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The extraction of atmospheric motion vectors (AMVs) from cloud and moisture features from successive geostationary satellite images is an established and important data source for numerical weather prediction (NWP). So far the extraction of AMVs has been confined to the main synoptic times only, which grossly underutilizes the potential of these satellite-derived data. The advent of four-dimensional variational assimilation techniques provides the opportunity to utilize data derived at asynoptic times. This will enhance the capabilities of geostationary satellite systems that can provide continuous and near-real time observations. The new assimilation schemes are able to digest data representing various scales and with variable quality, which further enhances the usefulness of the satellite data. In order to fully exploit the AMVs derived with satellite data, it is imperative to accurately assess the quality and representativeness of individual wind vectors and to provide this information to the NWP centers as an integral part of the observations in near real time. The required high production and dissemination frequency cannot be met if manual intervention is required; hence, the emphasis has to be on fully automated schemes. This paper will describe the automatic quality control scheme implemented at EUMETSAT. It is based on the statistical properties of the derived AMVs and it provides a quality indicator (QI), describing the expected quality of every individual vector. The derived QIs are currently disseminated together with the derived vectors. The paper will also provide validation results based on collocated radiosonde statistics and report on first experiences by ECMWF in utilizing the QIs. [ABSTRACT FROM AUTHOR]

The early season rainfall (ESR) over southern China, usually occurring from April to June, is a prominent meteorological phenomenon of the East Asian monsoon system. In this paper, output from the 45-day hindcast by the NCEP Climate Forecast System, version 2 (CFSv2), and various observational datasets are analyzed to assess the predictability of the ESR and associated atmospheric circulation. Results show that CFSv2 can successfully predict the ESR and associated circulation patterns over southern China. The lower-tropospheric convergence and upper-tropospheric divergence as well as the local upward motion over southern China lead to the formation of ESR. Analysis of bias shows small differences and close relationships between the predicted and observed ESR values when the forecast lead time is less than 2 weeks. The skill in the ESR predictions by CFSv2 decreases significantly when the lead time is longer than 2 weeks. Overall, CFSv2 has a higher level of skill when predicting the southern China ESR compared to the rainfall over other Asian regions during the same period of time. [ABSTRACT FROM AUTHOR]

During the summer, strong surface heating combines with the terrain and land-water contrasts of the northwest United States to create a complex array of diurnal circulations. Though observational and modeling studies have described some of these circulations, advances in high-resolution numerical modeling allow for a more comprehensive and three-dimensional examination. To simulate typical summer conditions over the Pacific Northwest, 3-hourly Global Forecast System (GFS) model output for July and August 2009-11 was used to initialize and provide boundary conditions for a high-resolution Weather Research and Forecasting (WRF) Model run. To ensure the realism of the simulation, it was compared to observations from a collection of days representing typical summer conditions. Generally, it was found that the simulated diurnal wind, relative humidity, and temperature were close to the observations. It is shown that regional diurnal circulations over the Pacific Northwest occur on a number of interacting scales, ranging from upslope/downslope winds on local terrain features to larger-scale circulations such as between the Pacific Ocean and the western Oregon and Washington interiors. Such multiscale diurnal circulations occur concurrently, with the interactions producing complex structures, several of which are described in this paper. Wind speeds in the Strait of Juan de Fuca and downstream of the major Cascade Mountain gaps reach maxima between 2100 and 2400 local daylight time (LDT), while most other areas have peak winds earlier in the day. Localized nocturnal low-level wind maxima are described, including one over the northern Willamette Valley and another over the high plateau of eastern Oregon. [ABSTRACT FROM AUTHOR]

Transverse cirrus bands have commonly been observed in the outflow of thunderstorms, though little literature exists on the subject. The primary objective of this paper is to characterize the transverse band signature in satellite imagery with references to storm location, movement, and life cycle. The transverse band signature was observed in nearly half of all convective systems analyzed between May and August 2006, commonly in the mature and decay stages of the system. Storm size and propagation did not appear to influence transverse bands, though the bands did appear to be associated with negative 300-hPa relative vorticity and positive divergence. Transverse bands lasted an average duration of 9 h and generally occurred during the nighttime hours. The satellite analysis was combined with eddy dissipation rate (EDR) atmospheric turbulence observations collected by commercial aircraft. At least one observation of light (moderate) turbulence was found within transverse bands for 93% (44%) of events, indicating that the presence of transverse bands in satellite imagery is a strong indicator for aviation turbulence. [ABSTRACT FROM AUTHOR]

Extreme rainfall events contribute a large portion of wintertime precipitation to northern California. The motivations of this paper were to study the observed differences in the patterns between extreme and more commonly occurring lighter rainfall events, and to study whether anomaly fields might be used to discriminate between them. Daily (1200–1200 UTC) precipitation amounts were binned into three progressively heavier categories (12.5–50.0 mm, light; 50–100 mm, moderate; and >100 mm, heavy) in order to help identify the physical processes responsible for extreme precipitation in the Sierra Nevada range between 37.5° and 41.0°N. The composite fields revealed marked differences between the synoptic patterns associated with the three different groups. The heavy composites showed a much stronger, larger-scale, and slower-moving negative geopotential height anomaly off the Pacific coast of Oregon and Washington than was revealed in either of the other two composites. The heavy rainfall events were also typically associated with an atmospheric river with anomalously high precipitable water (PW) and 850-hPa moisture flux (MF) within it. The standardized PW and MF anomalies associated with the heavy grouping were higher and were slower moving than in either of the lighter bins. Three multiday heavy rainfall events were closely examined in order to ascertain whether anomaly patterns could provide forecast utility. Each of the multiday extreme rainfall events investigated was associated with atmospheric rivers that contained highly anomalous 850-hPa MF and PW within it. Each case was also associated with an unusually intense negative geopotential height anomaly that was similarly located off of the west coast of the United States. The similarities in the anomaly pattern among the three multiday extreme events suggest that standardized anomalies might be useful in predicting extreme multiday rainfall events in the northern Sierra range. [ABSTRACT FROM AUTHOR]