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Mitchell W. Moncrieff
and
Changhai Liu

Abstract

The mesoscale organization of precipitating convection is highly relevant to next-generation global numerical weather prediction models, which will have an intermediate horizontal resolution (grid spacing about 10 km). A primary issue is how to represent dynamical mechanisms that are conspicuously absent from contemporary convective parameterizations. A hybrid parameterization of mesoscale convection is developed, consisting of convective parameterization and explicit convectively driven circulations.

This kind of problem is addressed for warm-season convection over the continental United States, although it is argued to have more general application. A hierarchical strategy is adopted: cloud-system-resolving model simulations represent the mesoscale dynamics of convective organization explicitly and intermediate resolution simulations involve the hybrid approach. Numerically simulated systems are physically interpreted by a mechanistic dynamical model of organized propagating convection. This model is a formal basis for approximating mesoscale convective organization (stratiform heating and mesoscale downdraft) by a first-baroclinic heating couplet.

The hybrid strategy is implemented using a predictor–corrector strategy. Explicit dynamics is the predictor and the first-baroclinic heating couplet the corrector. The corrector strengthens the systematically weak mesoscale downdrafts that occur at intermediate resolution. When introduced to the Betts–Miller–Janjic convective parameterization, this new hybrid approach represents the propagation and dynamical structure of organized precipitating systems. Therefore, the predictor–corrector hybrid approach is an elementary practical framework for representing organized convection in models of intermediate resolution.

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Changhai Liu
and
Mitchell W. Moncrieff

Abstract

Numerical simulations are performed to investigate organized convection observed in the Asian summer monsoon and documented as a category of mesoscale convective systems (MCSs) over the U.S. continent during the warm season. In an idealized low-inhibition and unidirectional shear environment of the mei-yu moisture front, the structure of the simulated organized convection is distinct from that occurring in the classical quasi-two-dimensional, shear-perpendicular, and trailing stratiform (TS) MCS. Consisting of four airflow branches, a three-dimensional, eastward-propagating, downshear-tilted, shear-parallel MCS builds upshear by initiating new convection at its upstream end. The weak cold pool in the low-inhibition environment negligibly affects convection initiation, whereas convectively generated gravity waves are vital. Upstream-propagating gravity waves form a saturated or near-saturated moist tongue, and downstream-propagating waves control the initiation and growth of convection within a preexisting cloud layer. A sensitivity experiment wherein the weak cold pool is removed entirely intensifies the MCS and its interaction with the environment. The horizontal scale, rainfall rate, convective momentum transport, and transverse circulation are about double the respective value in the control simulation. The positive sign of the convective momentum transport contrasts with the negative sign for an eastward-propagating TS MCS. The structure of the simulated convective systems resembles shear-parallel organization in the intertropical convergence zone (ITCZ).

Open access
Xiaoqin Jing
,
Bart Geerts
,
Yonggang Wang
, and
Changhai Liu

Abstract

There are several high-resolution (1–12 km) gridded precipitation datasets covering the interior western United States. This study cross validates seasonal orographic precipitation estimates from the Snowpack Telemetry (SNOTEL) network; the national hourly multisensor precipitation analysis Stage IV dataset (NCEP IV); four gauge-driven gridded datasets; and a 10-yr, 4-km, convection-permitting Weather Research and Forecasting (WRF) Model simulation. The NCEP IV dataset, which uses the NEXRAD network and precipitation gauges, is challenged in this region because of blockage and lack of low-level radar coverage in complex terrain. The gauge-driven gridded datasets, which statistically interpolate gauge measurements over complex terrain to better estimate orographic precipitation, are challenged by the highly heterogeneous, weather-dependent nature of precipitation in complex terrain at scales finer than can be resolved by the gauge network, such as the SNOTEL network. Gauge-driven gridded precipitation estimates disagree in areas where SNOTEL gauges are sparse, especially at higher elevations. The WRF simulation captures wintertime orographic precipitation distribution and amount well, and biases over specific mountain ranges are identical to those in an independent WRF simulation, suggesting that these biases are at least partly due to errors in the snowfall measurements or the gridding of these measurements. The substantial disagreement between WRF and the gridded datasets over some mountains may motivate reevaluation of some gauge records and installation of new SNOTEL gauges in regions marked by large discrepancies between modeled and gauge-driven precipitation estimates.

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Xinyuan Feng
,
Changhai Liu
,
Roy Rasmussen
, and
Guangzhou Fan

Abstract

A plateau vortex refers to a shallow meso-α-scale cyclonic vortex that is usually confined to near-surface levels (500 hPa) over the Tibetan Plateau during warm seasons. It is the major precipitation-producing weather system over the plateau, but the knowledge of its climatology and understanding of generation mechanisms are limited because of the lack of adequate observations in this harsh mountainous region. In this study, the high-resolution NCEP Climate Forecast System Reanalysis data have been used to perform a statistical survey of these vortices over 10 warm seasons (April–October of 2000–09). The purpose is to document their climatological features, including genesis, size, life cycle, propagation, and diurnal variation.Results show that ~103 plateau vortices occur on average every year. Most are detected from May through August, with the maximum monthly count in July. The primary area of origin exhibits a west–east orientation in correspondence with a large-scale confluence zone, and the most concentrated source lies in the area of 33°–36°N, 84°–90°E in the high elevated central and western plateau. Significant diurnal variations are observed, characteristic of a preferential genesis during late afternoon to evening hours and a late night dissipation peak. The vortex events have an average life span of ~15 h and an average horizontal dimension (effective diameter) of ~280 km. In accordance with the steering environmental flow, an overwhelming majority travel eastward with a mean translation speed of ~10 m s−1. A small fraction of systems (approximately nine cases annually) move off the plateau, predominantly from the eastern edge.

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Xiaoqin Jing
,
Bart Geerts
,
Yonggang Wang
, and
Changhai Liu

Abstract

Two high-resolution (4 km) regional climate simulations over a 10-yr period are conducted to study the changes in wintertime precipitation distribution across mountain ranges in the interior western United States (IWUS) in a warming climate. One simulation represents the current climate, and another represents an ~2050 climate using a pseudo–global warming approach. The climate perturbations are derived from the ensemble mean of 15 global climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5). These simulations provide an estimate of average changes in wintertime orographic precipitation enhancement and finescale distribution across mountain ranges. The variability in these changes among CMIP5 models is quantified using statistical downscaling relations between orographic precipitation distribution and upstream conditions, developed in Part I. The CMIP5 guidance indicates a robust warming signal (~2 K) over the IWUS by ~2050 but minor changes in relative humidity and cloud-base height. The IWUS simulations reveal a widespread increase in precipitation on account of higher precipitation rates during winter storms in this warmer climate. This precipitation increase is most significant over the mountains rather than on the surrounding plains. The increase in precipitation rate is largely due to an increase in low-level cross-mountain moisture transport. The application of the statistical relations indicates that individual CMIP5 models disagree about the magnitude and distribution of orographic precipitation change in the IWUS, although most agree with the ensemble-mean-predicted orographic precipitation increase.

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Xiaoqin Jing
,
Bart Geerts
,
Yonggang Wang
, and
Changhai Liu

Abstract

This study analyzes the control of upstream conditions on the distribution of wintertime precipitation across mountain ranges in the interior western United States using 10 winters of high-resolution regional climate model data. Three mountain ranges, the Wind River Range, the Park Range, and the Teton Range, are selected to explore the statistical relations between the precipitation distribution and upstream wind, stability, and cloud conditions. A 4-km-resolution simulation is used for the former two ranges, and a 1.33-km-resolution simulation driven by the 4-km-resolution simulation is used for the Teton Range, which is smaller and steeper. Across all three mountain ranges, the dominant factor controlling precipitation is the mountain-normal low-level wind speed. Statistically, stronger wind results in heavier precipitation and a lower upwind precipitation fraction. The low-level wind generally veers with height during precipitation events, but the amount of veering does not unambiguously affect the precipitation distribution or intensity. The more the terrain blocks the upstream flow, the more the precipitation shifts toward the upstream side of the mountain and the weaker the overall precipitation rate is. A higher cloud-base temperature and a lower cloud-base height typically are associated with heavier precipitation. Deeper clouds tend to produce heavier precipitation and a slightly lower windward/leeward contrast. Convective precipitation proportionally falls more on the lee slopes than stratiform precipitation. The upstream and macroscale cloud conditions identified herein predict both the mean precipitation rate and the upwind precipitation fraction very well for the three ranges studied here.

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Mitchell W. Moncrieff
,
Changhai Liu
, and
Peter Bogenschutz

Abstract

A new approach for treating organized convection in global climate models (GCMs) referred to as multiscale coherent structure parameterization (MCSP) introduces physical and dynamical effects of organized convection that are missing from contemporary parameterizations. The effects of vertical shear are approximated by a nonlinear slantwise overturning model based on Lagrangian conservation principles. Simulation of the April 2009 Madden–Julian oscillation event during the Year of Tropical Convection (YOTC) over the Indian Ocean using the Weather Research and Forecasting (WRF) Model at 1.3-km grid spacing identifies self-similar properties for squall lines, MCSs, and superclusters embedded in equatorial waves. The slantwise overturning model approximates this observed self-similarity. The large-scale effects of MCSP are examined in two categories of GCM. First, large-scale convective systems simulated in an aquaplanet model are approximated by slantwise overturning with attention to convective momentum transport. Second, MCSP is utilized in the Community Atmosphere Model, version 5.5 (CAM5.5), as tendency equations for second-baroclinic heating and convective momentum transport. The difference between MCSP and CAM5.5 is a direct measure of the global effects of organized convection. Consistent with TRMM measurements, the MCSP generates large-scale precipitation patterns in the tropical warm pool and the adjoining locale; improves precipitation in the intertropical convergence zone (ITCZ), South Pacific convergence zone (SPCZ), and Maritime Continent regions; and affects tropical wave modes. In conclusion, the treatment of organized convection by MCSP is salient for the next generation of GCMs.

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Linlin Zheng
,
Jianhua Sun
,
Xiaoling Zhang
, and
Changhai Liu

Abstract

Composite reflectivity Doppler radar data from June to September of 2007–2010 were used to classify mesoscale convective systems (MCSs) over central east China into seven morphologies. The morphologies included one nonlinear mode (NL) and six linear modes: convective lines with no stratiform precipitation (NS), trailing stratiform precipitation (TS), leading stratiform precipitation (LS), parallel stratiform precipitation (PS), bow echoes (BE), and embedded lines (EL). Nonlinear and linear systems composed 44.7% and 55.3% of total MCSs, respectively, but there was no primary linear mode. All MCS morphologies attained their peak occurrence in July, except BE systems, which peaked in June. On average, TS and PS modes had relatively longer lifespans than did other modes.

Significant differences in MCS-produced severe weather existed between dry and moist environments. High winds and hail events were mainly observed in dry environments, and in contrast, short-term intense precipitation occurred more frequently in moist environments. BE systems generated the most severe weather on average, while most TS systems were attendant with short-term intense precipitation and high winds. EL and PS systems were most frequently associated with extreme short-time intense precipitation (≥50 mm h−1) as these systems preferentially developed in moist environments. BE systems generally occurred under strong low-level shear and intermediately moist conditions. LS systems were observed in weak low-level shear, whereas EL systems often developed in relatively stable conditions and weak low- to middle-level shear. The largest instability was present in the environment for NS systems. The environmental parameters for TS systems featured the largest differences between the dry and moist cases.

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Xinyuan Feng
,
Changhai Liu
,
Guangzhou Fan
,
Xiaodong Liu
, and
Caiyun Feng

Abstract

The southwest vortex (SWV) is a meso-α-scale cyclonic low pressure system originating in southwest China. It is a high-impact precipitation-generating weather system in this region and a large downstream area. A climatology of the SWV over 1979–2010 is presented here. Results indicate that the SWV is a common regional weather system with about 73 annual occurrences. Two primary genesis regions are identified, one in the Sichuan basin and another one in the southeast flank of the Tibetan Plateau. SWV genesis displays seasonality with a spring–summer preference and diurnal variations. The average life cycle, horizontal dimension, and translation speed are 15.1 h, 435 km, and 8.6 m s−1, respectively. SWVs are classified into four types that show regional and seasonal contrasts in structure. In type I, the winter–spring elevated dry vortex in the basin is vertically confined to a shallow layer between 850 and 600 hPa and tilts northeastward. The low-level vortex has a cold center, and the middle to upper levels feature baroclinicity. In type II, the nighttime warm-season precipitating vortex system in the basin has a deep structure with the cyclonic vorticity extending from the surface into the upper troposphere. The nonsevere precipitating vortex (type IIa) is weakly baroclinic and tilts northward with height, whereas the severe precipitating vortex (type IIb) is vertically aligned. For type III, in the southern mountains, the shallow surface-based vortex develops in a well-mixed boundary layer and vertically tilts in the upslope direction and has a warm and low-humidity core. For type IV, the heavy precipitating vortex in the mountainous region is large, deep, and nearly upright with a fairly barotropic environment.

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Changhai Liu
,
Mitchell W. Moncrieff
, and
Edward J. Zipser

Abstract

Based on environmental conditions of the 22 and 23 June squall lines during the Convection Profonde Tropicale in 1981 (COPT81) experiment in West Africa, a series of numerical simulations are performed with a two-dimensional nonhydrostatic cloud model to examine the dynamical effect of microphysics in tropical squall lines.

The role of ice-phase microphysics strongly depends on ambient conditions. For the environment of strong convective instability, the ice phase is important regarding the system-scale structure but is not important to the convective-scale dynamics. On the other hand, the ice influence is crucial to the squall-line convective system if the environment has a weak convective instability and is almost saturated at low levels.

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