Positive Convective Available Potential Energy Research Articles

The interaction of deep convection with the general circulation in Titan's atmosphere. Part 1: Cloud resolving simulations

The deep convective cloud–environment feedback loop is likely important to Titan's global methane, energy, and momentum cycles, just as it is for Earth's global water, energy, and momentum budgets. General circulation models of Titan's atmosphere are unable to explicitly simulate deep convection and must instead parameterize the impact of this important subgrid-scale phenomenon on the model-resolved atmospheric state. The goal of this study is to better quantify through cloud resolving modeling the effects of deep convective methane storms on their environment and to feed that information forward to improve parameterizations in global models. Dozens of atmospheric profiles unstable with respect to deep moist convection are extracted from the global Titan Atmospheric Model (TAM) and used to initialize the cloud-resolving Titan Regional Atmospheric Modeling System (TRAMS). Mean profiles of heating/cooling and moistening/drying of the large-scale environment in TRAMS indicate that Titan's deep convection forces the environment in a manner analogous to Earth: Large-scale subsidence of the environmental air warms and dries the environment, but clouds can also moisten the environment through the detrainment and evaporation of condensate near cloud top. Relative humidity profiles and characteristic convective time scales are derived to guide the tuning of the deep convective parameterization implemented in TAM, as described in a companion paper. The triggering of convection, the dry convective mixing of the planetary boundary layer, and the entrainment of environmental air into rising air parcels are found to be critical to determining whether a deep convective cloud will form. Only profiles with relatively large convective available potential energy (CAPE) and well mixed planetary boundary layers with high relative humidity were found to produce storms. Environments with low-level thermal inversions and planetary boundary layers with low relative humidity or rapidly decreasing moisture with height failed to generate deep convection in TRAMS despite positive CAPE.

Relationship of Convection Initiation and Subsequent Storm Strength to Ensemble Simulated Environmental Conditions during IOP3b of VORTEX Southeast 2017

AbstractA 50-member convection-allowing ensemble was used to examine environmental factors influencing afternoon convection initiation (CI) and subsequent severe weather on 5 April 2017 during intensive observing period (IOP) 3b of the Verification of the Origins of Rotation in Tornadoes Experiment in the Southeast (VORTEX-SE). This case produced several weak tornadoes (rated EF1 or less), and numerous reports of significant hail (diameter ≥ 2 in.; ≥~5 cm), ahead of an eastward-moving surface cold front over eastern Alabama and southern Tennessee. Both observed and simulated CI was facilitated by mesoscale lower-tropospheric ascent maximized several tens of kilometers ahead of the cold-frontal position, and the simulated mesoscale ascent was linked to surface frontogenesis in the ensemble mean. Simulated maximum 2–5 km AGL updraft helicity (UHmax) was used as a proxy for severe-weather-producing mesocyclones, and considerable variability in UHmax occurred among the ensemble members. Ensemble members with UHmax > 100 m2 s−2 had stronger mesoscale ascent than in members with UHmax < 75 m2 s−2, which facilitated timelier CI by producing greater adiabatic cooling and moisture increases above the PBL. After CI, storms in the larger UHmax members moved northeastward toward a mesoscale region with larger convective available potential energy (CAPE) than in smaller UHmax members. The CAPE differences among members were influenced by differences in the location of an antecedent mesoscale convective system, which had a thermodynamically stabilizing influence on the environment toward which storms were moving. Despite providing good overall guidance, the model ensemble overpredicted severe weather likelihoods in northeastern Alabama, where comparisons with VORTEX-SE soundings revealed a positive CAPE bias.

Contrasting Spring and Summer Large-Scale Environments Associated with Mesoscale Convective Systems over the U.S. Great Plains

AbstractMesoscale convective systems (MCSs) are frequently observed over the U.S. Great Plains during boreal spring and summer. Here, four types of synoptically favorable environments for spring MCSs and two types each of synoptically favorable and unfavorable environments for summer MCSs are identified using self-organizing maps (SOMs) with inputs from observational data. During spring, frontal systems providing a lifting mechanism and an enhanced Great Plains low-level jet (GPLLJ) providing anomalous moisture are important features identified by SOM analysis for creating favorable dynamical and thermodynamic environments for MCS development. During summer, the composite MCS environment shows small positive convective available potential energy (CAPE) and convective inhibition (CIN) anomalies, which are in stark contrast with the large positive CAPE and negative CIN anomalies in spring. This contrast suggests that summer convection may occur even with weak large-scale dynamical and thermodynamic perturbations so MCSs may be inherently less predictable in summer. The two synoptically favorable environments identified in summer have frontal characteristics and an enhanced GPLLJ, but both shift north compared to spring. The two synoptically unfavorable environments feature enhanced upper-level ridges, but differ in the strength of the GPLLJ. In both seasons, MCS precipitation amount, area, and rate are much larger in the frontal-related MCSs than in nonfrontal MCSs. A large-scale index constructed using pattern correlation between large-scale environments and the synoptically favorable SOM types is found to be skillful for estimating MCS number, precipitation rate, and area in spring, but its explanatory power decreases significantly in summer. The low predictability of summer MCSs deserves further investigation in the future.

Anomaly patterns about strong convective events in the tropics and midlatitudes: Observations from radiosondes and surface weather stations

AbstractWe use 13 years (1998–2010) of rainfall estimates from the Tropical Rainfall Measuring Mission to identify high rain events located close to radiosondes. This is done in four regions: the Western Tropical Pacific, Tropical Brazil, Southeast China, and Southeast U.S. We then construct composite anomaly patterns of temperature, relative humidity, surface pressure, convective available potential energy (CAPE), geopotential height, mass divergence, relative vorticity, and potential vorticity about these high rain events. One motivation of this analysis is to identify regional differences in the interaction between strong convective events and the background atmosphere. We find, overall, that the changes in meteorological variables which occur during the evolution of strong convective events in midlatitudes are similar to the changes that occur in the tropics. In midlatitudes, however, strong convective events are associated with stronger anomalies in surface pressure and geopotential height and exhibit a warm anomaly in the lower troposphere prior to peak rainfall. In the Southeast U.S., the near‐surface layer of positive CAPE that occurs prior to high rain events is thicker than in the Western Tropical Pacific. In the two midlatitude regions, the midlevel potential vorticity maximum that develops during the growth stage of high rain events acquires a downward tilt toward the surface during the decay stage, suggesting downward transport toward the surface. A conceptual model previously used to interpret the anomaly patterns of the 2 day equatorial wave is used to interpret the anomaly patterns associated with more general types of high rain events in the tropics.

A statistical analysis of sounding derived indices and parameters for extreme and non-extreme thunderstorm events over Cyprus

Abstract. The main purpose of this study is to provide a simple statistical analysis of several stability indices and parameters for extreme and non-extreme thunderstorm events during the period 1997 to 2001 in Cyprus. For this study, radiosonde data from Athalassa station (35°1´ N, 33°4´ E) were analyzed during the aforementioned period. The stability indices and parameters set under study are the K index, the Total Totals (TT) index, the Convective Available Potential Energy related parameters such as Convective Available Potential Energy (CAPE), Downdraft CAPE (DCAPE) and the Convective Inhibition (CIN), the Vorticity Generator Parameter (VGP), the Bulk Richardson Number (BRN), the BRN Shear and the Storm Relative Helicity (SRH). An event is categorized as extreme, if primarily, CAPE was non zero and secondary, if values of both the K and the TotalTotals (TT) indices exceeded 26.9 and 50, respectively. The cases with positive CAPE but lower values of the other indices, were identified as non-extreme. By calculating the median, the lower and upper limits, as well as the lower and upper quartiles of the values of these indices, the main characteristics of their distribution were determined.

Impact of a revised convective triggering mechanism on Community Atmosphere Model, Version 2, simulations: Results from short‐range weather forecasts

This study implements a revised convective triggering condition in the National Center for Atmospheric Research (NCAR) Community Atmosphere Model, Version 2 (CAM2), model to reduce its excessive warm season daytime precipitation over land. The new triggering mechanism introduces a simple dynamic constraint on the initiation of convection that emulates the collective effects of lower level moistening and upward motion of the large‐scale circulation. It requires a positive contribution from the large‐scale advection of temperature and moisture to the existing positive convective available potential energy (CAPE) for model convection to start. In contrast, the original convection triggering function in CAM2 assumes that convection is triggered whenever there is positive CAPE, which results in too frequent warm season convection over land arising from strong diurnal variation of solar radiation. We examine the impact of the new trigger on CAM2 simulations by running the climate model in numerical weather prediction (NWP) mode so that more available observations and high‐frequency NWP analysis data can be used to evaluate model performance. We show that the modified triggering mechanism has led to considerable improvements in the simulation of precipitation, temperature, moisture, clouds, radiations, surface temperature, and surface sensible and latent heat fluxes when compared to the data collected from the Atmospheric Radiation Measurement (ARM) Program at its Southern Great Plains (SGP) site. Similar improvements are also seen over other parts of the globe. In particular, the surface precipitation simulation has been significantly improved over both the continental United States and around the globe; the overestimation of high clouds in the equatorial tropics has been substantially reduced; and the temperature, moisture, and zonal wind are more realistically simulated. Results from this study also show that some systematic errors in the CAM2 climate simulations can be detected in the early stage of model integration. Examples are the extremely overestimated high clouds in the tropics in the vicinity of Intertropical Convergence Zone and the spurious precipitation maximum to the east of the Rockies. This has important implications in studies of these model errors since running the climate model in NWP mode allows us to perform a more in‐depth analysis during a short time period where more observations are available and different model errors from various processes have not compensated for the systematic errors.

Origin of Lapse Rate Changes in the Upper Tropical Troposphere

Vertical motions in clouds arise from a variety of thermodynamic processes, including latent heat release, evaporative cooling, melting, and cloud radiative heating. In the Tropics, the net upward vertical mass flux from convective systems should approximately balance subsidence in clear sky regions associated with radiative cooling, provided the exchange of mass with midlatitudes can be assumed small. Tropical climatologies of temperature, water vapor, and ozone are used to calculate the clear sky radiative mass flux, and the derivative of this mass flux with respect to potential temperature, dMr(θ)/dθ, is used as a proxy for net convective outflow. Convective outflow increases rapidly at 345 K (∼11.3 km). This corresponds to the pseudoequivalent potential temperature θe at which air parcels near the surface first attain positive convective available potential energy (CAPE). The rate at which dMr(θ)/dθ decreases above 345 K is similar to the rate at which the near surface θe probability distribution function (PDF) decreases. This behavior is referred to as “scaling.” It suggests that the timescale for removal of an air parcel from the convective boundary layer is independent of θe (once it has positive CAPE), and that the residual vertical mass flux from convective clouds can be described as if air parcels detrain near their level of neutral buoyancy (LNB). It is also suggested that the mean tropical temperature profile above 345 K is controlled, not by mixing, but by the need for the vertical variation in net convective outflow to be consistent with the near-surface θe PDF, and that this accounts for the fact that the mean temperature profile above 345 K increasingly deviates from a moist adiabat. It is also argued that there are sufficient high θe air parcels near the surface to sustain the Brewer–Dobson circulation by detrainment at the LNB followed by radiative ascent into the stratosphere.

Impact of the convection triggering function on single‐column model simulations

We analyze the causes of the large thermal biases in the simulation of the National Center for Atmospheric Research's Community Climate Model Version 3 (CCM3) single‐column model (SCM) when it is forced with the Atmospheric Radiation Measurement Program's (ARM) summer 1995 Intensive Observing Period (IOP) data. We have found that deficiencies of the convection triggering function used in the model can explain a significant part of the biases. In the model, convection is triggered whenever there is positive convective available potential energy (CAPE), which happens to occur during daytime due to solar heating of the land surface. In observations, however, convection takes place only when the large‐scale dynamic condition is favorable to the release of CAPE. On the basis of observations we propose a new triggering function for the CCM3 deep convection scheme. We assume that model convection occurs only when the large‐scale dynamic forcing makes positive contributions to the existing positive CAPE. Improved simulation results are obtained when the new triggering function is implemented in the model. We further test the new triggering function using the ARM Southern Great Plains summer 1997 IOP data and the Global Atmospheric Research Program's Atlantic Tropical Experiment phase III observations. These tests confirm the results obtained from the ARM 1995 experiments.

The Importance of Understanding Mesoscale Model Parameterization Schemes for Weather Forecasting

Abstract A severe weather outbreak that occurred on 21–23 November 1992 in the southern United States is used to illustrate how an understanding of model parameterization schemes can help in the evaluation and utilization of mesoscale model output. Results from a mesoscale model simulation show that although the model accurately simulated many of the observed mesoscale features, there are several aspects of the model simulation that are not perfect. Through an understanding of the model parameterization schemes, these model imperfections are analyzed and found to have little effect on the overall skill of the model forecast in this case. Mesoscale model output also is used to provide guidance to evaluate the severe weather threat. By using the model output to produce hourly calculations of convective available potential energy (CAPE) and storm relative environmental helicity (SREH), it is found that regions with positive CAPE, SREH greater than 150 m2 s−2, and model-produced convective rainfall correspond...

Thunderstorms above Frontal Surfaces in Environments without Positive CAPE. Part I: A Climatology

The first of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on the climatology of such storms for the conterminous United States. The dataset used consists of 1093 observations made over a 4-year period. The events were selected using conventional network data and a set of criteria that eliminated thunderstorms rooted in the boundary layer. A composite of the dataset shows that the typical “elevated” thunderstorm occurs northeast of an associated surface low-pressure center, and north of a surface warm front in a region with northeasterly surface winds. The planetary boundary layer is generally very stable as determined by comparisons with both the 50-kPa and 85-kPa air. The thunderstorms are usually found in the left exit region of a low-level wind maximum (an area of horizontal deformation). The large-scale environment is strongly baroclinic with large vertical wind shear and warm advection. Several of the identified characteristics suggest that frequently elevated thunderstorms are the result of physical mechanisms different from those fundamental to surface-based thunderstorms. The most striking of these is that for elevated thunderstorms there is generally very little, if any, positive CAPE in the environment, as the atmosphere is slightly more stable than moist adiabatic above the frontal inversion. The annual frequency distribution of elevated thunderstorms is bimodal, with a primary peak in April and a secondary peak in September. The events are concentrated in an area extending northward from the central Gulf Coast along the Mississippi River valley. The data further show that nearly all winter-season (December through February) thunderstorms east of the Rocky Mountains are of the elevated type. The primary exception involves those over the Florida Peninsula, where surface-based convection persists throughout the year. Most of the winter-season elevated thunderstorms occur near the Gulf Coast downstream from migrating cyclones.

Thunderstorms above Frontal Surfaces in Environments without Positive CAPE. Part II: Organization and Instability Mechanisms

Abstract The second of two papers describing thunderstorms that occur above frontal surfaces, frequently in environments without positive convective available potential energy (CAPE), focuses on an impressive outbreak of elevated thunderstorms during AVE-SESAME I. It is shown that the thunderstorms occurred in three convective impulses, each of which developed in the warm sector before propagating onto the frontal surface; subsequent thunderstorms developed over the frontal surface. While in the warm sector, the convection was supported by an extremely unstable boundary layer. However, this convective energy quickly diminished above the frontal surface and thunderstorms continued and developed for many hours in an essentially stable hydrostatic environment. During the lifetime of these impulses, mesoscale updrafts developed and moved with the convective areas, maintaining nearly steady-state systems with strong low-level inflow. The environment was found to be symmetrically neutral in the region of the in...