Showing papers on "Convective available potential energy published in 2023"

The impacts of dust aerosol and convective available potential energy on precipitation vertical structure in southeastern China as seen from multisource observations

Abstract: Abstract. The potential impacts of dust aerosols and atmospheric convective available potential energy (CAPE) on the vertical development of precipitating clouds in southeastern China (20–30∘ N, 110–125∘ E) in June, July, and August from 2000 to 2013 were studied using multisource observations. In southeastern China, heavy-dust conditions are coupled with strong northerly winds that transport air masses containing high concentrations of mineral dust particles, with cold temperatures, and with strong wind shear. This leads to weaker CAPE on dusty days compared with that on pristine days. Based on satellite observations, precipitating drops under dusty conditions grow faster in the middle atmospheric layers (with a temperature of between −5 and +2 ∘C) but slower in the upper and lower layers compared with their pristine counterparts. For a given precipitation top height (PTH), the precipitation rate under dusty conditions is lower in the upper layer but higher in the middle and lower layers. Moreover, the associated latent heating rate released by precipitation in the middle layer is higher. The precipitation top temperature (PTT) shows a fairly good linear relationship with the near-surface rain rate (NSRR): the linear regression slope between the PTT and NSRR is stable under dusty and pristine conditions. However, the PTT0 (the PTT related to rain onset) at the onset of precipitation is highly affected by both the CAPE and aerosol conditions. On pristine days, a stronger CAPE facilitates the vertical development of precipitation and leads to a decrease in PTT0, at a rate of −0.65 ∘C per 100 J kg−1 of CAPE for deep convective precipitation (with a variation of 15 %) and at a rate of −0.41 ∘C per 100 J kg−1 of CAPE for stratiform precipitation (with variation of 12 %). After removing the impacts of CAPE on PTT, dust aerosols led to an increase in PTT0, at a rate of +4.19 ∘C per unit aerosol optical depth (AOD) for deep convective precipitation and at a rate of +0.35 ∘C per unit AOD for stratiform precipitation. This study showed clear evidence that meteorological conditions and aerosol conditions combine to impact the vertical development of precipitation clouds. A quantitative estimation of the sensitivity of PTT to CAPE and dust was also provided.

The role of helicity and fire–atmosphere turbulent energy transport in potential wildfire behaviour

Abstract: Background Understanding near-surface fire–atmosphere interactions at turbulence scale is fundamental for predicting fire spread behaviour. Aims This study aims to investigate the fire–atmosphere interaction and the accompanying energy transport processes within the convective boundary layer. Methods Three groups of large eddy simulations representing common ranges of convective boundary layer conditions and fire intensities were used to examine how ambient buoyancy-induced atmospheric turbulence impacts fire region energy transport. Key results In a relatively weak convective boundary layer, the fire-induced buoyancy force could impose substantial changes to the near-surface atmospheric turbulence and cause an anticorrelation of the helicity between the ambient atmosphere and the fire-induced flow. Fire-induced impact became much smaller in a stronger convective environment, with ambient atmospheric flow maintaining coherent structures across the fire heating region. A high-efficiency heat transport zone above the fire line was found in all fire cases. The work also found counter-gradient transport zones of both momentum and heat in fire cases in the weak convective boundary layer group. Conclusions We conclude that fire region energy transport can be affected by convective boundary layer conditions. Implications Ambient atmospheric turbulence can impact fire behaviour through the energy transport process. The counter-gradient transport might also indicate the existence of strong buoyancy-induced mixing processes.

Revisiting the Land‐Ocean Contrasts in Deep Convective Cloud Intensity Using Global Satellite Observations

Abstract: Tropical convection tends to be more intense over land than ocean, but why? Numerous previous studies have investigated the causes of this difference. This paper revisits this question using CloudSat data and focuses on interconnecting various environmental parameters and cloud properties, which have often been examined in a piecemeal way in the past. Our analysis shows that if convection is treated as a process by which potential energy (convective available potential energy) is converted to kinetic energy (vertical velocity), then the conversion is more efficient over land than ocean. A key factor that affects this conversion efficiency is the lifting condensation level (LCL). Higher LCLs over land give rise to broader dry boundary layer thermals that transition to wider deep convective cores. Wider cores, in turn, are better protected from the dilution by entrainment, thus leading to stronger updrafts. This study highlights the importance of the dry stage of convection.

Subseasonal Variations of Convective and Microphysical Characteristics of Extreme Precipitation Over the Pearl River Delta at Monsoon Coast

Abstract: The large-scale flows and rainfall pattern change markedly with the progression of the summer monsoon, but how the convective and microphysical characteristics of monsoon extreme precipitation vary subseasonally remains elusive. This study compares 16,844 extreme precipitation features (EPFs) among the pre-monsoon, active-monsoon, post-monsoon periods, and tropical cyclone (TC) days using multisource data including dual-polarimetric radar observations over the Pearl River Delta in South China. Results indicate that the environmental dynamics and thermodynamics significantly influence the EPFs' convective and microphysical properties. Despite the similar environmental conditions such as abundant moisture and moderate to large convective available potential energy (CAPE) under which the EPFs occur, differences among the four periods are noticed. During the pre-monsoon period, precipitation systems are the largest in area but their EPFs are the least frequent and have the lowest raindrop concentration, likely due to the colder and drier environment with large vertical wind shear (VWS). The onset of the summer monsoon increases both the frequency and the convective intensity of EPFs, leading to an increase in raindrop size, which is consistent with the substantial increases of CAPE and moisture during the active-monsoon period. The EPFs share similar convective intensity and raindrop size distribution (RSD) between the post-monsoon and the active-monsoon periods, although the post-monsoon EPFs are slightly less frequent and have a smaller horizontal scale related to the reduced 0–6-km VWS. Interestingly, EPFs associated with TCs have the weakest convective intensity but the most active warm-rain processes with the RSD being closer to the maritime regime.

Can GNSS help satellite measurements for thunderstorm nowcasting?

Abstract: <p>An accurate assessment of atmospheric water vapour is useful for the nowcasting of thunderstorms and heavy precipitation. Meteosat Third Generation (MTG) will offer new possibilities with the Flexible Combined Imager (FCI) channel at 0.91 µm for low level moisture and geostationary sounding using the Infra-Red Sounder (IRS), in addition to the "classic" water vapour imaging channels at 6.3 µm and 7.3 µm. Water vapour profiles offer better utility for nowcasting than integrated or single-level products. Profiles allow for the calculation of stability parameters such as Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN). They are regularly derived from radiosondes, satellite sounding, and numerical weather prediction (NWP). However, radiosonde measurements are expensive and sparse; satellite soundings have a limited vertical resolution and are relatively poor in the boundary layer; and forecasts have inherent uncertainties compared to measurements.</p> <p>This study looks into the value of total precipitable water (TPW) measurements from ground-based Global Navigational Satellite Systems (GNSS) in combination with satellite soundings and images. Central Europe has a dense network of GNSS stations with hourly measurements. Although GNSS cannot measure profiles, TPW measurements (calculated from bending angles) are highly accurate. When the NWP forecast sounding disagrees with the satellite sounding and no radiosonde sounding is available, GNSS TPW may help to assess which profile is more consistent with TPW and may be more accurate for CAPE or CIN. In addition, the study looks into the added value of the reflectance at 0.91 µm. Note that this study is still in its early stages and any results presented are highly preliminary. While waiting for FCI and IRS measurements, OLCI and IASI are used as proxies.</p>

Verification of Rapid Refresh and High-Resolution Rapid Refresh Model Variables in Tornadic Tropical Cyclones

Abstract: Tropical cyclone tornadoes (TCTORs) are a hazard to life and property during landfalling tropical cyclones (TCs). The threat is often spread over a wide area within the TC envelope, and must be continually evaluated as the TC moves inland and dissipates. To anticipate the risk of TCTORs, forecasters may use high-resolution, rapidly updating model analyses and short-range forecasts such as the Rapid Refresh (RAP) and High-resolution Rapid Refresh (HRRR), and an ingredients-based approach similar to that used for forecasting continental midlatitude tornadoes. Though RAP and HRRR errors have been identified in typical midlatitude convective environments, this study evaluates the performance of the RAP and the HRRR within the TC envelope, with particular attention given to sounding-derived parameters previously identified as useful for TCTOR forecasting. A sample of 1,730 observed upper-air soundings is sourced from 13 TCs that made landfall along the US coastline between 2017–2019. The observed soundings are paired with their corresponding model gridpoint soundings from the RAP analysis, RAP 12-hour forecast, and HRRR 12-hour forecast. Model errors are calculated for both the raw sounding variables of temperature, dewpoint, and wind speed, as well as for the selected sounding-derived parameters. Results show a moist bias that worsens with height across all model runs. There are also statistically significant underpredictions in stability-related parameters such as convective available potential energy (CAPE) and kinematic parameters such as vertical wind shear.

Recent developments in convection forecasting at ECMWF

Abstract: <p>The representation of convection in ECMWF’s forecasting system has been improved in recent years by advances in computing, substantial upgrades of both horizontal and vertical resolutions, and by major changes in the moist processes in the model. These developments have also opened up the opportunity for improvements of existing convective products and the development of new ones, such as lightning density diagnostics. As part of this ongoing initiative ECMWF is partnering with the European Severe Storms Laboratory (ESSL) on a number of projects. The computation of Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) parameters from the model has been revised and more versions of CAPE and CIN have been implemented and are made available to the forecasting community. CAPE and composite CAPE-shear parameters have been included in ECMWF’s Extreme Forecast Index (EFI) to help forecasting outbreaks of severe convection in the medium range. Alongside objective statistical verification, the convective EFI has been evaluated at ESSL’s Testbed recently. ECMWF has implemented ensemble vertical profiles to facilitate forecasting convection among other applications. ECMWF is working with ESSL on providing more parameters and products for forecasting deep, moist convection and its attendant severe weather. These include Storm Relative Helicity (SRH) and post-processed probabilities of various convective hazards such as large hail and severe wind gusts. Following its open data policy, ECMWF has also provided more probabilistic and deterministic graphical convective products on its website. This presentation will provide a brief overview of all these recent developments.</p>

Analysis of the severe thunderstorm in Croatia on September 15th 2022.

Abstract: On September 15th 2022, in the afternoon hours, central part of Croatia was hit by a violent thunderstorm, with gale-  and possibly hurricane-force winds, hail and heavy rain - and caused severe damage to properties and particularly to the forests. Testimonies of local witnesses pointed even to possibility of occurence of a tornado. A comprehensive synoptic and mesoscale analysis was performed, including radiosounding, satelite and new radar observations from the recently established radar network.   Synoptic situation was characterized by the passage of a cold front, bringing cold air in the upper layers of atmosphere which enhanced  atmospheric instabiliy. Sounding analysis pointed out high value of most unstable convective available potential energy, accompanied by pronounced deep layer wind shear, as well as significant low layer shear (0-1 km) which is quite favorable for the formation of a tornado. Severity of the phenomenon was confirmed by lightning and satelite measurements (overshooting tops) and particularly by the radar image, pointing to hail areas and exhibiting so-called 'bow-apex' feature, indicating development of a supercell.  In order to clarify some doubts - for the first time in our practice – a detailed in situ inspection of damaged area was also carried out, and aerial footage was also consulted. Although the atmospheric conditions were prone to development of a tornado, no material proof lead to presence of a tornado vortex, since no evidence of wind spinnig was found, and trees were usually knocked down in the same direction.  The risk for severe thunderstorm activity was well forecasted, and corresponding operational alert was issued by the Met Service. Furthermore – as an early warning for the incoming weather change - a special announcement was issued on our web page. 

Grid spacing effects on convection initiation and aerosol-cloud interactions: A case study of a supercell storm from the Swabian MOSES 2021 field campaign

Abstract: The predictability of deep moist convection is subject to large uncertainties, mainly due to inaccurate initial and boundary data, incomplete description of physical processes, or uncertainties in microphysical parameterizations. In this study, we present hindcasts of a supercell storm that occurred during the Swabian MOSES field campaign in southwestern Germany in summer 2021. The supercell storm of 23 June 2021 passed directly over the main observation site equipped with various instruments, allowing a detailed comparison of simulations and observations. The preconvective radiosonde observations revealed suitable conditions for supercell development, i.e., low convective inhibition, moderate convective available potential energy, sufficient deep-layer shear, and a Bulk Richardson number of 22. Numerical simulations were performed with the ICOsahedral Non-hydrostatic (ICON) model using two horizontal grid spacings (i.e., 2 km/1 km) with a single-moment and a double-moment microphysics scheme. The double-moment scheme allows us to study aerosol effects on clouds and precipitation with cloud condensation nuclei (CCN) concentrations ranging from low to very high. Numerical results show that all 2-km model realizations do not simulate convective precipitation at the correct location and time. For the 1-km grid spacing, changes in aerosol concentration resulted in large changes in convective precipitation, causing the supercell to disappear completely in some simulations. Only the 1-km model run, which assumes a clean environment, is able to realistically capture the supercell storm. During the Swabian MOSES field campaign, aerosol particle concentrations and size distributions were continuously measured with an optical particle counter from June to August 2021. The day of the supercell storm was characterized by the lowest potential CCN values of the month, suggesting that the low aerosol concentration in the successful model run is a reasonable assumption for this case study. Possible reasons for the discrepant model results, i.e., effects of grid spacing on convection initiation and detailed analyses of microphysical process rates, are discussed. These results demonstrate the benefits of using an aerosol-aware double-moment microphysics scheme at high model resolution for convection initiation and cloud evolution, and that the use of different CCN concentrations can determine whether a supercell is successfully simulated or not.

Comment on egusphere-2023-72

Abstract: Abstract. Atmospheric convection plays a key role in tracer transport from the planetary boundary layer to the free troposphere. Lagrangian transport simulations driven by global meteorological input data such as the European Centre for Medium-Range Weather Forecasts (ECMWF's) ERA5 and ERA-Interim reanalysis typically lack proper explicit representations of convective up- and downdrafts because of the limited spatiotemporal resolution of the input data. Lagrangian transport simulations for the troposphere can be improved by applying parametrizations to better represent the effects of unresolved convective transport in the global meteorological reanalysis data. Here, we implemented and assessed the effects of the extreme convection parametrization (ECP) in the Massive Parallel Trajectory Calculations (MPTRAC) model. The ECP is conceptually simple. It requires the convective available potential energy (CAPE) and the height of the equilibrium level (EL) of the meteorological data for input. Assuming that unresolved convective events yield well-mixed vertical columns of air, the ECP randomly redistributes the air parcels vertically between the surface and the EL, if CAPE is present. We analyzed statistics of explicitly resolved and parametrized convective updrafts and found that the frequencies of strong updrafts due to the ECP, i.e., 20 K potential temperature increase over 6 h or more, increase by 2 to 3 orders of magnitude for ERA5 and 3 to 5 orders of magnitude for ERA-Interim compared to the explicitly resolved updrafts. To assess the effects of the ECP on tropospheric tracer transport, we conducted transport simulations for the artificial tracer e90, which is released globally near the surface and has a constant e-folding lifetime of 90 days throughout the atmosphere. The e90 simulations were conducted for the year 2017 with both, ERA5 and ERA-Interim data. Next to sensitivity tests on the choice of the CAPE threshold, an important tuning parameter of the ECP, we suggest a possible improvement of the ECP method, i.e., to take into account the convective inhibition (CIN) indicating the presence of warm, stable layers that prevent convective updrafts in the real atmosphere. While ERA5 has higher spatiotemporal resolution and explicitly resolves more convective updrafts than ERA-Interim, we found there is still a need for both reanalyses to apply a convection parametrization such as the ECP to better represent tracer transport from the planetary boundary layer into the free troposphere on the global scale.

Evaluation of tornadic environments and their trends and projected changes in Japan

Abstract: Abstract Tornadoes are responsible for several high impact weather related disasters in Japan. However, little is known about how these events have changed over the last several decades or how they may change in future climates. This study examines environmental conditions associated with tornados in Japan using pseudo-soundings from the high-resolution fifth-generation ECMWF reanalysis. We first determine appropriate discriminators of F2 + tornadoes using thermodynamic (convective available potential energy, convective inhibition, lifting condensation level, and the K-index), kinematic (deep layer shear and storm-relative helicity), and multivariate tornado parameters (energy helicity index, K-helicity index, and the significant tornado parameter), and confirm that F2 + tornadoes occur in environments with higher instability and helicity, but are better distinguished using multivariate parameters. Recent trends indicate that in some of the most densely populated regions, F2 + environments have increased significantly over the last four decades. We also examined future changes for each parameter using a large ensemble 2-K warming experiment. Robust increases in strong tornado environments are depicted in many regions in Japan, particularly on the Sea of Japan side and the Kanto region. This indicates that despite projected decreases in deep layer shear and higher convective inhibition, significant increases in atmospheric instability compensate, leading to more days with F2 + tornado potential.

Lightning Prediction Using Electric Field Measurements Associated with Convective Events at a Tropical Location

Abstract: Nowcasting of lightning occurrences is essential in tropical locations as lightening causes severe damage to life and property. This study attempts to nowcast lightning events during convective phenomena using an electric field monitor (EFM) at a tropical urban location, Kolkata (22.65°N, 88.45°E). Before the onset of heavy lightning occurrences, definite changes in the atmospheric electric field (EF) are observed, which in turn are associated with high cloud liquid water content (LWC) and low cloud base height (CBH). A model has been proposed to nowcast lightning strikes within about 17.5 km radius of the present location based on the EF standard deviation (EFSD) values. The proposed technique is tested with the lightning data provided by a collocated lightning detector, which yields a prediction efficiency of ~ 0.91 (~ 0.86), a false alarm rate of ~ 0.23 (~ 0.18), and a critical success index of ~ 0.71 (~ 0.72) with an optimal range of other performance parameters during the monsoon (pre-monsoon) periods, thereby generating an alarm 45 min before lightning events.

Polarimetric Radar Observations of a Long-lived Supercell and Associated Tornadoes on 10–11 December 2021

Abstract: Abstract We present environmental and polarimetric radar observations of a long-lived December supercell which tracked approximately 750 km from Arkansas to northern Kentucky. The storm was associated with two long-track EF4 tornadoes, one of which was among the longest-tracked tornadoes recorded in the United States. The supercell’s life cycle is documented from 2000 UTC on 10 December 2021 – 0700 UTC on 11 December 2021, using data from five operational polarimetric weather radars. After convection initiation in central Arkansas, it took nearly 4 hours for a supercell to develop. Afterward, the storm’s Z DR column and arc became anomalously large leading up to genesis of the first EF4 tornado. During this time, the storm’s environment had moderate convective available potential energy (CAPE) and strong deep-layer shear. A cell interaction at about 0200 UTC disrupted the supercell updraft, weakening the Z DR arc and column and initiating the largest radar-implied hailfall event observed with this storm. The remnant circulation associated with the first EF4 tornado did not fully dissipate, and it appeared to merge with the low-level mesocyclone on the nose of a rear flank downdraft surge likely initiated by the hailfall. It is hypothesized that this merger was important to the intensification of the storm’s second EF4 tornado, which lasted nearly 3 hours and traveled approximately 267 km. During the second EF4 tornado the storm experienced decreasing CAPE and increasing storm relative helicity. Increasing interactions with other cells eventually weakened the storm, and its original updraft was obscured before the storm’s remnants dissipated in northern Kentucky.

Is Amazon deforestation decreasing the number of thunderstorms over South America?

Abstract: Lightning activity has been predicted to increase with global warming, though estimates of lightning sensitivity to a change of temperature vary widely. Since lightning is a small scale process, it must be represented by parameterizations in climate models. This paper uses large-scale meteorological parameters tied to thunderstorm generation to improve existing empirical models that simulate regional thunderstorm behavior. The response of the number of thunderstorms (as presented here) to climate change is rarely analysed in global studies. This study focuses on Tropical America, and uses the ERA5 higher resolution reanalysis data (ERA5) to develop our empirical model. Thunderstorm data were taken from the World Wide Lightning Location Network (WWLLN) and processed using the clustering algorithm developed by Mezuman et al. (2014). The two meteorological parameters that correlated best with thunderstorm clusters in Tropical America were specific humidity (SH) and convective available potential energy (CAPE). The resulting empirical model was run from 1979-2019 using ERA5 reanalysis data as input. This approach approximates the long-term trends in the behavior of thunderstorms in the regions, in the absence of a complete historical lightning record. To our surprise, Tropical American thunderstorms exhibited a negative trend over this period, with a ~8% decrease in thunderstorm clusters since the 1980s even with a rise of 1K in temperature over the same period. The regions of largest decreases in thunderstorm activity align well with estimates of deforestation. We estimate that for every 1 Tg C lost due to deforestation, there is a 10% decrease in thunderstorm number.

Spatio-Temporal Climatology and Trends of Convective Available Potential Energy (CAPE) over Bangladesh, including three lightning hotspots during 40 years (1982-2021)

Abstract: Abstract This study investigates the climatology of Convective Available Potential Energy (CAPE) over Bangladesh and its eight administrative divisions, along with three lightning hotspots (Sherpur, Shahjadpur, and Bajitpur), using monthly, seasonal, and annual data for 40 years (1982–2021). The monthly CAPE data at 0000 UTC and 1200 UTC has been collected from the fifth generation of the European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis data (ERA5) at 0.25° resolution. The study reveals that the increasing CAPE trend over Bangladesh may be responsible for the increased frequency of extreme events. Significant CAPE values were observed in Bangladesh’s south-west and southern parts from March to May. In April, there was a notable increasing trend in CAPE values, particularly in the north-western region. The average CAPE values for Bangladesh’s three lightning hotspot regions (Sherpur, Shahjadpur, and Bajitpur) is higher than 1500 J/kg during the pre-monsoon at 0000 UTC, directly correlating with the lightning and thunderstorm. The Mann-Kendall test has been employed to follow yearly and seasonal trends. Overall, this study provides valuable insights into the spatial distribution of CAPE and its association with thunderstorms in Bangladesh, which can inform the development of effective strategies to manage weather-related hazards in the country.

Reply on RC2

Abstract: Abstract. 3-D fields of temperature (T) and specific humidity (q) retrieved by instruments such as the Atmospheric Infrared Sounder (AIRS) are predictive of convection, but convection often triggers during the multi-hour gaps between satellite overpasses. Here we fill the hours after AIRS overpasses by treating AIRS retrievals as air parcels which are moved adiabatically along Numerical Weather Prediction (NWP) wind trajectories. The approach is tested in a simulation experiment that samples 3-D European Reanalysis-5 (ERA5) T and q following the real-world AIRS time-space sampling from March–November 2019 over much of the Continental U.S. Our time-resolved product is named ERA5-FCST, in correspondence to the AIRS forecast product we are using it to test, named AIRS-FCST. ERA5-FCST errors may arise since processes such as radiative heating and NWP sub-grid convection are ignored. For bulk atmospheric layers, ERA5-FCST captures 59–94 % of local hourly variation in T and q. We then consider the relationship between convective available potential energy (CAPE), convective inhibition (CIN), and ERA5 precipitation. The 1° latitude-longitude ERA5-FCST grid cells in our highest CAPE and lowest CIN bin are more than 50 times as likely to develop heavy precipitation (> 4 mm hr⁻¹), compared with the baseline probability from randomly selecting a location. This is a substantial improvement compared with using the original CAPE and CIN values at overpass time. The results support development of similar FCST products for operational atmospheric sounders to provide time-resolved thermodynamics in rapidly changing pre-convective atmospheres.

Characteristics of Diagnostics for Identifying Elevated Convection over the British Isles in a Convection-Allowing Model

Abstract: Identifying modes of convection can be useful in both forecasting and research. For example, it allows for potentially different impacts to be determined in forecasting contexts and stratification of model behavior in research contexts. One area where identification could be particularly beneficial is elevated convection. Elevated convection is not routinely examined (outside of an operational environment) within a physical-process perspective in operational numerical weather prediction model evaluation or verification. Using convection-allowing model (CAM) output the characteristics of four elevated convection diagnostics (based on boundary layer, Convective Available Potential Energy (CAPE) ratios, downdraft, and inflow layer properties) are examined in operational forecasts during the UK Testbed Summer 2021 run at the Met Office. A survey of the practical use of these diagnostics in a simulated operational environment revealed that diagnostics based on CAPE ratios and inflow layer properties were preferred. These diagnostics were the smoothest varying in both space and time. Treating the CAPE ratio and downdraft properties diagnostics as proxies for updrafts and downdrafts, respectively, showed that updrafts were slightly more likely to be resolved than downdrafts. However, a substantial proportion of both are unresolved in current CAMs. Filtering the CAPE ratios by the inflow layer properties led to improved spatial and temporal characteristics, and thus indicates a potentially useful diagnostic for both research and forecasting.

Adverse impact of terrain steepness on thermally-driven initiation of orographic convection

Abstract: Abstract. Diurnal mountain winds precondition the environment for deep moist convection through horizontal and vertical transport of heat and moisture. They also play a key role in convection initiation, especially in strongly inhibited environments, by lifting air parcels above the level of free convection. Despite its relevance, the impact of these thermally-driven circulations on convection initiation has yet to be examined systematically. Using idealized large-eddy simulations with the WRF model, we study the effect of cross-valley circulations on convection initiation under synoptically undisturbed and convectively inhibited conditions, considering quasi-2D mountain ranges of different heights and widths. In particular, we contrast convection initiation over relatively steep mountains (20 % average slope) and moderately steep ones (10 %). One distinctive finding is that, under identical environmental conditions, relatively steep mountain ranges lead to a delayed onset and lower intensity of deep moist convection, although they cause stronger thermal updrafts at ridge tops. This finding cannot be explained considering the temporal evolution of convective indices, such as convective inhibition and convective available potential energy. Analysis of the ridgetop moisture budget reveals the competing effects of moisture advection by the mean thermally-driven circulation and turbulent moisture transport. In general, at mountaintops, the divergence of the turbulent moisture flux offsets the convergence of the advective moisture flux almost entirely. The weaker total moistening found over steep mountains can be explained by considering that buoyant updrafts over their ridgetops are on average relatively narrow. Thus, they are more strongly affected by the turbulent entrainment of environmental air, which depletes their moisture and cloud water content and makes them less effective at initiating deep convection. In our simulations, convective updrafts over moderately steep mountains, on the other hand, gain more moisture from the vapor flux at cloud base and lose less moisture due to horizontal vapor fluxes over the course of the day, leading to significantly higher moisture accumulation. The precipitation efficiency, a measure of how much of the condensed water eventually precipitates, is also considerably larger over the moderately steep mountains. The weaker convection over steep mountains is a robust finding, valid over a range of background environmental stratifications and mountain sizes.

Lagrangian transport simulations using the extreme convection parametrization: an assessment for the ECMWF reanalyses

Abstract: Abstract. Atmospheric convection plays a key role in tracer transport from the planetary boundary layer to the free troposphere. Lagrangian transport simulations driven by global meteorological input data such as the European Centre for Medium-Range Weather Forecasts (ECMWF's) ERA5 and ERA-Interim reanalysis typically lack proper explicit representations of convective up- and downdrafts because of the limited spatiotemporal resolution of the input data. Lagrangian transport simulations for the troposphere can be improved by applying parametrizations to better represent the effects of unresolved convective transport in the global meteorological reanalysis data. Here, we implemented and assessed the effects of the extreme convection parametrization (ECP) in the Massive Parallel Trajectory Calculations (MPTRAC) model. The ECP is conceptually simple. It requires the convective available potential energy (CAPE) and the height of the equilibrium level (EL) of the meteorological data for input. Assuming that unresolved convective events yield well-mixed vertical columns of air, the ECP randomly redistributes the air parcels vertically between the surface and the EL, if CAPE is present. We analyzed statistics of explicitly resolved and parametrized convective updrafts and found that the frequencies of strong updrafts due to the ECP, i.e., 20 K potential temperature increase over 6 h or more, increase by 2 to 3 orders of magnitude for ERA5 and 3 to 5 orders of magnitude for ERA-Interim compared to the explicitly resolved updrafts. To assess the effects of the ECP on tropospheric tracer transport, we conducted transport simulations for the artificial tracer e90, which is released globally near the surface and has a constant e-folding lifetime of 90 days throughout the atmosphere. The e90 simulations were conducted for the year 2017 with both, ERA5 and ERA-Interim data. Next to sensitivity tests on the choice of the CAPE threshold, an important tuning parameter of the ECP, we suggest a possible improvement of the ECP method, i.e., to take into account the convective inhibition (CIN) indicating the presence of warm, stable layers that prevent convective updrafts in the real atmosphere. While ERA5 has higher spatiotemporal resolution and explicitly resolves more convective updrafts than ERA-Interim, we found there is still a need for both reanalyses to apply a convection parametrization such as the ECP to better represent tracer transport from the planetary boundary layer into the free troposphere on the global scale.

Tracking the Convection Potential Based on a Foundation Remote Sensing Air Sounding Profile System

Abstract: Using ground-based remote sensing equipment (wind profile radar and microwave radiometer) data and ground automatic station data, a ground-based remote sensing sounding profile system (FAS) is constructed, which aims to make use of its advantages of high resolution, high accuracy, and low cost to make up for the lack of space–time density in existing conventional sounding layer information. The retrieval results of remote sensing sounding profiles in Beijing from May 2021 to September 2022 were tested and evaluated. The results show that the correlation coefficient between FAS and conventional sounding specific humidity is 0.89, the root–mean–square deviation is 1.53 g/kg, and the evolution trends of different data sources of convective available potential energy (CAPE) and vertical wind shear are synchronous. A case study was conducted to evaluate the effectiveness of 40 severe convective processes in the Beijing Plain area. The results show that, due to the minute-level time resolution of FAS, the retrieved convective parameters could track the evolution trend of the atmospheric state with high timeliness, dynamically describe the configuration of thermodynamic parameters, and indicate the time-varying local convective potential and instability level. Therefore, it has certain short-term forecasting significance for the occurrence time, intensity, and convection type.

An analytic formula for entraining CAPE in mid-latitude storm environments

Abstract: This article introduces an analytic formula for entraining convective available potential energy (ECAPE) with an entrainment rate that is determined directly from an environmental sounding, rather than prescribed by the formula user. Entrainment is connected to the background environment using an eddy diffusivity approximation for lateral mixing, updraft geometry assumptions, and mass continuity. These approximations result in a direct correspondence between the storm relative flow and the updraft radius and an inverse scaling between the updraft radius squared and entrainment rate. The aforementioned concepts, combined with the assumption of adiabatic conservation of moist static energy, yield an explicit analytic equation for ECAPE that depends entirely on state variables in an atmospheric profile and a few constant parameters with values that are established in past literature. Using a simplified Bernoulli-like equation, the ECAPE formula is modified to account for updraft enhancement via kinetic energy extracted from the cloud’s background environment. CAPE and ECAPE can be viewed as predictors of the maximum vertical velocity wmax in an updraft. Hence, these formulas are evaluated using wmax from past numerical modeling studies. Both of the new formulas improve predictions of wmax substantially over commonly used diagnostic parameters, including undiluted CAPE and ECAPE with a constant prescribed entrainment rate. The formula that incorporates environmental kinetic energy contribution to the updraft correctly predicts instances of exceedance of by wmax, and provides a conceptual explanation for why such exceedance is rare among past simulations. These formulas are potentially useful in nowcasting and forecasting thunderstorms and as thunderstorm proxies in climate change studies.

Does big CAPE equate to big tornado potential in supercell thunderstorms?

Abstract: <p>Convective available potential energy (CAPE), when considered by itself, is not a skillful discriminator of tornadic supercells from their nontornadic counterparts.  However, there is a longstanding notion that large CAPE may allow strong-to-violent tornadoes to occur in environments with otherwise weak vertical wind shear.  There is also anecdotal evidence that the largest tornadoes often occur on days with moderate-to-extreme CAPE.  These notions and anecdotes prompt us to ask whether large CAPE is <em>conditionally</em> supportive for significant tornado formation?</p> <p>We use a matrix of large-eddy simulations of supercells to address this question, wherein the vertical wind shear and the magnitude and vertical distribution of CAPE are independently varied.  Our analysis of these simulations identifies two influences of CAPE on storm evolution that potentially facilitate both tornadogenesis, and the formation of large and intense tornadoes.  Large CAPE generally fostered more negatively buoyant and expansive cold pools.  In environments with strong shear and low-level storm-relative flow, a more negatively buoyant rear-flank downdraft amid large CAPE fortified the rear-flank convergence zone beneath the updraft, leading to a more efficient projection of initially horizontal streamwise vorticity into the vertical.  This led to more intense mesocyclones and tornadoes, when compared to simulations with the same wind profile but smaller CAPE.  In environments with weak vertical wind shear and large CAPE, storms were more resistant to the negative effects of entrainment and thereby more prone to sustaining mesocyclones and producing tornadoes, when compared to environments with both weak shear and CAPE.  We argue that these processes explain past anecdotes about the role of CAPE in tornadogenesis.</p>

Comment on egusphere-2023-97

Abstract: Abstract. 3-D fields of temperature (T) and specific humidity (q) retrieved by instruments such as the Atmospheric Infrared Sounder (AIRS) are predictive of convection, but convection often triggers during the multi-hour gaps between satellite overpasses. Here we fill the hours after AIRS overpasses by treating AIRS retrievals as air parcels which are moved adiabatically along Numerical Weather Prediction (NWP) wind trajectories. The approach is tested in a simulation experiment that samples 3-D European Reanalysis-5 (ERA5) T and q following the real-world AIRS time-space sampling from March–November 2019 over much of the Continental U.S. Our time-resolved product is named ERA5-FCST, in correspondence to the AIRS forecast product we are using it to test, named AIRS-FCST. ERA5-FCST errors may arise since processes such as radiative heating and NWP sub-grid convection are ignored. For bulk atmospheric layers, ERA5-FCST captures 59–94 % of local hourly variation in T and q. We then consider the relationship between convective available potential energy (CAPE), convective inhibition (CIN), and ERA5 precipitation. The 1° latitude-longitude ERA5-FCST grid cells in our highest CAPE and lowest CIN bin are more than 50 times as likely to develop heavy precipitation (> 4 mm hr⁻¹), compared with the baseline probability from randomly selecting a location. This is a substantial improvement compared with using the original CAPE and CIN values at overpass time. The results support development of similar FCST products for operational atmospheric sounders to provide time-resolved thermodynamics in rapidly changing pre-convective atmospheres.

ERA5 Reanalysis of Environments Conducive to Lightning-Ignited Wildfires in Catalonia

Abstract: In the climate change context, wildfires are an increasing hazard in the Mediterranean Basin, especially those triggered by lightning. Although lightning activity can be predicted with a reasonable level of confidence, the challenge remains in forecasting the thunderstorm’s probability of ignition. The present work aims to characterise the most suitable predictors to forecast lightning-ignited wildfires. Several ERA5 parameters were calculated and compared for two different samples, thunderstorm episodes that caused a wildfire (n = 961) and ordinary thunderstorms (n = 1023) that occurred in Catalonia (NE Iberian Peninsula) in the 2006–2020 period. Lightning wildfires are mostly associated with dry thunderstorms, characterised by: weak-to-moderate Mixed-Layer Convective Available Potential Energy (MLCAPE, 150–1100 J kg−1), significant Dew Point Depression at 850 hPa (DPD850, 3.3–10.1 °C), high Most-Unstable Lifted Condensation Level (MULCL, 580–1450 m) and steep 500–700 hPa Lapse Rate (LR, −7.0–−6.3 °C). Under these conditions, with relatively dry air at lower levels, thunderstorms tend to be high-based, the rain evaporating before reaching the ground and lightning occurring without significant rainfall. Specifically forecasting the probability of LIW occurrence would be of great assistance to the forest protection tactical decision-making process, preparing for “dry” thunderstorm days where multiple ignitions can be expected.

Identifying the Deep-inflow Mixing Features in Orographically Locked Diurnal Convection

Abstract: This study focuses on the deep-inflow mixing features of the orographically locked diurnal convection, involving interactions between local circulation and the thermodynamic environment of the convection. Under the weak synoptic weather regime, orographically locked diurnal convection is a typical summertime phenomenon in Taiwan, a tropical island in the Asian monsoon region. Numerical simulations are carried out using the vector vorticity equation model with high-resolution Taiwan topography (TaiwanVVM), which can appropriately simulate the characteristics of diurnal convection and the evolution of boundary layer and local circulation. The semi-realistic approach, simplified by observed soundings as the uniform initial condition over the entire domain, emphasizes the decisive environmental factors that modulate the development of convection, representing the variability of the background environment by the ensembles. The analyses by the deep-inflow mixing framework, including the locally-derived convective structures and the upstream moist static energy (MSE) transport, improve the understanding of the interactive physical processes in the boundary layer development and local circulation evolution of orographically locked diurnal convection over complex topography. The convective structures of the deep-inflow mixing, increasing vertical velocity and convective mass flux with height through a deep lower-tropospheric inflow layer, are found in strong convective updraft columns within heavily-precipitating systems over precipitation hotspots. While the topography constrains the location of the convection, enhanced convective development is associated with higher upstream MSE transport through this deep-inflow layer via local circulation, augmenting the rain rate by 35% in precipitation hotspots. The results highlight the importance of non-local dynamical entrainment of the deep-inflow, transporting MSE via local circulation to supply the growth of orographically locked diurnal convection. Thus, the deep-inflow mixing framework can serve as the theoretical basis for describing the orographic locking feature of diurnal convection over complex topography. Guided by the simulations, the Storm Tracker mini-radiosondes are released upstream of the precipitation hotspot, targeting observations within the most common deep-inflow path. Initial field measurements support the presence of high MSE transport within the deep-inflow layer when organized convection occurs at the precipitation hotspot.

Comment on egusphere-2023-72

Abstract: Abstract. Atmospheric convection plays a key role in tracer transport from the planetary boundary layer to the free troposphere. Lagrangian transport simulations driven by global meteorological input data such as the European Centre for Medium-Range Weather Forecasts (ECMWF's) ERA5 and ERA-Interim reanalysis typically lack proper explicit representations of convective up- and downdrafts because of the limited spatiotemporal resolution of the input data. Lagrangian transport simulations for the troposphere can be improved by applying parametrizations to better represent the effects of unresolved convective transport in the global meteorological reanalysis data. Here, we implemented and assessed the effects of the extreme convection parametrization (ECP) in the Massive Parallel Trajectory Calculations (MPTRAC) model. The ECP is conceptually simple. It requires the convective available potential energy (CAPE) and the height of the equilibrium level (EL) of the meteorological data for input. Assuming that unresolved convective events yield well-mixed vertical columns of air, the ECP randomly redistributes the air parcels vertically between the surface and the EL, if CAPE is present. We analyzed statistics of explicitly resolved and parametrized convective updrafts and found that the frequencies of strong updrafts due to the ECP, i.e., 20 K potential temperature increase over 6 h or more, increase by 2 to 3 orders of magnitude for ERA5 and 3 to 5 orders of magnitude for ERA-Interim compared to the explicitly resolved updrafts. To assess the effects of the ECP on tropospheric tracer transport, we conducted transport simulations for the artificial tracer e90, which is released globally near the surface and has a constant e-folding lifetime of 90 days throughout the atmosphere. The e90 simulations were conducted for the year 2017 with both, ERA5 and ERA-Interim data. Next to sensitivity tests on the choice of the CAPE threshold, an important tuning parameter of the ECP, we suggest a possible improvement of the ECP method, i.e., to take into account the convective inhibition (CIN) indicating the presence of warm, stable layers that prevent convective updrafts in the real atmosphere. While ERA5 has higher spatiotemporal resolution and explicitly resolves more convective updrafts than ERA-Interim, we found there is still a need for both reanalyses to apply a convection parametrization such as the ECP to better represent tracer transport from the planetary boundary layer into the free troposphere on the global scale.

Application of a Three-Dimensional Wind Field from a Phased-Array Weather Radar Network in Severe Convection Weather

Abstract: In 2019–2020, an array weather radar (AWR) network consisting of seven X-band phased-array radars (PARs), with a detection spatial resolution of 30 m and a temporal resolution of 30 s, was built in the city of Foshan in China’s Guangdong Province. The detection time deviation in the same space is within 5 s. Through variational data assimilation, the three-dimensional wind field inside the storm can be obtained. This study selected instances of hail, thunderstorms, strong winds, and short-duration heavy precipitation in 2020 to conduct a detailed analysis. The results show the following: (1) The fine detection ability enables phased-array radars to detect the complete evolution process of convective storms, including development, strengthening, and weakening, providing a useful reference for judging the future variation trends of convective storms. (2) Through evolutionary analysis of the three-dimensional wind field, the dynamic mechanisms of storm strengthening and weakening could be obtained, which could serve as a reference to predict the development of storms. The gust wind index and convection index calculated based on the three-dimensional wind field could provide advanced warning for nowcasting. When the gust wind index was greater than 263, the probability of gale-force wind (above 17.0 m/s) was determined to be high. Moreover, the warning could be provided 10–20 min in advance. A convection index greater than 35 and the presence of concentrated contour lines were found to be conducive to the strengthening and formation of a convection, and the warning could be provided 20 min in advance. These results show that the application of PAR can provide important technical support for nowcasting severe convective weather.

Deep convective cloud system size and structure across the global tropics and subtropics

Abstract: Abstract. A new database is constructed comprising millions of deep convective clouds that spans the global tropics and subtropics for the entire record of the MODIS instruments on the Terra and Aqua satellites. The database is a collection of individual cloud objects ranging from isolated convective cells to mesoscale convective cloud systems spanning hundreds of thousands of square kilometers in cloud area. By matching clouds in the database with the MERRA-2 reanalysis dataset and microwave imager brightness temperatures from the AMSR-E instrument, the database is designed to explore the relationships among the horizontal scale of cloud systems, the thermodynamic environment within which the cloud resides, the amount of aerosol in the environment, and indicators of the microphysical structure of the clouds. We find that the maximum values of convective available potential energy and vertical shear of the horizontal wind associated with a cloud impose a strong constraint on the size attained by convective cloud system, although the relationship varies geographically. The cloud database provides a means of empirical study of the factors that determine the spatial structure and coverage of convective cloud systems, which are strongly related to the overall radiative forcing by cloud systems. The observed relationships between cloud system size and structure from this database can be compared with similar relationships derived from simulated clouds in atmospheric models to evaluate the representation of clouds and convection in weather forecast and climate projection simulations, including whether models exhibit the same relationships between the atmospheric environment and cloud system size and structure. Furthermore, the dataset is designed to probe the impacts of aerosols on the size and structure of deep convective cloud systems.