Difei Deng

Influence of the Summer NAO on the Spring-NAO-Based Predictability of the East Asian Summer Monsoon

AbstractThe dominant mode of atmospheric circulation over the North Atlantic region is the North Atlantic Oscillation (NAO). The boreal spring NAO may imprint its signal on contemporaneous sea surface temperature (SST), leading to a North Atlantic SST tripolar pattern (NAST). This pattern persists into the following summer and modulates the East Asian summer monsoon (EASM). Previous studies have shown that the summer NAST is caused mainly by the preceding spring NAO, whereas the contemporaneous summer NAO plays a secondary role. The results of this study illustrate that, even if the summer NAO plays a secondary role, it may also perturb summer SST anomalies caused by the spring NAO. There are two types of perturbation caused by the summer NAO. If the spring and summer NAO patterns have the same (opposite) polarities, the summer NAST tends to be enhanced (reduced) by the summer NAO, and the correlation between the spring NAO and EASM is usually stronger (weaker). In the former (latter) case, the spring-NAO...

Applications of Bivariate Fourier Series for Solving the Poisson Equation in Limited-Area Modeling of the Atmosphere: Higher Accuracy with a Boundary Buffer Strip Discarded and an Improved Order-Raising Procedure

AbstractBivariate Fourier series have many benefits in limited-area modeling (LAM), weather forecasting, and meteorological data analysis. However, atmospheric data are not spatially periodic on the LAM domain (“window”), which can be normalized to the unit square (x, y) ∈ [0, 1] ⊗ [0, 1] by rescaling the coordinates. Most Fourier LAM meteorology has employed rather low-order methods that have been quite successful in spite of Gibbs phenomenon at the boundaries of the artificial periodicity window. In this article, the authors explain why. Because data near the boundary between the high-resolution LAM window and the low-resolution global model are necessarily suspect, corrupted by the discontinuity in resolution, meteorologists routinely ignore LAM results in a buffer strip of nondimensional width D, and analyze only the Fourier sums in the smaller domain (x, y) ∈ [D, 1 − D] ⊗ [D, 1 − D]. It is shown that the error in a one-dimensional Fourier series with N terms or in a two-dimensional series with N2 ter...

Rainfall Characteristics of Recurving Tropical Cyclones over the Western North Pacific

AbstractA dataset of 88 recurving western North Pacific tropical cyclones from 2004 to 2015 is investigated for rainfall characteristics during their period of recurvature. The TCs are categorized into two groups based on different large-scale patterns from empirical orthogonal function analysis. Group 1 is characterized by an intense midlatitude baroclinic zone and close distance between the zone and TC, while Group 2 is characterized by a weaker midlatitude baroclinic zone and more remote distance between the zone and TC at the time of recurvature. The results show the large-scale environment has substantial impact on TC rainfall patterns. In Group 1, as the TC approaches and is embedded into the baroclinic zone, a relatively strong interaction between the TC and midlatitudes occurs, which is reflected by a rapid increase of environmental vertical wind shear and TC translation speed, the alignment of the shear vector and motion vector, and a sharp contrast of temperature and moisture. Higher rainfall an...

Potential Vorticity Perspective of Vortex Structure Changes of Tropical Cyclone Bilis (2006) during a Heavy Rain Event following Landfall

AbstractTropical Cyclone (TC) Bilis made landfall on the China coast at 0500 UTC 14 July 2006. Following the landfall, sudden and unforecast torrential rain commenced some 400 km southwest of the weakening circulation center at around 1200 UTC 14 July 2006. At least 843 people were killed and the direct economic loss was estimated at up to

The regional differences of Tibetan convective systems in boreal summer

5 billion (U.S. dollars) in this event.Prior to the rain event, as the environmental fields evolved, the vertical vorticity weakened and deformation increased around Bilis’s circulation. It is illustrated that a strong gradient wind imbalance (GWI) through midlevels became established over the northwestern quadrant of Bilis, from which a large quantity of air with high potential vorticity (PV) was redistributed from the inner circulation to the outer radii. Both backward and forward Lagrangian trajectories show this redistribution as an outward bulge of midlevel PV toward the rainfall areas. The transport of midlevel PV from inner to outer radii provides a dynamical re...

Multi-scale characteristics of moisture transport during a rainstorm process in North China

The Tibetan Convective Systems (TCSs) observed over the southeastern Tibetan Plateau (TP) in boreal summer show two local maximum centers in the central and eastern TP. TCSs identified by the International Satellite Cloud Climatology Project (ISCCP) Convection Tracking Database (CTD) are utilized to study the regional differences of TCSs. Results indicate that there are more TCSs in the eastern TP than central TP. 20-30 % of TCSs over the eastern TP are associated with shear line, vortex, and low pressure systems, while 70-80 % of TCSs are formed locally. In contrast, 50-70 % of TCSs over the central TP are linked to synoptic weather systems. TCSs over the eastern TP tend to have smaller size, more humidity and more rainfall than those over central TP, and they are also more likely to move out of TP. TCSs are mostly generated in mid-afternoon with more rainfall in evening over the central TP and in midnight over the eastern TP. Results from the third Tibetan Plateau Atmospheric Scientific Experiment (TIPEX-III) confirm that there are more complicated properties of TCSs’ diurnal variability over the eastern TP than central TP. Results also show that TP plays an important role in summer precipitation in both its downstream regions and the whole China.