Search Results

You are looking at 1 - 10 of 42 items for

  • Author or Editor: Lin Zhao x
  • Refine by Access: All Content x
Clear All Modify Search
Wenchao Chu
,
Yanluan Lin
, and
Ming Zhao

Abstract

Performance of global climate models (GCMs) is strongly affected by the cumulus parameterization (CP) used. Similar to the approach in GFDL AM4, a double-plume CP, which unifies the deep and shallow convection in one framework, is implemented and tested in the NCAR Community Atmospheric Model version 5 (CAM5). Based on the University of Washington (UW) shallow convection scheme, an additional plume was added to represent the deep convection. The shallow and deep plumes share the same cloud model, but use different triggers, fractional mixing rates, and closures. The scheme was tested in single-column, short-term hindcast, and AMIP simulations. Compared with the default combination of the Zhang–McFarlane scheme and UW scheme in CAM5, the new scheme tends to produce a top-heavy mass flux profile during the active monsoon period in the single-column simulations. The scheme increases the intensity of tropical precipitation, closer to TRMM observations. The new scheme increased subtropical marine boundary layer clouds and high clouds over the deep tropics, both in better agreement with observations. Sensitivity tests indicate that regime-dependent fractional entrainment rates of the deep plume are desired to improve tropical precipitation distribution and upper troposphere temperature. This study suggests that a double-plume approach is a promising way to combine shallow and deep convections in a unified framework.

Full access
Lin Zhao
,
S.-Y. Simon Wang
, and
Jonathan Meyer

Abstract

Using observed and reanalysis data, the pronounced interdecadal variations of Lake Qinghai (LQH) water levels and associated climate factors were diagnosed. From the 1960s to the early 2000s, the water level of LQH in the Tibetan Plateau has experienced a continual decline of 3 m but has since increased considerably. A water budget analysis of the LQH watershed suggested that the water vapor flux divergence is the dominant atmospheric process modulating precipitation and subsequently the lake volume change . The marked interdecadal variability in and was found to be related to the North Pacific (NP) and Pacific decadal oscillation (PDO) modes during the cold season (November–March). Through empirical orthogonal function (EOF) and regression analyses, the water vapor sink over the LQH watershed also responds significantly to the summer Eurasian wave train modulated by the low-frequency variability associated with the cold season NP and PDO modes. Removal of these variability modes (NP, PDO, and the Eurasian wave train) led to a residual uptrend in the hydrological variables of , , and precipitation, corresponding to the net water level increase. Attribution analysis using the Coupled Model Intercomparison Project phase 5 (CMIP5) single-forcing experiments shows that the simulations driven by greenhouse gas forcing produced a significant increase in the LQH precipitation, while anthropogenic aerosols generated a minor wetting trend as well.

Full access
Lin Liu
,
Guang Yang
,
Xia Zhao
,
Lin Feng
,
Guoqing Han
,
Yue Wu
, and
Weidong Yu

Abstract

The Indian Ocean witnessed a weak positive Indian Ocean dipole (IOD) event from the boreal summer to autumn in 2015, while an extreme El Niño occurred over the tropical Pacific. This was different from the case in 1997/98, when an extreme El Niño and the strongest IOD took place simultaneously. The analysis here suggests that the unique sea surface temperature anomaly (SSTA) pattern of El Niño in 2015 might have contributed to the weak IOD that year. El Niño in 2015 had a complex SSTA pattern, with positive warming over the central and eastern tropical Pacific. Such a combination of the classic El Niño (also known as cold-tongue El Niño) and the recently identified central Pacific El Niño (also known as El Niño Modoki II) had opposite remote influences on the tropical Indian Ocean. The classic El Niño reduced the strength of the Walker circulation over the tropical Indian Ocean, but this was offset by El Niño Modoki II. This study points out that the IOD can be strongly modulated by combined El Niño types in some circumstances, as in 2015.

Full access
Kexin Song
,
Jiuwei Zhao
,
Ruifen Zhan
,
Li Tao
, and
Lin Chen

Abstract

Confidence and uncertainty issues of simulations were seldom evaluated in previous studies although the climate models are widely used. This study evaluates the performance of the CMIP6-HighResMIP simulations in presenting long-term variability of tropical cyclone (TC) genesis frequency (TCGF) and track density (TCTD) and quantifies the relative contributions of internal and external forcing to TC activities during the 1950–2014. There is overall poor model performance in simulating long-term changes in TC activities over the Northern Hemisphere, including interdecadal variabilities and long-term linear trends. The simulated long-term changes in TCGF and TCTD over the eastern North Pacific (ENP) in six high-resolution models show opposite characteristics to the observations. Moreover, most models cannot capture the variabilities of TCGF and TCTD over the western part of the western North Pacific (WNP) and northern part of the North Atlantic (NA). However, these models show a high degree of confidence in reproducing the interdecadal variabilities and linear trends of TCGF and TCTD over the eastern part of the WNP and the tropical NA. Quantitative evaluations further show that there are the opposite relative contributions of long-term climate variabilities to TCGF and TCTD changes over the ENP between the observations and the multimodel ensemble mean, followed by large model biases over the western WNP and the northern NA, but relatively consistent contributions over the southern NA and the Caribbean. These results help us cope with contrasting and consistent future TC changes among the model projections.

Significance Statement

While climate models have been widely used to project future changes in tropical cyclone (TC) activity, few studies have examined to what extent we can trust these model projections. We used the CMIP6-HighResMIP simulations to quantify the model biases in presenting TC activity, and evaluate the relative contributions of internal and external forcing to TC activities. In general, the HighResMIP has large discrepancies in representing longer-term climate variability of TC activity. However, the models can capture well TC activity over the eastern part of the western North Pacific and tropical Atlantic, which is attributed to good performance of models in reproducing the relationship between long-term climate variabilities beyond interannual scale and TC activity. These results highlight confidence and uncertainty in future TC changes among the model projections.

Open access
Ming Zhao
,
Isaac M. Held
, and
Shian-Jiann Lin

Abstract

High-resolution global climate models (GCMs) have been increasingly utilized for simulations of the global number and distribution of tropical cyclones (TCs), and how they might change with changing climate. In contrast, there is a lack of published studies on the sensitivity of TC genesis to parameterized processes in these GCMs. The uncertainties in these formulations might be an important source of uncertainty in the future projections of TC statistics.

This study investigates the sensitivity of the global number of TCs in present-day simulations using the Geophysical Fluid Dynamics Laboratory High Resolution Atmospheric Model (GFDL HIRAM) to alterations in physical parameterizations. Two parameters are identified to be important in TC genesis frequency in this model: the horizontal cumulus mixing rate, which controls the entrainment into convective cores within the convection parameterization, and the strength of the damping of the divergent component of the horizontal flow. The simulated global number of TCs exhibits nonintuitive response to incremental changes of both parameters. As the cumulus mixing rate increases, the model produces nonmonotonic response in global TC frequency with an initial sharp increase and then a decrease. However, storm mean intensity rises monotonically with the mixing rate. As the strength of the divergence damping increases, the model produces a continuous increase of global number of TCs and hurricanes with little change in storm mean intensity. Mechanisms for explaining these nonintuitive responses are discussed.

Full access
Dingchi Zhao
,
Wenhao Dong
,
Yanluan Lin
,
Yang Hu
, and
Dianbin Cao

Abstract

Using abundant rainfall gauge measurements and Global Precipitation Mission (GPM) data, spatial patterns of rainfall diurnal cycles and their seasonality over high mountain Asia (HMA) were examined. Spatial distributions of rainfall diurnal cycles over the HMA have a prominent seasonality regulated by circulations at different spatiotemporal scales, within which large regional contrasts are embedded. Rainfall diurnal variability is relatively weak in the premonsoon season, with larger amplitude over the western HMA, the southeastern HMA, as well as southern periphery regions, characterized by a dominant late afternoon to morning rainfall preference. The pattern of rainfall spatial distributions is closely related to the midlatitude westerlies. Both the mean rainfall and amplitudes of diurnal cycles become more pronounced with the advance of monsoon season but weaken during postmonsoon. The widespread late afternoon to night pattern over HMA migrating with seasonal atmospheric circulation is consistent with the lifetime of convective systems, which become active from the afternoon due to radiative heating and decay during the night. Stationary terrain-dependent night-to-morning rainfall patterns are visible in those east–west-orientated valleys over HMA and the Qaidam basin throughout the seasons. This salient geographical dependence is associated with local circulation produced by the strong differential thermal conditions over mountains and valleys, which can lift the warm moist air at the mouth of the valley and trigger nocturnal convection.

Significance Statement

The main purpose of this study is to explore how spatial patterns of rainfall diurnal cycles over high mountain Asia vary with the seasons. Our results show that the widespread late afternoon to night rainfall over high mountain Asia migrating with seasonal atmospheric circulation is consistent with the lifetime of convective systems. Stationary terrain-dependent night-to-morning rainfall patterns are visible in those east–west-orientated valleys over high mountain Asia and the Qaidam basin throughout the seasons. These results highlight the importance of large-scale atmospheric circulation and local circulation on precipitation, which is critical for water resources over high mountain Asia.

Free access
Guang Yang
,
Xia Zhao
,
Dongliang Yuan
,
Yazhou Zhang
,
Lin Liu
, and
Shiqiu Peng

Abstract

Previous studies have indicated that boreal winter-to-spring sea surface temperature anomalies (SSTA) over the tropical Atlantic or Indian Ocean can trigger the central-Pacific (CP) type of ENSO in the following winter due to winds over the western Pacific. Here, with the aid of observational data and CMIP5 model simulations, we demonstrate that the ability of the winter-to-spring north tropical Atlantic (NTA) SSTA or Indian Ocean Basin (IOB) mode to initiate CP ENSO events in the following winter may strongly depend on each other. Most warming events of the IOB and NTA, which are followed by CP La Niña events, are concomitant. The synergistic effect of the IOB and NTA SSTA may produce greater CP ENSO events in the subsequent winter via Walker circulation adjustments. The impacts between warming and cooling events of the IOB and NTA SSTA are asymmetric. IOB and NTA warmings appear to contribute to the subsequent CP La Niña development, which is much greater than IOB and NTA cooling contributing to CP El Niño. Overall, a combination of the IOB and NTA SSTA precursors may improve predictions of La Niña events.

Significance Statement

Although boreal winter-to-spring sea surface temperature anomalies over the tropical Atlantic or Indian Ocean can trigger central-Pacific (CP) ENSO in the following winter, it is not yet clear whether the effects of these two basins are independent. The purpose of this study is to better understand the joint effect of these two basins on CP ENSO events. We demonstrate that the ability of the north tropical Atlantic (NTA) SSTA to initiate CP ENSO events in the following winter may strongly depend on the state of the Indian Ocean Basin mode (IOB). The synergistic impact of these two basins may produce stronger CP ENSO events. These results highlight the role of three-ocean interactions in ENSO diversity and prediction.

Restricted access
Wenli Wang
,
Kun Yang
,
Long Zhao
,
Ziyan Zheng
,
Hui Lu
,
Ali Mamtimin
,
Baohong Ding
,
Xin Li
,
Lin Zhao
,
Hongyi Li
,
Tao Che
, and
John C. Moore

Abstract

Snow depth on the interior of Tibetan Plateau (TP) in state-of-the-art reanalysis products is almost an order of magnitude higher than observed. This huge bias stems primarily from excessive snowfall, but inappropriate process representation of shallow snow also causes excessive snow depth and snow cover. This study investigated the issue with respect to the parameterization of fresh snow albedo. The characteristics of TP snowfall were investigated using ground truth data. Snow in the interior of the TP is usually only some centimeters in depth. The albedo of fresh snow depends on snow depth, and is frequently less than 0.4. Such low albedo values contrast with the high values (~0.8) used in the existing snow schemes of land surface models. The SNICAR radiative transfer model can reproduce the observations that fresh shallow snow has a low albedo value, based on which a fresh snow albedo scheme was derived in this study. Finally, the impact of the fresh snow albedo on snow ablation was examined at 45 meteorological stations on TP using the land surface model Noah-MP which incorporated the new scheme. Allowing albedo to change with snow depth can produce quite realistic snow depths compared with observations. In contrast, the typically assumed fresh snow albedo of 0.82 leads to too large snow depths in the snow ablation period averaged across 45 stations. The shallow snow transparency impact on snow ablation is therefore particularly important in the TP interior, where snow is rather thin and radiation is strong.

Free access
Yunpeng Shan
,
Eric M. Wilcox
,
Lan Gao
,
Lin Lin
,
David L. Mitchell
,
Yan Yin
,
Tianliang Zhao
,
Lei Zhang
,
Hongrong Shi
, and
Meng Gao

Abstract

Significant uncertainty lies in representing the rain droplet size distribution (DSD) in bulk cloud microphysics schemes and in the derivation of parameters of the function fit to the spectrum from the varying moments of a DSD. Here we evaluate the suitability of gamma distribution functions (GDFs) for fitting rain DSDs against observed disdrometer data. Results illustrate that double-parameter GDFs with prescribed or diagnosed positive spectral shape parameters μ fit rain DSDs better than the Marshall–Palmer distribution function (with μ = 0). The relative errors of fitting the spectrum moments (especially high-order moments) decrease by an order of magnitude [from O(102) to O(101)]. Moreover, introduction of a triple-parameter GDF with mathematically solved μ decreases the relative errors to O(100). Based on further investigation of potential combinations of the three prognostic moments for triple-moment cloud microphysical schemes, it is found that the GDF with parameters determined from predictions of the zeroth, third, and fourth moments (the 034 GDF) exhibits the best fit to rain DSDs compared to other moment combinations. Therefore, we suggest that the 034 prognostic moment group should replace the widely accepted 036 group to represent rain DSDs in triple-moment cloud microphysics schemes. An evaluation of the capability of GDFs to represent rain DSDs demonstrates that 034 GDF exhibits accurate fits to all observed DSDs except for rarely occurring extremely wide spectra from heavy precipitation and extremely narrow spectra from drizzle. The knowledge gained from this assessment can also be used to improve cloud microphysics retrieval schemes and data assimilation.

Free access
Baoqiang Xiang
,
Ming Zhao
,
Xianan Jiang
,
Shian-Jiann Lin
,
Tim Li
,
Xiouhua Fu
, and
Gabriel Vecchi

Abstract

Based on a new version of the Geophysical Fluid Dynamics Laboratory (GFDL) coupled model, the Madden–Julian oscillation (MJO) prediction skill in boreal wintertime (November–April) is evaluated by analyzing 11 years (2003–13) of hindcast experiments. The initial conditions are obtained by applying a simple nudging technique toward observations. Using the real-time multivariate MJO (RMM) index as a predictand, it is demonstrated that the MJO prediction skill can reach out to 27 days before the anomaly correlation coefficient (ACC) decreases to 0.5. The MJO forecast skill also shows relatively larger contrasts between target strong and weak cases (32 versus 7 days) than between initially strong and weak cases (29 versus 24 days). Meanwhile, a strong dependence on target phases is found, as opposed to relative skill independence from different initial phases. The MJO prediction skill is also shown to be about 29 days during the Dynamics of the MJO/Cooperative Indian Ocean Experiment on Intraseasonal Variability in Year 2011 (DYNAMO/CINDY) field campaign period. This model’s potential predictability, the upper bound of prediction skill, extends out to 42 days, revealing a considerable unutilized predictability and a great potential for improving current MJO prediction.

Full access