Updraft Width Research Articles

An Analytic Description of the Structure and Evolution of Growing Deep Cumulus Updrafts

Abstract New theoretical analytic expressions are derived for the evolution of a passive scalar, buoyancy, and vertical velocity in growing, entraining moist deep convective updrafts. These expressions are a function of updraft radius, height, convective available potential energy (CAPE), and environmental relative humidity RH. They are quantitatively consistent with idealized three-dimensional moist updraft simulations with varying updraft sizes and in environments with differing RH. In particular, the analytic expressions capture the rapid decrease of buoyancy with height due to entrainment for narrow updrafts in a dry environment despite large CAPE. In contrast to the standard entraining-plume model, the theoretical expressions also describe the effects of engulfment of environmental air between the level of free convection (LFC) and height of maximum buoyancy (HMB) required by mass continuity to balance upward acceleration of updraft air (i.e., dynamic entrainment). This organized inflow sharpens horizontal gradients, thereby enhancing smaller-scale lateral turbulent mixing below the HMB. For narrow updrafts in a dry environment, this enhanced mixing leads to a negatively buoyant region between the LFC and HMB, effectively cutting off the region of positive buoyancy at the HMB from below so that the updraft structure resembles a rising thermal rather than a plume. Thus, it is proposed that a transition from plume-like to thermal-like structure is driven by dynamic entrainment and depends on updraft width (relative to height) and environmental RH. These results help to bridge the entraining-plume and rising-thermal conceptual models of moist convection.

Concurrent Sensitivities of an Idealized Deep Convective Storm to Parameterization of Microphysics, Horizontal Grid Resolution, and Environmental Static Stability

Abstract This study investigated the sensitivity of idealized deep convective storm simulations to microphysics parameterization, horizontal grid spacing (Δx), and environmental static stability. Three different bulk microphysics schemes in the Weather Research and Forecasting Model were tested for Δx between 0.125 and 2 km and three different environmental soundings, modified by altering static stability above 5 km. Horizontally and temporally averaged condensation and surface precipitation rates and convective updraft mass flux were sensitive to microphysics scheme and Δx for all environmental soundings. Microphysical sensitivities were similar for 0.125 < Δx < 1 km, but they varied for different soundings. Sensitivities of these quantities to Δx were less robust and varied with microphysics scheme. Other statistical convective characteristics, such as the mean updraft width and strength, exhibited similar sensitivities to Δx for all of the microphysics schemes. Microphysical sensitivities were primarily attributed to interactions between microphysics, cold pools, and dynamics that affected the spatial coverage of convective updrafts and hence the horizontally averaged convective mass flux, condensation rate, and surface precipitation. However, these linkages were less clear for the lowest convective available potential energy (CAPE) sounding, and in this case other mechanisms compensated to give a similar spatial coverage of convective updrafts even in simulations without a cold pool. For higher CAPE, there was considerable production of rimed ice from all of the microphysics schemes and its assumed characteristics, especially the fall speed, were important in explaining sensitivity via microphysical impacts on the cold pool. These results highlight the need for continued improvement in representing the production of rimed ice and its characteristics in microphysics schemes.

Fitting Measurements of Thunderstorm Updraft Profiles to Model Profiles

Abstract Twelve thunderstorm updrafts were analyzed in detail and a composite average derived. The updrafts were normalized in width and maximum intensity so they could he compared on the same scale. The composite average was compared with the Gaussian and the polynomial curves frequently used by modelers to describe some updraft characteristics. The composite average was also compared with an axially symmetric jet model. Favorable results were found in all three cases with good results in the case of the axially symmetric jet model. The relationship between updraft width and maximum vertical velocity is almost linear for smaller updrafts, but deviates from linear for the wider updrafts. It was found that a sensible temperature excess exists in the updrafts at the altitude where measurements were taken (5.0 to 6.2 km).