Claudia Christine Stephan

Realistic simulations of atmospheric gravity waves over the continental U.S. using precipitation radar data

Convectively forced gravity waves can affect the dynamics of the upper troposphere and middle atmosphere on local to global scales. Simulating these waves requires cloud-resolving models, which are computationally expensive and therefore limited to case studies. Furthermore, full-physics models cannot accurately reproduce the locations, timing, and intensity of individual convective rain cells, limiting the validation of simulated waves. Here, we present a new modeling approach that retains the spatial scope of larger-scale models but permits direct validation of the modeled waves with individual cases of observed waves. Full-physics cloud-resolving model simulations are used to develop an algorithm for converting instantaneous radar precipitation rates over the U.S. into a high-resolution latent heating/cooling field. This heating field is used to force an idealized dry version of the WRF model. Wave patterns and amplitudes observed in individual satellite overpasses are reproduced with remarkable quantitative agreement. The relative simplicity of the new model permits longer simulations with much larger and deeper domains needed to simulate wave horizontal/vertical propagation. Eliminating the complicating factors of cloud physics and radiation this approach provides a link between conceptual and full-physics models and is suitable for studying wave-driven far-field circulation patterns.

Summer Season Squall-Line Simulations: Sensitivity of Gravity Waves to Physics Parameterization and Implications for Their Parameterization in Global Climate Models

AbstractGravity waves have important effects on the middle atmosphere circulation, and those generated by convection are prevalent in the tropics and summer midlatitudes. Numerous case studies have been carried out to investigate their characteristics in high-resolution simulations. Here, the impact of the choice of physics parameterizations on the generation and spectral properties of these waves in models is investigated. Using the Weather Research and Forecasting Model (WRF) a summertime squall line over the Great Plains is simulated in a three-dimensional, nonlinear, and nonhydrostatic mesoscale framework. The distributions of precipitation strength and echo tops in the simulations are compared with radar data. Unsurprisingly, those storm features are most sensitive to the microphysics scheme. However, it is found that these variations in storm morphology have little influence on the simulated stratospheric momentum flux spectra. These results support the fundamental idea behind climate model paramete...

A Case Study on the Far-Field Properties of Propagating Tropospheric Gravity Waves

AbstractMesoscale gravity waves were observed by barometers deployed as part of the USArray Transportable Array on 29 June 2011 near two mesoscale convective systems in the Great Plains region of the United States. Simultaneously, AIRS satellite data indicated stratospheric gravity waves propagating away from the location of active convection. Peak perturbation pressure values associated with waves propagating outside of regions where there was precipitation reached amplitudes close to 400 Pa at the surface. Here the origins of the waves and their relationship to observed precipitation are investigated with a specialized model study. Simulations with a 4-km resolution dry numerical model reproduce the propagation characteristics and amplitudes of the observed waves with a high degree of quantitative similarity despite the absence of any boundary layer processes, surface topography, or moist physics in the model. The model is forced with a three-dimensional, time-dependent latent heating/cooling field that...

Characteristics of Gravity Waves from Convection and Implications for Their Parameterization in Global Circulation Models

AbstractCharacteristic properties of gravity waves from convection over the continental United States are derived from idealized high-resolution numerical simulations. In a unique modeling approach, waves are forced by a realistic thermodynamic source based on observed precipitation data. The square of the precipitation rate and the gravity wave momentum fluxes both show lognormal occurrence distributions, with long tails of extreme events. Convectively generated waves can give forces in the lower stratosphere that at times rival orographic wave forcing. Throughout the stratosphere, zonal forces due to convective wave drag are much stronger than accounted for by current gravity wave drag parameterizations, so their contribution to the summer branch of the stratospheric Brewer–Dobson circulation is in fact much larger than models predict. A comparison of these forces to previous estimates of the total drag implies that convectively generated gravity waves are a primary source of summer-hemisphere stratosph...

A comprehensive analysis of coherent rainfall patterns in China and potential drivers. Part II: intraseasonal variability

The causes of subseasonal precipitation variability in China are investigated using observations and reanalysis data for extended winter (November–April) and summer (May–October) seasons from 1982 to 2007. For each season, the three dominant regions of coherent intraseasonal variability are identified with Empirical Orthogonal Teleconnection (EOT) analysis. While previous studies have focused on particular causes for precipitation variability or on specific regions, here a comprehensive analysis is carried out with an objective method. Furthermore, the associated rainfall anomaly timeseries are tied to specific locations in China, which facilitates their interpretation. To understand the underlying processes associated with spatially coherent patterns of rainfall variability, fields from observations and reanalysis are regressed onto EOT timeseries. The three dominant patterns in winter together explain 43% of the total space–time variance and have their origins in midlatitude disturbances that appear two pentads in advance. Winter precipitation variability along the Yangtze River is associated with wave trains originating over the Atlantic and northern Europe, while precipitation variability in southeast China is connected to the Mediterranean storm track. In summer, all patterns have a strong relationship with the Boreal Summer Intraseasonal Oscillation and are modulated by the seasonal cycle of the East Asian summer monsoon. The wet and dry phases of the regional patterns can substantially modulate the frequency of daily rainfall across China. The discovered links between weather patterns, precursors, and effects on local and remote precipitation may provide a valuable basis for hydrological risk assessments and the evaluation of numerical weather prediction models.

MJO‐Related Intraseasonal Variation in the Stratosphere: Gravity Waves and Zonal Winds

Previous work has shown eastward migrating regions of enhanced temperature variance due to long-vertical wavelength stratospheric gravity waves that are in sync with intraseasonal precipitation and tropopause wind anomalies associated with the Madden-Julian Oscillation (MJO). Here the origin of these intraseasonal gravity wave variations is investigated with a set of idealized gravity wave-resolving model experiments. The experiments specifically test whether tropopause winds act to control gravity wave propagation into the stratosphere by a critical level filtering mechanism or play a role in gravity wave generation through an obstacle source effect. All experiments use identical convective latent heating variability but the large-scale horizontal wind profile is varied to investigate relationships between stratospheric gravity waves and zonal winds at different levels. Results show that the observed long vertical wavelength gravity waves are primarily sensitive to stratospheric zonal wind variations, while tropopause wind variations have only a very small effect. Thus neither the critical level filter mechanism nor the obstacle source play much of a role in the observed intraseasonal gravity wave variations. Instead the results suggest that the stratospheric waves follow the MJO precipitation sources, and tropopause wind anomalies follow the same sources. We further find evidence of intraseasonal wave drag effects on the stratospheric circulation in reanalyzed winds. The results suggest that waves drive intraseasonal stratospheric zonal wind anomalies that descend in altitude with increasing MJO phases 3 through 7. Eastward anomalies descend further than westward, suggesting that MJO-related stratospheric waves cause larger eastward drag forces.

On Northern Hemisphere Wave Patterns Associated with Winter Rainfall Events in China

During extended winter (November–April), 43% of the intraseasonal rainfall variability in China is explained by three spatial patterns of temporally coherent rainfall. These patterns were identified with empirical orthogonal teleconnection (EOT) analysis of observed 1982–2007 pentad rainfall anomalies and connected to midlatitude disturbances. However, examination of individual strong EOT events shows that there is substantial inter-event variability in their dynamical evolution, which implies that precursor patterns found in regressions cannot serve as useful predictors. To understand the physical nature and origins of the extratropical precursors, the EOT technique is applied to six simulations of the Met Office Unified Model at horizontal resolutions of 200–40 km, with and without air–sea coupling. All simulations reproduce the observed precursor patterns in regressions, indicating robust underlying dynamical processes. Further investigation into the dynamics associated with observed patterns shows that Rossby wave dynamics can explain the large inter-event variability. The results suggest that the apparently slowly evolving or quasi-stationary waves in regression analysis are a statistical amalgamation of more rapidly propagating waves with a variety of origins and properties.摘要本文通过对1982-2007年冬季(11月–4月)逐候降水异常开展经验正交遥相关(EOT)分解, 发现中国冬季降水季节内变化的三个主导型态, 可以解释其季节内变率的43%, 且与中纬度扰动有关. 然而, 对强EOT事件的分析发现, 不同EOT事件对应的波动动力演变过程存在较大差异, 说明回归分析得到的前期信号不能作为有用的预测因子. 为了理解热带外预测因子的物理本质, 该文进一步对比分析了英国气象局一体化模式(MetUM)不同分辨率的大气环流模式和耦合模式模拟结果. 所有模拟均能模拟出观测中基于回归方法得到的前期预报因子, 说明了相应动力机制的可靠性. 对观测中降水型动力过程的进一步诊断指出, Rossby波可以解释不同EOT事件间动力过程差异的产生原因. 该文研究表明, 回归分析得到的是明显的演变缓慢的波动或准静止波, 这是统计合并多种起源、多种特性的快速传播的波动的结果.

Relationships Between Gravity Waves Observed at Earth's Surface and in the Stratosphere Over the Central and Eastern United States

Observations of tropospheric gravity waves (GWs) made by the new and extensive USArray Transportable Array (TA) barometric network located east of the Rockies, in the central and eastern United States and of stratospheric (30-40 km above sealevel) GWs made by the Atmospheric InfraRed Sounder (AIRS) are compared over a 5-year time span from 2010 through 2014. GW detections in the period band from 2-6 hours made at the Earths surface during the thunderstorm season from May through August each year exhibit the same broad spatial and temporal patterns as observed at stratospheric altitudes. At both levels, the occurrence frequency of GWs is higher at night than during the day and is highest to the west of the Great Lakes. Statistically significant correlations between the variance of the pressure at the TA, which is a proxy for GWs at ground level, with 8.1 μm brightness temperature measurements from AIRS and rain radar precipitation data, which are both proxies for convective activity, indicate that GWs observed at the TA are related to convective sources. There is little, if any, time lag between the two. Correlations between GWs in the stratosphere and at ground level are weaker, possibly due to the AIRS observational filter effect, but are still statistically significant at nighttime. We conclude that convective activity to the west of the Great Lakes is the dominant source of GWs both at ground level and within the stratosphere.