The largest convective clouds are mesoscale convective systems, which account for a large portion of Earth's cloud cover and precipitation, and the patterns of wind and weather associated with mesoscale convective systems are important local phenomena that often must be forecast on short timescales. They often produce floods. Mesoscale convective systems are generally much larger than the individual cumulonimbus and lines of cumulonimbus discussed in Chapter 8 . They develop circulations on the mesoscale, which are larger in scale than the updrafts and downdrafts of individual cumulonimbus clouds. The mesoscale circulations produce large regions of stratiform (nimbostratus) precipitation of the type discussed in Chapter 6 . Often the stratiform precipitation regions trail a squall line consisting of convective cells, and a mesoscale convective vortex tends to form in the stratiform region. The heating profile in the stratiform region is positive at upper levels and negative at lower levels due to evaporation and melting of the precipitation particles. The dynamics of mesoscale circulations involve a joint adjustment to the wind shear and thermodynamic stratification of the large scale environment. Gravity-wave dynamics also contribute to the maintenance of mesoscale convective systems. This chapter reviews both the observed structure of mesoscale systems and their unique dynamics.

/pdf/mesoscale-convective-systems-144jge62r4.pdf

Mesoscale convective systems

PROTRACED COUNTERINSURGENCY: CHINESE COIN STRATEGY IN XINJIANG by MAJ J. Scott LaRonde, USA, 95 pages. In 1949, following the conclusion of its revolutionary war against the Chinese Nationalist forces, the People’s Liberation Army (PLA) peacefully occupied China’s western most province of Xinjiang. For nearly sixty years, the PLA has conducted a counterinsurgency against several, mostly Uyghur-led, separatist movements. Despite periods of significant violence, particularly in the early 1950s and again in the 1990s, the separatist forces have not gained momentum and remained at a level one insurgency. Mao ZeDeng is revered as a master insurgent and the father of Fourth Generation Warfare. Strategists in armies worldwide study his writings on revolutionary and guerilla warfare. This monograph concludes that Mao, as well as the communist leaders who followed him, was also successful at waging protracted counterinsurgency. For nearly sixty years, separatist movements in Xinjiang, Tibet, and Taiwan have all failed. This monograph analyzes the conflict in Xinjiang and concludes that the Chinese continue to defeat the separatist movement in Xinjiang through a strategy that counters Mao’s seven fundamentals of revolutionary warfare.

/pdf/approved-for-public-release-distribution-is-unlimited-2qs7y34q3g.pdf

Approved for Public Release; Distribution is Unlimited

The representation of convective processes within mesoscale models with horizontal grid sizes smaller than 20 km has become a major concern for the simulation of mesoscale weather systems. In this paper, the authors investigate the effects of grid resolution on convective processes using a nonhydrostatic cloud model to help clarify the capabilities and limitations of using explicit physics to resolve convection in mesoscale models. By varying the horizontal grid interval between 1 and 12 km, the degradation in model response as the resolution is decreased is documented and the processes that are not properly represented with the coarser resolutions are identified. Results from quasi-three-dimensional squall-line simulations for midlatitude-type environments suggest that resolutions of 4 km are sufficient to reproduce much of the mesoscale structure and evolution of the squall-line-type convective systems produced in 1-km simulations. The evolution at coarser resolutions is characteristically slow...

The resolution dependence of explicitly modeled convective systems

A formal approach is presented to couple small-scale processes associated with atmospheric moist convection with the large-scale dynamics. The approach involves applying a two-dimensional cloud-resolving model in each column of a three-dimensional large-scale model. In the spirit of classical convection parameterization, which assumes scale separation between convection and the large-scale flow, the cloud-resolving models from neighboring columns interact only through the large-scale dynamics. This approach is referred to as Cloud-Resolving Convection Parameterization (CRCP). In short, CRCP involves many two-dimensional cloud-resolving models interacting in a manner consistent with the large-scale dynamics. The approach is first applied to the idealized problem of a convective–radiative equilibrium of a two-dimensional nonrotating atmosphere in the presence of SST gradients. This simple dynamical setup allows comparison of CRCP simulations with the cloud-resolving model results. In these tests, t...

https://opensky.ucar.edu:/islandora/object/articles%3A10250/datastream/PDF/view

Coupling Cloud Processes with the Large-Scale Dynamics Using the Cloud-Resolving Convection Parameterization (CRCP)

We have constructed profiles of lightning NOχ mass distribution for use in specifying the effective lightning NOχ source in global and regional chemical transport models. The profiles have been estimated for midlatitude continental, tropical continental, and tropical marine regimes based on profiles computed for individual storms in each regime. In order to construct these profiles we have developed a parameterization for lightning occurrence, lightning type, flash placement, and NOχ production in a cloud-scale tracer transport model using variables computed in the two-dimensional Goddard Cumulus Ensemble (GCE) model. Wind fields from the GCE model are used to redistribute the lightning NOχ throughout the duration of the storm. Our method produces reasonable results in terms of computed flash rates and NOχ mixing ratios compared with observations. The profiles for each storm are computed by integrating the lightning NOχ mass across the cloud model domain for each model layer at the end of the storm. The results for all three regimes show a maximum in the mass profile in the upper troposphere, usually within 2–4 km of the tropopause. Downdrafts appear to be the strongest in the simulated midlatitude continental systems, evidenced by substantial lightning NOχ mass (up to 23%) in the lowest kilometer. Tropical systems, particularly those over marine areas, tended to have a greater fraction of intracloud flashes and weaker downdrafts, causing only minor amounts of NOχ to remain in the boundary layer following a storm. Minima appear in the profiles typically in the 2–5 km layer. Even though a substantial portion of the NOχ is produced by cloud-to-ground flashes in the lowest 6 km, at the end of the storm most of the NOχ is in the upper troposphere (55–75% above 8 km) in agreement with observations. With most of the effective lightning NOχ source in the upper troposphere where the NOχ lifetime is several days, substantial photochemical O3 production is expected in this layer downstream of regions of deep convection containing lightning. We demonstrate that the effect on upper tropospheric NOχ and O3 is substantial when the vertical distribution of the lightning NOχ source in a global model is changed from uniform to being specified by our profiles. Uncertainties in a number of aspects of our parameterization are discussed.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98JD02651

Vertical distributions of lightning NOx for use in regional and global chemical transport models

An intense rear-inflow jet, surface pressure perturbations, and stratiform precipitation associated with a squall line during 10–11 June 1985 are examined using a three-dimensional mesoscale nested-grid model. It is found that the large-scale baroclinity provides favorable and deep rear-to-front flow within the upper half of the troposphere and the mesoscale response to convective forcing helps enhance the trailing extensive rear inflow. However, latent cooling and water loading are directly responsible for the generation of the descending portion of the rear inflow. The role of the rear inflow is generally to produce convergence ahead and divergence behind the system, and thus assist the rapid acceleration of the leading convection when the prestorm environment is convectively favorable and the rapid dissipation of the convection when encountering unfavorable conditions. In this case study, the rear-inflow jet appears to have caused the splitting of the surface wake low as well as the organized ...

Numerical Simulation of an Intense Squall Line during 10–11 June 1985 PRE-STORM. Part II: Rear Inflow, Surface Pressure Perturbations and Stratiform Precipitation

A 24-h nested-grid simulation of an intense squall line during the 1985 PRE-STORM experiment is presented using an improved version of the Pennsylvania State University/National Center for Atmospheric Research three-dimensional mesoscale model. Although the model is initialized at 1200 UTC 10 June 1985 with conventional meteorological observations, it reproduces remarkably well many observed meso-β scale features that are analyzed from the high-resolution network data. These include 1) the generation of two areas of deep convection at the model initial time; 2) the timing of the initiation of the squall line along a surface front 9 h into the model integration; 3) the development of several convective bands at 2100 UTC; 4) the rapid intensification and rapid dissipation processes of the squall line as it entered and moved out of the network, respectively; 5) the generation of a presquall mesolow, a squall-induced mesohigh and a wake low as well as corresponding multiple surface convergence-diverg...

Numerical Simulation of an Intense Squall Line during 10–11 June 1985 PRE-STORM. Part I: Model Verification

Comments on ``Parameterization of Convective Precipitation in Mesoscale Numerical Models: A Critical Review''

A mesoβ-scale momentum budget and its effect on larger-scale mean flow in a midlatitude mesoscale convective system are investigated using a numerical simulation of an intense squall line that occurred during 10–11 June 1985 PRE-STORM. It is found that the momentum generation normal to the line associated with the latent heating and cooling contributes most significantly to the momentum budget and determines the mesoβ-scale internal structure and evolution of the squall line. The momentum generation along the line contributes to the initial development of a mesovortex but has little effect on the final vertical structure of the along-line flow. Both vertical and horizontal momentum advection have significant contributions, particularly to the vertical mixing of the along-line flow; and this component of the horizontal momentum is locally transported down-gradient. It is also found that for midlatitude convective systems, convectively generated downdrafts can play as prominent a role as updrafts i...

Mesoscale Momentum Budget in a Midlatitude Squall Line: A Numerical Case Study

A subseasonal-to-seasonal (S2S) prediction system was recently developed using the GFDL Seamless System for Prediction and Earth System Research (SPEAR) global coupled model. Based on 20-yr hindcast results (2000–19), the boreal wintertime (November–April) Madden– Julian oscillation (MJO) prediction skill is revealed to reach 30 days measured before the anomaly correlation coefficient of the real-time multivariate (RMM) index drops to 0.5. However, when the MJO is partitioned into four distinct propagation patterns, the prediction range extends to 38, 31, and 31 days for the fast-propagating, slow-propagating, and jumping MJO patterns, respectively, but falls to 23 days for the standing MJO. A further improvement of MJO prediction requires attention to the standing MJO given its large gap with its potential predictability (38 days). The slow-propagating MJO detours southward when traversing the Maritime Continent (MC), and confronts the MC prediction barrier in the model, while the fast-propagating MJO moves across the central MC without this prediction barrier. The MJO diversity is modulated by stratospheric quasi-biennial oscillation (QBO): the standing (slow-propagating) MJO coincides with significant westerly (easterly) phases of QBO, partially explaining the contrasting MJO prediction skill between these two QBO phases. The SPEAR model shows its capability, beyond the propagation, in predicting their initiation for different types of MJO along with discrete precursory convection anomalies. The SPEAR model skillfully predicts the observed distinct teleconnections over the North Pacific and North America related to the standing, jumping, and fast-propagating MJO, but not the slow-propagating MJO. These findings highlight the complexities and challenges of incorporating MJO prediction into the operational prediction of meteorological variables.

MJO Diversity and Teleconnections

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