Numerical simulations of extratropical tropopause‐penetrating convection: Sensitivities to grid resolution

Abstract

Deep extratropical convection that penetrates and overshoots the altitude of the tropopause has important implications both for chemistry-climate interactions through stratosphere-troposphere exchange and for hazardous weather at the Earth's surface. In this study, the sensitivity of tropopause structure and cross-tropopause transport to the choice of numerical model resolution in simulations with explicitly resolved convection (i.e., no convective parameterization) is examined. For an observed case of overshooting convection, the Advanced Research Weather Research and Forecasting (ARW-WRF) model is run for all possible combinations of three horizontal (3 km, 1 km, and 333.33 m) and vertical (600 m, 300 m, and 150 m) grid resolutions. Although ARW-WRF is successful in producing tropopause-penetrating convection in each case, the depth of overshooting and cross-tropopause transport are found to increase with refinement in the horizontal dimension and decrease with refinement in the vertical dimension. These results are related to changes in storm intensity and the sharpness of the tropopause, where the former is found to increase with refinement in the horizontal dimension and the latter is found to increase with refinement in the vertical dimension. Comparisons of simulated storm altitudes with those observed from ground-based radar reveal large positive biases in simulations where the horizontal resolution is fine and the vertical resolution is coarse.

Simulations of Vertical Water Vapor Transport for TC Ingrid (2013)

“…He attributed these effects to the competing influences of changes in simulated storm intensity and sharpness of the tropopause, where the former increased with smaller horizontal grid spacing and the latter increased with smaller vertical grid spacing. He recommended that future model simulations with explicitly resolved tropopause‐penetrating convection use Δ x = O (1 km) and Δ z ≤ 300 m. Our grid spacings of 1.33 km (horizontal) and 440 m (vertical in the UTLS layer) are only slightly larger than the Homeyer () recommendations.…”

“…If the vertical grid spacing is too coarse in the UTLS, the stability transition at the tropopause is poorly represented, and the simulated overshooting and associated changes in water vapor are exaggerated. Homeyer () examined extratropical overshooting convection using WRF with horizontal grid spacings ranging from 3 km to 333 m and vertical spacings ranging from 600 to 150 m. Results showed that the depth of overshooting and cross‐tropopause transport increased with smaller horizontal grid spacing but decreased with smaller vertical grid spacing. He attributed these effects to the competing influences of changes in simulated storm intensity and sharpness of the tropopause, where the former increased with smaller horizontal grid spacing and the latter increased with smaller vertical grid spacing.…”

Convection‐Permitting Simulation of a Heavy Rainfall Event in Armenia Using the WRF Model

“…Therefore, the observed cloud top was at 12 km above sea level which is close to tropopause. Very deep convective systems that could penetrate into the stratosphere contributing to troposphere to stratosphere transport are of high importance (Dauhut et al, ; Homeyer, ). In the absence of sounding observations in Armenia in July 2016 the height of tropopause level was estimated from Diyarbakir station located in Turkey (around 430‐km southwest from Talin station) using data from the University of Wyoming (http://weather.uwyo.edu/upperair/sounding.html).…”

Sensitivity of simulated convection‐driven stratosphere‐troposphere exchange in WRF‐Chem to the choice of physical and chemical parameterization

“…Simulated cloud top altitudes are all higher than the observed echo top, which is expected since NEXRAD WSR‐88D radars detect only precipitation‐sized particles. This result is in agreement with previous studies [e.g., Homeyer et al ., ; Homeyer , ] of simulated tropopause‐penetrating convection, which show that model simulated echo tops are typically lower than observed, while cloud top altitudes are higher. In contrast to the model simulated echo tops, which showed little variability in the upper bounds (75% and maximum) of the echo top distribution, there are notable differences between the BMPs.…”

“…However, since the evolution, vertical extent, and intensity of convection are sensitive to the model design, it is important to determine the choices that best reproduce the physical and chemical transport characteristics of observed storms. Model sensitivity tests for the choice of horizontal and vertical grid resolution have recently been completed and show that the depth of overshooting and cross‐tropopause transport increase with finer horizontal grid spacing but decrease with finer vertical grid spacing [ Homeyer , ]. This paper will present results of model sensitivity to the choice of physical and chemical parameterization.…”