Search Results

You are looking at 1 - 10 of 52 items for

  • Author or Editor: Bo Wang x
  • Refine by Access: All Content x
Clear All Modify Search
Bo Huang
and
Xuguang Wang

Abstract

Valid-time-shifting (VTS) ensembles, either in the form of full ensemble members (VTSM) or ensemble perturbations (VTSP), were investigated as inexpensive means to increase ensemble size in the NCEP Global Forecast System (GFS) hybrid four-dimensional ensemble–variational (4DEnVar) data assimilation system. VTSM is designed to sample timing and/or phase errors, while VTSP can eliminate spurious covariances through temporal smoothing. When applying a shifting time interval (τ = 1, 2, or 3 h), VTSM and VTSP triple the baseline background ensemble size from 80 (ENS80) to 240 (ENS240) in the EnVar variational update, where the overall cost is only increased by 23%–27%, depending on the selected τ. Experiments during a 10-week summer period show the best-performing VTSP with τ = 2 h improves global temperature and wind forecasts out to 5 days over ENS80. This could be attributed to the improved background ensemble distribution, ensemble correlation accuracy, and increased effective rank in the populated background ensemble. VTSM generally degrades global forecasts in the troposphere. Improved global forecasts above 100 hPa by VTSM may benefit from the increased spread that alleviates the underdispersiveness of the original background ensemble at such levels. Both VTSM and VTSP improve tropical cyclone track forecasts over ENS80. Although VTSM and VTSP are much less expensive than directly running a 240-member background ensemble, owing to the improved ensemble covariances, the best-performing VTSP with τ = 1 h performs comparably or only slightly worse than ENS240. The best-performing VTSM with τ = 3 h even shows more accurate track forecasts than ENS240, likely contributed to by its better sampling of timing and/or phase errors for cases with small ensemble track spread.

Full access
Bo Sun
and
Huijun Wang

Abstract

In this study, a Lagrangian particle dispersion model, Flexible Particle (FLEXPART), is employed to simulate the trajectories of global air parcels during 2000–09 with the purpose of revealing the moisture sources of the semiarid grasslands of China, especially on precipitation days. Based on land-cover and precipitation data, two areas of semiarid grasslands are identified: one in North China and one in the Tibetan Plateau. Using the FLEXPART simulation results, air parcels reaching these two target regions are traced back for 10 days to examine their temporal variations in position (longitude, latitude, and altitude) and specific humidity. The moisture sources of these semiarid grasslands are discussed for different precipitation categories. Moreover, the contributions of different moisture sources to the precipitation in the target regions are computed and compared. The results indicate that the moisture released in the target regions is substantially from the Eurasian continent, in both summer and winter. During May–September, the southern and eastern adjacent land areas seem to be the main moisture sources of rainfall in the grasslands of North China, while the Eurasian continent on the north and west tends to be the predominant contributor to the rainfall over the grasslands of the eastern Tibetan Plateau. During October–April, moistures released in both target regions principally originate from the Eurasian continent on the north and west. Overall, although the moisture uptake over oceanic sources is also considerable, most released moisture over the target regions is from the Eurasian continent throughout the year, while little of the contribution of oceanic sources is due to great loss of moisture en route.

Full access
Bo Sun
and
Huijun Wang

Abstract

This study aims to identify the distinct characteristics of water vapor transport (WVT) and its role in supplying moisture for widespread snowfall (WS) events in northeastern China (NEC). Fifty WS events in NEC were selected based on cumulative precipitation gauge data taken at 12-h intervals from 1980 to 2009 and a qualified set of criteria. The evolution of WVT during WS events in NEC was analyzed using 6-h ECMWF Interim Re-Analysis (ERA-Interim) data and discussed in regard to WVT paths and water vapor budgets over NEC. The results of this analysis indicate that southerly WVT, which carries moisture over eastern China, its adjacent seas, and the Sea of Japan, has played a key role in supplying water vapor for WS, which is quite different from the climatology of winter WVT. Moreover, the results indicate that there tends to be an 18-h lag between the WVT budget and precipitation, resulting in a great amount of water vapor accumulating over NEC before WS. The amount of preaccumulated water vapor could account for about 47% of the total precipitation, whereas synchronous WVT could only supply a limited amount of moisture that could hardly sustain WS. In addition, the original atmospheric moisture over NEC has likely made a considerable contribution to WS. The lag between the WVT budget and precipitation appears to be an outcome of the cooperation between the atmospheric flow field and the specific humidity field.

Full access
Bo Sun
and
Huijun Wang

Abstract

This study analyzes the interannual and interdecadal variability of spring and summer precipitation over the Three River Source (TRS) region in China using four datasets. A general consistency is revealed among the four datasets with regard to the interannual and interdecadal variability of TRS precipitation during 1979–2015, demonstrating a confidence of the four datasets in representing the precipitation variability over the TRS region. The TRS spring and summer precipitation shows distinct interannual and interdecadal variability, with an overall increasing trend in the spring precipitation and an interdecadal oscillation in the summer precipitation. The regimes associated with the interannual variability of TRS spring and summer precipitation are further investigated. The interannual variability of TRS spring precipitation is essentially modulated by an anomalous easterly water vapor transport (WVT) branch associated with the leading mode of Eurasian spring circulation. El Niño–Southern Oscillation (ENSO) may affect the interannual variability of TRS spring precipitation by causing southerly WVT anomalies toward the TRS region. The interannual variability of TRS summer precipitation is essentially modulated by an anomalous southwesterly WVT branch over the TRS region, which is mainly associated with a Eurasian wave train connected with the summer North Atlantic Oscillation. A strong East Asian summer monsoon and an El Niño–decaying summer may also contribute to the southwesterly WVT anomalies over the TRS region.

Full access
Bo Yang
,
Yuqing Wang
, and
Bin Wang

Abstract

In a quiescent environment on an f plane, the internal dynamic processes of a tropical cyclone (TC) can generate axially asymmetric circulations (asymmetries) in its inner-core region. The present study investigates how these inner-core asymmetries affect TC intensity. For this purpose, a three-dimensional (3D) TC model and its axisymmetric (2D) version were used. Both have identical model vertical structure and use the same set of parameters and the same initial conditions. The differences between the two model runs are considered to be due to mainly the effects of the TC asymmetries. The results show that the presence of asymmetries in the 3D run reduces the TC final intensity by about 15% compared with the 2D run, suggesting that the TC asymmetry is a limiting factor to the potential intensity (PI).

In the 2D run without asymmetries, the convective heating in the eyewall generates an annular tower of high potential vorticity (PV) with relatively low PV in the eye. The eyewall tilts outward with height significantly. Underneath the tilted eyewall the downdrafts induced by evaporation of rain and melting of snow and graupel make the subcloud-layer inflow dry and cool, which lowers the boundary layer equivalent potential temperature (θe ), thus increasing the entropy difference between the air and sea in the vicinity of the radius of maximum wind (RMW). The increased air–sea entropy deficit leads to more energy input into TC from the underlying ocean and thus a greater final intensity. On the other hand, in the 3D run, the model-resolved asymmetric eddies, which are characterized by the vortex Rossby waves in the mid-lower troposphere, play important roles in modifying the symmetric structure of the TC. Potential vorticity and θe budgets indicate that significant inward PV mixing from the eyewall into the eye results in a less-tilted eyewall, which in turn limits the drying and cooling effects of downdrafts in the subcloud layer and reduces the air–sea entropy deficit under the eyewall, thereby reducing the TC intensity. The angular momentum budget analysis shows that the asymmetric eddies tend to reduce the strength of the primary circulation in the vicinity of the RMW. This eddy contribution to the azimuthal mean angular momentum budget is larger than the parameterized horizontal diffusion contribution in the 3D run, suggesting an overall diffusive effect of the asymmetric eddies on the symmetric circulation.

Full access
Pao-Shin Chu
and
Jian-Bo Wang

Abstract

Recent climate change in tropical convection in the western Pacific and Indian Ocean regions is inferred from the outgoing longwave radiation (OLR) records. The systematic bias in the OLR series is first corrected and results of the rotated empirical orthogonal function analysis indicate that the bias, to a first approximation, has been corrected.

Linear regression analysis and nonparametric Mann–Kendall rank statistics are employed to detect trends. From 1974 to 1992, trend analyses based on the entire consecutive monthly records suggest a significant decrease in OLR over the tropical central–western Pacific and a large portion of the Indian Ocean. In contrast, northern Australia shows the largest increase in OLR over time. The significance of the local linear trend pattern has been determined via a Monte Carlo simulation technique that scrambles OLR time series at each grid point “simultaneously” and results show the field significance.

An increase in convection shows a preference to occur in the summer hemisphere. During the boreal summer half-year, this is seen in a region extending from the Arabian Sea across southeast Asia eastward to the northwest Pacific, with the largest value over the Bay of Bengal. More summer monsoon rainfall is likely to have occurred in these regions. For the austral summer half-year, enhanced convection is found over the equatorial south-central Pacific and the south-central Indian Ocean. Time series of tropical cyclone counts in the northwest Pacific, the Bay of Bengal, and the south-central Indian Ocean also reveal a general level of increase. Regardless of seasonality, a positive trend in OLR is always observed over a large portion of tropical Australia.

A sensitivity test is conducted to investigate the change in linear trend patterns by removing the years during which the El Niño–Southern Oscillation phenomenon occurred. Although the enhanced convection over the Bay of Bengal, the south Indian Ocean, and the northwest Pacific are still noticeable, it is much weaker over the equatorial south-central Pacific than when the complete duration series were used. Other sensitivity tests are also conducted to examine the change in linear trend patterns by varying data lengths and by skipping the missing 10-month observation in the OLR time series; results are basically similar to those when complete data are used. The authors speculate that monsoon convection over the tropical western Pacific and the Indian Ocean has undergone a change in the climate mean state, probably on a decadal timescale.

Full access
Bo Sun
,
Huijun Wang
, and
Botao Zhou

Abstract

This study examined the interdecadal variations in the relationship between the East Asian water vapor transport (WVT) and the central and eastern tropical Pacific (CETP) sea surface temperatures (SSTs) in January during 1951–2018, focusing on the meridional WVT over East Asia, which is critical for the East Asian winter precipitation. The results indicate that before the 1980s, an increased southerly WVT over East Asia was generally associated with warm SST anomalies in the CETP during January, whereas, after the mid-1980s, an increased southerly WVT over East Asia was mostly associated with cold SST anomalies in the central tropical Pacific during January. The underlying mechanisms are discussed based on a comparison on the climate anomalies associated with the East Asian meridional WVT between the periods of 1951–79 and 1986–2018. During 1951–79, the meridional WVT over East Asia was mainly modulated by the Pacific–East Asian (PEA) teleconnection, which would induce an anomalous southerly WVT over East Asia corresponding to warm SST anomalies in the CETP. Whereas, during 1986–2018, the connection between the PEA teleconnection and the East Asian meridional WVT was weakened. The connection among the CETP SSTs, the anomalous zonal circulation over the North Pacific, and the East Asian meridional WVT was enhanced. Additionally, the connection among the CETP SSTs, the circumglobal teleconnection in the Northern Hemisphere, and the East Asian meridional WVT was enhanced. The above two enhanced connections opposed the effect of the PEA teleconnection and would induce an anomalous southerly WVT over East Asia corresponding to cold SST anomalies in the central tropical Pacific.

Full access
Tianjun Zhou
,
Bo Wu
, and
Bin Wang

Abstract

The authors evaluate the performances of 11 AGCMs that participated in the Atmospheric Model Intercomparison Project II (AMIP II) and that were run in an AGCM-alone way forced by historical sea surface temperature covering the period 1979–99 and their multimodel ensemble (MME) simulation of the interannual variability of the Asian–Australian monsoon (AAM). The authors explore to what extent these models can reproduce two observed major modes of AAM rainfall for the period 1979–99, which account for about 38% of the total interannual variances. It is shown that the MME SST-forced simulation of the seasonal rainfall anomalies reproduces the first two leading modes of variability with a skill that is comparable to the NCEP/Department of Energy Global Reanalysis 2 (NCEP-2) in terms of the spatial patterns and the corresponding temporal variations as well as their relationships with ENSO evolution. Both the biennial tendency and low-frequency components of the two leading modes are captured reasonably in MME. The skill of AMIP simulation is seasonally dependent. December–February (DJF) [July–August (JJA)] has the highest (lowest) skill. Over the extratropical western North Pacific and South China Sea, where ocean–atmosphere coupling may be critical for modeling the monsoon rainfall, the MME fails to demonstrate any skill in JJA, while the reanalysis has higher skills. The MME has deficiencies in simulating the seasonal phase of two anticyclones associated with the first mode, which are not in phase with ENSO forcing in observations but strictly match that of Niño-3.4 SST in MME. While the success of MME in capturing essential features of the first mode suggests the dominance of remote El Niño forcing in producing the predictable portion of AAM rainfall variability, the deficiency in capturing the seasonal phase implies the importance of local air–sea coupling effects. The first mode generally concurs with the turnabout of El Niño; meanwhile, the second mode is driven by La Niña at decaying stage. Multimodel intercomparison shows that there are good relationships between the simulated climatology and anomaly in terms of the degree of accuracy.

Full access
Craig H. Bishop
,
Bo Huang
, and
Xuguang Wang

Abstract

A consistent hybrid ensemble filter (CHEF) for using hybrid forecast error covariance matrices that linearly combine aspects of both climatological and flow-dependent matrices within a nonvariational ensemble data assimilation scheme is described. The CHEF accommodates the ensemble data assimilation enhancements of (i) model space ensemble covariance localization for satellite data assimilation and (ii) Hodyss’s method for improving accuracy using ensemble skewness. Like the local ensemble transform Kalman filter (LETKF), the CHEF is computationally scalable because it updates local patches of the atmosphere independently of others. Like the sequential ensemble Kalman filter (EnKF), it serially assimilates batches of observations and uses perturbed observations to create ensembles of analyses. It differs from the deterministic (no perturbed observations) ensemble square root filter (ESRF) and the EnKF in that (i) its analysis correction is unaffected by the order in which observations are assimilated even when localization is required, (ii) it uses accurate high-rank solutions for the posterior error covariance matrix to serially assimilate observations, and (iii) it accommodates high-rank hybrid error covariance models. Experiments were performed to assess the effect on CHEF and ESRF analysis accuracy of these differences. In the case where both the CHEF and the ESRF used tuned localized ensemble covariances for the forecast error covariance model, the CHEF’s advantage over the ESRF increased with observational density. In the case where the CHEF used a hybrid error covariance model but the ESRF did not, the CHEF had a substantial advantage for all observational densities.

Full access
Bo Huang
,
Xuguang Wang
, and
Craig H. Bishop

Abstract

The ensemble Kalman filter is typically implemented either by applying the localization on the background error covariance matrix (B-localization) or by inflating the observation error variances (R-localization). A mathematical demonstration suggests that for the same effective localization function, the background error covariance matrix from the B-localization method shows a higher rank than the R-localization method. The B-localization method is realized in the ensemble transform Kalman filter (ETKF) by extending the background ensemble perturbations through modulation (MP-localization). Specifically, the modulation functions are constructed from the leading eigenvalues and eigenvectors of the original B-localization matrix. Because of its higher rank than the classic R-localized ETKF, the B-/MP-localized ETKF is termed as the high-rank ETKF (HETKF). The performances of the HETKF and R-localized ETKF were compared through cycled data assimilation experiments using the Lorenz model II. The results show that the HETKF outperforms the R-localized ETKF especially for a small ensemble. The improved analysis in the HETKF is likely associated with the higher rank from the B-/MP-localization method, since its higher rank is expected to contribute more positively to alleviating the rank deficiency issue and thus improve the analysis for a small ensemble. The HETKF is less sensitive to the localization length scales and inflation factors. Furthermore, the experiments suggest that the above conclusion comparing the HETKF and R-localized ETKF does not depend on how the analyzed ensemble perturbations are subselected in the HETKF.

Full access