainment zone have been very limited. In our study, we apply a new technique of estimating entrainment zone depth from weather radar data across the United States. We also evaluate the performance of a simple model for estimating entrainment zone depth and offer conclusions on how it could be improved.