Search Results

You are looking at 1 - 10 of 21 items for

  • Author or Editor: Weidong Yu x
  • Refine by Access: All Content x
Clear All Modify Search
Lin Liu
,
Weidong Yu
, and
Tim Li

Abstract

The performance of 23 World Climate Research Programme (WCRP) Coupled Model Intercomparison Project, phase 3 (CMIP3) models in the simulation of the Indian Ocean dipole (IOD) is evaluated, and the results show large diversity in the simulated IOD intensity. A detailed diagnosis is carried out to understand the role of the Bjerknes dynamic air–sea feedback and the thermodynamic air–sea coupling in shaping the different model behaviors. The Bjerknes feedback processes include the equatorial zonal wind response to SST, the thermocline response to the equatorial zonal wind, and the ocean subsurface temperature response to the thermocline variation. The thermodynamic feedback examined includes the wind–evaporation–SST and cloud–radiation–SST feedbacks. A combined Bjerknes and thermodynamic feedback intensity index is introduced. This index well reflects the simulated IOD strength contrast among the strong, moderate, and weak model groups. It gives a quantitative measure of the relative contribution of the dynamic and thermodynamic feedback processes.

The distinctive features in the dynamic and thermodynamic coupling strength are closely related to the mean state difference in the coupled models. A shallower (deeper) equatorial mean thermocline, a stronger (weaker) background vertical temperature gradient, and a greater (smaller) mean vertical upwelling velocity are found in the strong (weak) IOD simulation group. Thus, the mean state biases greatly affect the air–sea coupling strength on the interannual time scale. A number of models failed to simulate the observed positive wind–evaporation–SST feedback during the IOD developing phase. Analysis indicates that the bias arises from a greater contribution to the surface latent heat flux anomaly by the sea–air specific humidity difference than by the wind speed anomaly.

Full access
Sha Lu
,
Weidong Guo
,
Jun Ge
, and
Yu Zhang

Abstract

The arid and semiarid areas of the Loess Plateau are extremely sensitive to climate change. Land–atmosphere interactions of these regions play an important role in the regional climate. However, most present land surface models (LSMs) are not reasonable and accurate enough to describe the surface characteristics in these regions. In this study, we investigate the effects of three key land surface parameters including surface albedo, soil thermal conductivity, and additional damping on the Noah LSM in simulating the land surface characteristics. The observational data from June to September from 2007 to 2009 collected at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) station in northwest China are used to validate the Noah LSM simulations. The results suggest that the retrieved values of surface albedo, soil thermal conductivity, and additional damping based on observations are in closer agreement with those of the MULT scheme for surface albedo, the J75_NOAH scheme for soil thermal conductivity, and the Y08 scheme for additional damping, respectively. Furthermore, the model performance is not obviously affected by surface albedo parameterization schemes, while the scheme of soil thermal conductivity is vital to simulations of latent heat flux and soil temperature and the scheme of additional damping is crucial for simulating net radiation flux, sensible heat flux, and surface soil temperature. A set of optimal parameterizations is proposed for the offline Noah LSM at the SACOL station when the MULT scheme for surface albedo, the J75_NOAH scheme for soil thermal conductivity, and the Y08 scheme for additional damping are combined simultaneously, especially in the case of sensible heat flux and surface soil temperature simulations.

Full access
Yang Yang
,
Tim Li
,
Kuiping Li
, and
Weidong Yu

Abstract

Recent in situ buoy observations revealed interesting seasonal features of the diurnal sea surface temperature cycle (DSST) in the eastern tropical Indian Ocean. Composite analysis shows that areas away from the equator exhibit stronger seasonal variations of DSST, while weaker seasonal variations appear near the equator. The most interesting characteristic is the distinctive contrast of the seasonal variations of DSST between the Bay of Bengal (BOB) and the region south of the equator (particularly around 12°S). While the range of DSST is weakest in the BOB during boreal summer, it has its largest range around 12°S in austral summer. Furthermore, BOB DSST exhibits two peaks that occur during the monsoon transitions (March–April and October), whereas DSST south of the equator shows only a single peak in its annual cycle.

Using a one-dimensional, oceanic, mixed layer model, the authors examined the cause of the distinctive annual cycles of DSST north and south of the equator. Two parallel experiments were conducted at buoy sites 12°N, 90°E and 12°S, 80.5°E driven by surface forcing from the Modern-Era Retrospective Analysis for Research and Applications (MERRA) product. The results demonstrated that, in the BOB, both surface shortwave radiation and wind stress contribute to the March maximum, whereas the wind stress alone drives the October maximum. In contrast, the seasonal variation of DSST south of the equator is primarily caused by the annual cycle of the wind stress, which is extremely weak in austral summer near the intertropical convergence zone (ITCZ). How the monsoon and ITCZ modulate the distinctive annual cycles of DSST is discussed.

Full access
Baoqiang Xiang
,
Ming Zhao
,
Yi Ming
,
Weidong Yu
, and
Sarah M. Kang

Abstract

Most current climate models suffer from pronounced cloud and radiation biases in the Southern Ocean (SO) and in the tropics. Using one GFDL climate model, this study investigates the migration of the intertropical convergence zone (ITCZ) with prescribed top-of-the-atmosphere (TOA) shortwave radiative heating in the SO (50°–80°S) versus the southern tropics (ST; 0°–20°S). Results demonstrate that the ITCZ position response to the ST forcing is twice as strong as the SO forcing, which is primarily driven by the contrasting sea surface temperature (SST) gradient over the tropics; however, the mechanism for the formation of the SST pattern remains elusive. Energy budget analysis reveals that the conventional energetic constraint framework is inadequate in explaining the ITCZ shift in these two perturbed experiments. For both cases, the anomalous Hadley circulation does not contribute to transport the imposed energy from the Southern Hemisphere to the Northern Hemisphere, given a positive mean gross moist stability in the equatorial region. Changes in the cross-equatorial atmospheric energy are primarily transported by atmospheric transient eddies when the anomalous ITCZ shift is most pronounced during December–May. The partitioning of energy transport between the atmosphere and ocean shows latitudinal dependence: the atmosphere and ocean play an overall equivalent role in transporting the imposed energy for the extratropical SO forcing, while for the ST forcing, the imposed energy is nearly completely transported by the atmosphere. This contrast originates from the different ocean heat uptake and also the different meridional scale of the anomalous ocean circulation.

Full access
Kuiping Li
,
Qin He
,
Yang Yang
,
Zhi Li
, and
Weidong Yu

Abstract

The atmospheric 10–20-day quasi-biweekly mode (QBWM) significantly modulates the active–break spells of the South Asian monsoon. Current knowledge, however, is limited concerning the diversity of the QBWM in the Indian Ocean (IO). Based on extended empirical orthogonal function analysis, two dominant summer modes are constructed in the IO. The first mode (QBWM1) generally depicts IO basin-dependent variability, while the second mode (QBWM2) exhibits a close relationship with the northwestern Pacific. QBWM1 initiates in the equatorial western IO and propagates toward the eastern IO along the equator. Two Rossby wave cells evolve in the off-equatorial eastern IO when convection encounters the Maritime Continent, and subsequently the northern cell develops and moves westward in the South Asian monsoon region. In contrast, QBWM2 originates in the northwestern Pacific and passes westward across the South Asian monsoon region in the form of convectively coupled Rossby waves. The maintenance mechanism of the peculiar IO basin-dependent QBWM1 is understood in terms of moisture dynamics. Significant moisture anomalies are found to precondition convection initiation in the western IO and subsequent eastward movement along the equator. Afterward, two off-equatorial moisture centers are generated in the double Rossby wave cells along with convection dissipation in the eastern IO, and the moisture anomalies are delivered from the southern cell toward the convection initiation area in the western IO via a moisture conveyor belt without coupling with convection. Moisture budget analysis indicates that the horizontal moisture advection associated with QBWM1 is regulated by the mean clockwise circulation in the tropical IO.

Full access
Zhi Li
,
Weidong Yu
,
Kuiping Li
,
Huiwu Wang
, and
Yanliang Liu

Abstract

Globally, the highest formation rate of super tropical cyclones (TCs) occurs over the Bay of Bengal (BoB) during the premonsoon transition period (PMT), but TC genesis has a low frequency here. TCs have occurred over the BoB in only 20 of the past 36 years of PMTs (1981–2016). This study investigates which environmental conditions modulate TC formation during the PMT over the BoB by conducting a quantitative analysis based on the genesis potential parameter, vorticity tendency equation, and specific humidity budget equation. The results show that there is a cyclonic anomaly in the TC genesis group compared to the non-TC genesis group, which is mainly due to the divergence term. A significant difference in vorticity contributes to TC formation over the BoB during the PMT. Furthermore, anomalous cyclonic flow enhances ascending motion, transporting moisture to the midlevel atmosphere. A change in specific humidity (SH) causes an increase in relative humidity, which contributes positively to TC formation. The vertical wind shear also makes a small positive contribution. In contrast to the previous three terms, the contribution from the instability term associated with 500- and 850-hPa air temperatures is negative and almost negligible. In addition, the synoptic-scale disturbance energy is more powerful in the TC genesis group than in the non-TC genesis group, which is favorable for TC breeding. Together, these conditions determine whether TCs are generated over the BoB during the PMT.

Open access
Kuiping Li
,
Yang Yang
,
Lin Feng
,
Weidong Yu
, and
Shouhua Liu

Abstract

This study investigates the northward-propagating quasi-biweekly oscillation (QBWO) in the western North Pacific by examining the composite meridional structures. Using newly released reanalysis and remote sensing data, the northward propagation is understood in terms of the meridional contrasts in the planetary boundary layer (PBL) moisture and the column-integrated moist static energy (MSE). The meridional contrast in the PBL moisture, with larger values north of the convection center, is predominantly attributed to the moisture convergence associated with barotropic vorticity anomalies. A secondary contribution comes from the meridional moisture advection, for which advections by mean and perturbation winds are almost equally important. The meridional contrast in the MSE tendency, due to the recharge in the front of convection and discharge in the rear of convection, is jointly contributed by the meridional and vertical MSE advections. The meridional MSE advection mainly depends on the moisture processes particularly in the PBL, and the vertical MSE advection largely results from the advection of the mean MSE by vertical velocity anomalies, wherein the upper-troposphere ascending motion related to the stratiform heating in the rear of the convection plays the major role. In addition, partial feedback from sea surface temperature (SST) anomalies is evaluated on the basis of MSE budget analysis. SST anomalies tend to enhance the surface turbulent heat fluxes ahead of the convention center and suppress them behind the convention center, thus positively contributing approximately 20% of the meridional contrast in the MSE tendency.

Free access
Jinhui Xie
,
Pang-Chi Hsu
,
Pallav Ray
,
Kuiping Li
, and
Weidong Yu

Abstract

As rainfed agriculture remains India’s critical source of livelihood, improving our understanding of rainy season onset timing in the region is of great importance for a better prediction. Using a new gridded dataset of rainy season characteristics, we found a clear phase relationship between the Madden–Julian oscillation (MJO) and the onset timing of the rainy season over the Indian subcontinent. A significantly high probability of rainy season onset is observed when the MJO convection stays over the western-central Indian Ocean. On the other hand, the rainy season onset is infrequent when the MJO is over the Maritime Continent and western Pacific. The MJO-associated convective instability with anomalous warm and moist air in the lower troposphere appears and grows during the period 10 days prior to the onset of rainy season, and drops substantially after the start of rainy season, suggesting its role as a trigger of rainy season onset. In contrast, the low-frequency background state (LFBS) with a period > 90 days favors a convectively unstable stratification even after the onset of the rainy season, supporting the succeeding precipitation during the entire rainy season. Based on the scale-decomposed moisture budget diagnosis, we further found that the key processes inducing the abrupt transition from a dry to a wet condition come mainly from two processes: 1) convergence of LFBS moisture by MJO-related circulation perturbations and 2) advection of MJO moisture anomalies by the background cross-equatorial flow toward the Indian subcontinent. The results may help provide a better and longer lead-time prediction of the rainy season onset over the Indian subcontinent.

Full access
Yang Yang
,
Yanliang Liu
,
Kuiping Li
,
Lin Liu
, and
Weidong Yu

Abstract

The 10–20-day quasi-biweekly oscillation (QBWO) is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with comprehensive analyses of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. In this study, the diversity of the austral summer QBWO in the SWIO is examined based on K-means cluster analysis, which objectively classifies two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). For the EM (PM), an active convection center originates from the subtropical ocean (tropical ocean) and exhibits an eastward (poleward) propagation path. Moisture budget analysis reveals that positive moisture time tendency anomalies show a phase-leading relationship relative to both QBWO convection centers. This phase leading in moisture tendency anomalies is mainly due to horizontal moisture advection. Further analysis demonstrates that meridional moisture transport (i.e., the summer mean moisture advected by the meridional quasi-biweekly wind) is fundamentally responsible for moisture phase leading in both QBWO modes in their mature phases. The combined scale interaction among low frequency, quasi-biweekly, and high frequency contributes to the initial movement for both modes in the growing phases. Although the two modes in the SWIO are initiated in different regions and exhibit distinct evolutionary features, they are regulated by similar moisture dynamics: the northerlies (northeasterlies) of the cyclonic wind response bring higher mean moisture levels east (south) of the convective center, which leads to the eastward (southward) movement of the EM (PM).

Significance Statement

The quasi-biweekly oscillation (QBWO), which can affect extreme weather events, such as extreme precipitation and heat waves, is active in the southwestern Indian Ocean (SWIO) during austral summer. Compared with previous studies of the QBWO in the Asian monsoon regions during boreal summer, studies focusing on the austral summer QBWO in the SWIO are relatively scarce. Specifically, we objectively classify the austral summer QBWO in the SWIO into two distinct modes: an eastward-propagating mode (EM) and a poleward-propagating mode (PM). Through moisture tendency diagnosis, we find that the two QBWO modes are regulated by similar moisture dynamics, although they are initiated in different regions and exhibit distinct evolutionary features. This improved understanding may provide insights into the monitoring and prediction of the QBWO.

Restricted access
Xing Luo
,
Jun Ge
,
Weidong Guo
,
Lei Fan
,
Chaorong Chen
,
Yu Liu
, and
Limei Yang

Abstract

Deforestation can impact precipitation through biophysical processes and such effects are commonly examined by models. However, previous studies mostly conduct deforestation experiments with a single model and the simulated precipitation responses to deforestation diverge across studies. In this study, 11 Earth system models are used to robustly examine the biophysical impacts of deforestation on precipitation, precipitation extremes, and the seasonal pattern of the rainy season through a comparison of a control simulation and an idealized global deforestation simulation with clearings of 20 million km2 of forests. The multimodel mean suggests decreased precipitation, reduced frequency and intensity of heavy precipitation, and shortened duration of rainy seasons over deforested areas. The deforestation effects can even propagate to some regions that are remote from deforested areas (e.g., the tropical and subtropical oceans and the Arctic Ocean). Nevertheless, the 11 models do not fully agree on the precipitation changes almost everywhere. In general, the models exhibit higher consistency over the deforested areas and a few regions outside the deforested areas (e.g., the subtropical oceans) but lower consistency over other regions. Such intermodel spread mostly results from divergent responses of evapotranspiration and atmospheric moisture convergence to deforestation across the models. One of the models that has multiple simulation members also reveals considerable spread of the precipitation responses to deforestation across the members due to internal model variability. This study highlights the necessity of robustly examining precipitation responses to deforestation based on multiple models and each model with multiple simulation members.

Full access