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M. Issa Lélé and Peter J. Lamb

Abstract

The Intertropical Front (ITF) is a fundamental feature of the atmospheric circulation over West Africa. It separates the wedge of warm moist southwesterly monsoon flow off the tropical Atlantic from much hotter and very dry northeasterly wind from the Sahara Desert. Here, the daily temperature, humidity, and rainfall data for 1974–2003 are analyzed to document the northward advance and southward retreat of the ITF between boreal spring and autumn, and assess its role in Sudan–Sahel (10°–20°N) rainfall patterns. Using largely dekadal (10 day) and monthly resolutions, analyses are performed for the 30-yr-average seasonal time scale and sets of extreme years, with a major focus on concurrent monthly ITF–rainfall relations. The seasonal rainfall predictive potential of the early season ITF latitude is also investigated, as is the secular variation of ITF latitude–weather system–rainfall associations during 1974–2003.

The northward advance of the ITF across the Sudan–Sahel from April to early August is relatively slow, averaging 0.8° latitude dekad−1 (8.8 km day−1). The southward ITF retreat between mid-August and mid-November is almost twice as fast, averaging 1.4° latitude dekad−1 (15.5 km day−1). Coupled with the ITF advance, the monsoon rainbelt migrates northward and intensifies. However, its northern boundary (1 mm day−1 monthly average isohyet) lags 100–250 km south of the ITF, while the most useful rainfall for society (>3 or 4 mm day−1 monthly average) generally occurs more than 400 km south of the ITF. There, the monsoon wedge is thickest and the horizontal velocity and moisture convergence are maximized in a regional ITCZ. The rapid ITF retreat during September–October is preceded by a similar rainbelt displacement. During both ITF advance and retreat, rainfall over the Sudan–Sahel region is positively related to the ITF’s latitude. The association is strongest during the early (April–June) and late (October) rainy season months (linear correlation, r = +0.74 to +0.81), when the ITF is located to the south and rainfall is low. It is weaker during the July–September rainy season core when the ITF is farthest north (r = +0.50 to +0.58). This concurrent rainfall dependence on ITF latitude is established further by contingency analyses for the 30-yr study period and by investigation of several extremely dry and wet individual seasons. The April ITF latitude anomaly is a moderately consistent indicator of the subsequent ITF latitude and associated rainfall anomaly through the first core rainy season month (July). This seasonal prediction potential does not persist into the rainy season peak (August), when the concurrent ITF–rainfall relationship is weakest (r = +0.50), the monsoon wedge is thickest, and rain-producing mesoscale dynamical processes are developed fully. However, because the ITF tends to retreat early (late) in seasons when it advanced early (late), the April ITF latitude specification of the September–October ITF latitude and rainfall (negative) is almost as consistent as that for July (positive). The secular variation of ITF latitude during 1974–2003 strongly influenced mesoscale weather systems and rainfall variability on decadal time scales.

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Jianhua Sun and Sixiong Zhao

Abstract

This paper investigated the interactions between the synoptic patterns, quasi-stationary fronts, eastward-propagating cloud clusters from the Tibetan Plateau, surface conditions, and atmospheric stratification processes associated with a 20-day event of freezing precipitation over southern China from January to February 2008. It was found that the long duration of the freezing precipitation process was primarily caused by stationary and anomalous synoptic weather patterns such as a blocking high pressure system in the northern branch and a trough in the south branch of the westerlies, which resulted in the convergence of cold air from northern China and warm, moist air from the south. The cloud clusters over the Tibetan Plateau propagated eastward and showed noticeable impacts in the local areas when they moved over southern China during several similar cloud propagation processes from January to February 2008. An east–west-oriented quasi-stationary front system in southern China, which is rare during the Asian winter monsoon season, is responsible for producing freezing precipitation and snowstorms. A stronger horizontal gradient of the isolines of the pseudo-equivalent potential temperature and higher temperatures at the inversion layer in the western part of the front than that in its eastern part can be found. At the same time, low-level moisture convergence ahead of the front enhanced the formation, development, and persistence of freezing precipitation in the west part of the front. The thickness of the warm layer and the temperature inversion layer also modulated the intensity and duration of freezing rain and ice pellets. Temperature from about −1° to −3°C and weak winds were found to be favorable meteorological factors at the surface level for freezing precipitation. These analysis results are synthesized into a conceptual model that coherently describes the physics processes associated with the synoptic features and quasi-stationary front system as well as the atmospheric stratification process during the freezing precipitation event.

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Hamish A. Ramsay, Lance M. Leslie, and Jeffrey D. Kepert

Abstract

Advances in observations, theory, and modeling have revealed that inner-core asymmetries are a common feature of tropical cyclones (TCs). In this study, the inner-core asymmetries of a severe Southern Hemisphere tropical cyclone, TC Larry (2006), are investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5) and the Kepert–Wang boundary layer model. The MM5-simulated TC exhibited significant asymmetries in the inner-core region, including rainfall distribution, surface convergence, and low-level vertical motion. The near-core environment was characterized by very low environmental vertical shear and consequently the TC vortex had almost no vertical tilt. It was found that, prior to landfall, the rainfall asymmetry was very pronounced with precipitation maxima consistently to the right of the westward direction of motion. Persistent maxima in low-level convergence and vertical motion formed ahead of the translating TC, resulting in deep convection and associated hydrometeor maxima at about 500 hPa. The asymmetry in frictional convergence was mainly due to the storm motion at the eyewall, but was dominated by the proximity to land at larger radii. The displacement of about 30°–120° of azimuth between the surface and midlevel hydrometeor maxima is explained by the rapid cyclonic advection of hydrometeors by the tangential winds in the TC core. These results for TC Larry support earlier studies that show that frictional convergence in the boundary layer can play a significant role in determining the asymmetrical structures, particularly when the environmental vertical shear is weak or absent.

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Howard B. Bluestein

Abstract

During the evening of 4 May 2007, a large, powerful tornado devastated Greensburg, Kansas. The synoptic and mesoscale environments of the parent supercell that spawned this and other tornadoes are described from operational data. The formation and early evolution of this long-track supercell, within the context of its larger-scale environment, are documented on the basis of Weather Surveillance Radar-1988 Doppler (WSR-88D) data and mobile Doppler radar data. The storm produced tornadoes cyclically for about 30 min before producing a large, long-lived tornado. It is shown that in order to have forecasted the severe weather locations and times accurately, it would have been necessary to have predicted 1) the localized formation of an isolated convective storm near/east of a dryline, 2) the subsequent splitting and resplitting of the storm several times, 3) the growth of a new storm along the right-rear flank of an existing storm, and 4) the transition from the cyclic production of small tornadoes to the production of one, large, long-track tornado. It is therefore suggested that both extreme sensitivity to initial conditions associated with storm formation and the uncertainty of storm behavior made it unusually difficult to forecast this event accurately.

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Rong-Hua Zhang and Antonio J. Busalacchi

Abstract

High-resolution space-based observations reveal significant two-way air–sea interactions associated with tropical instability waves (TIWs); their roles in budgets of heat, salt, momentum, and biogeochemical fields in the tropical oceans have been recently demonstrated. However, dynamical model-based simulations of the atmospheric response to TIW-induced sea surface temperature (SSTTIW) perturbations remain a great challenge because of the limitation in spatial resolution and realistic representations of the related processes in the atmospheric planetary boundary layer (PBL) and their interactions with the overlying free troposphere. Using microwave remote sensing data, an empirical model is derived to depict wind stress perturbations induced by TIW-related SST forcing in the eastern tropical Pacific Ocean. Wind data are based on space–time blending of Quick Scatterometer (QuikSCAT) Direction Interval Retrieval with Thresholded Nudging (DIRTH) satellite observations and NCEP analysis fields; SST data are from the Tropical Rainfall Measuring Mission (TRMM) Microwave Imager (TMI). These daily data are first subject to a spatial filter of 12° moving average in the zonal direction to extract TIW-related wind stress (τ TIW) and SSTTIW perturbations. A combined singular value decomposition (SVD) analysis is then applied to these zonal high-pass-filtered τ TIW and SSTTIW fields. It is demonstrated that the SVD-based analysis technique can effectively extract TIW-induced covariability patterns in the atmosphere and ocean, acting as a filter by passing wind signals that are directly related with the SSTTIW forcing over the TIW active regions. As a result, the empirical model can well represent TIW-induced wind stress responses as revealed directly from satellite measurements (e.g., the structure and phase), but the amplitude can be underestimated significantly. Validation and sensitivity experiments are performed to illustrate the robustness of the empirical τ TIW model. Further applications are discussed for taking into account the TIW-induced wind responses and feedback effects that are missing in large-scale climate models and atmospheric reanalysis data, as well as for uncoupled ocean and coupled mesoscale and large-scale air–sea modeling studies.

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Huijun Wang and Ke Fan

Abstract

A new scheme is developed to improve the seasonal prediction of summer precipitation in the East Asian and western Pacific region. The scheme is applied to the Development of a European Multimodel Ensemble System for Seasonal to Interannual Prediction (DEMETER) results. The new scheme is designed to consider both model predictions and observed spatial patterns of historical “analog years.” In this paper, the anomaly pattern correlation coefficient (ACC) between the prediction and the observation, as well as the root-mean-square error, is used to measure the prediction skill. For the prediction of summer precipitation in East Asia and the western Pacific (0°–40°N, 80°–130°E), the prediction skill for the six model ensemble hindcasts for the years of 1979–2001 was increased to 0.22 by using the new scheme from 0.12 for the original scheme. All models were initiated in May and were composed of nine member predictions, and all showed improvement when applying the new scheme. The skill levels of the predictions for the six models increased from 0.08, 0.08, 0.01, 0.14, −0.07, and 0.07 for the original scheme to 0.11, 0.14, 0.10, 0.22, 0.04, and 0.13, respectively, for the new scheme.

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