Gui-Ying Yang - Academia.edu (original) (raw)

Papers by Gui-Ying Yang

Quarterly journal of the Royal Meteorological Society, Feb 27, 2024

Tellus B, Feb 1, 1999

An account is given of a number of recent studies with idealised models whose aim is to further u... more An account is given of a number of recent studies with idealised models whose aim is to further understanding of the large-scale tropical atmospheric circulation. Initial-value integrations with a model with imposed heating are used to discuss aspects of the Asian summer monsoon, including constraints on cross-equatorial flow into the monsoon. The summer descent in the Mediterranean region and on the eastern sides of the summer subtropical anticyclones are seen to be associated with the monsoons to their east. An aqua-planet GCM is used to investigate the relationship between simple SST distributions and tropical convection and circulation. The existence of strong equatorial convection and Hadley cells is found to depend sensitively on the curvature of the meridional profile in SST. Zonally confined SST maxima produce convective maxima centred to the west and suppression of convection elsewhere. Strong equatorial zonal flow changes are found in some experiments and three mechanisms for producing these are investigated in a model with imposed heating.

Journal of Climate, Jul 21, 2023

The global perspective presented here is built on earlier papers discussing the dynamics of the u... more The global perspective presented here is built on earlier papers discussing the dynamics of the upper branch of the Hadley cell in the two solsticial seasons. The role of the tropics is made explicit in a conceptual model that is presented and evaluated. The fluctuation of deep tropical convection in longitude and time is seen as crucial. The filamentary outflows from such convective events move westward and across the equator deep into the winter hemisphere. The horizontal tilt of the cross-equatorial flow implies a significant upper-tropospheric flux of westerly momentum from the winter tropics to the summer hemisphere. These properties are related to the cross-equatorial propagation of wave activity triggered by deep tropical convection in the summer hemisphere. The filaments carry with them near-equatorial values of absolute vorticity and potential vorticity. After turning anticyclonically, some filaments move eastward and poleward to the equatorial edge of the winter subtropical jet. There is strong evidence they can interact with the eddies on this jet and enhance their poleward westerly momentum flux. In the global perspective, tropical and extratropical systems and the interaction between them are all important for the dynamics of the upper branch of the Hadley cell. significance statement The Hadley cell is the large-scale overturning in the atmosphere with air in the upper troposphere moving from the equatorial region to near 30° in the winter hemisphere. In the standard view it is midlatitude weather systems that are responsible for removing angular momentum from this upper branch of the Hadley cell. Here it is proposed that tropical systems and their interaction with the midlatitude systems are also important. Insight into the role of the tropics in the dynamics of the Hadley cell can be obtained by considering it as the sum of many events of active deep convection occurring in different longitudes and at different times.

Quarterly Journal of the Royal Meteorological Society, 2020

The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for th... more The seminal theory for the Hadley Cells has demonstrated that their existence is necessary for the reduction of tropical temperature gradients to a value such that the implied zonal winds are realisable. At the heart of the theory is the notion of angular momentum conservation in the upper branch of the Hadley Cells. Eddy mixing associated with extratropical systems is invoked to give continuity at the edge of the Hadley Cell and to reduce the subtropical jet by a factor of three or more to those observed. In this article a detailed view is presented of the dynamics of the June-August Hadley Cell, as given by ERA data for the period 1981-2010, with an emphasis on the dynamics of the upper branch. The steady and transient northward fluxes of angular momentum have a very similar structure, both having a maximum on the Equator and a reversal in sign near 12 • S, with the transient flux merging into that associated with eddies on the winter subtropical jet. In the northward absolute vorticity flux, the Coriolis torque is balanced by both the steady and transient fluxes. The overturning circulations that average to give the Hadley Cell are confined to specific longitudinal regions, as are the steady and transient momentum fluxes. In these regions, both intraseasonal and synoptic variations are important. The dominant contributor to the Hadley Cell is from the Indian Ocean and west Pacific regions, and the maxima in OLR variability and meridional wind in these regions have a characteristic structure associated with the Westward-moving Mixed Rossby-Gravity wave. Much of the upper tropospheric motion into the winter hemisphere occurs in filaments of air from the summer equatorial region. These filaments can reach the winter subtropical jet, leading to the strengthening of it and of the eddies on it, implying strong tropical-extratropical interaction.

Journal of Climate, 2006

The atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is descri... more The atmospheric component of the new Hadley Centre Global Environmental Model (HadGEM1) is described and an assessment of its mean climatology presented. HadGEM1 includes substantially improved representations of physical processes, increased functionality, and higher resolution than its predecessor, the Third Hadley Centre Coupled Ocean–Atmosphere General Circulation Model (HadCM3). Major developments are the use of semi-Lagrangian instead of Eulerian advection for both dynamical and tracer fields; new boundary layer, gravity wave drag, microphysics, and sea ice schemes; and major changes to the convection, land surface (including tiled surface characteristics), and cloud schemes. There is better coupling between the atmosphere, land, ocean, and sea ice subcomponents and the model includes an interactive aerosol scheme, representing both the first and second indirect effects. Particular focus has been placed on improving the processes (such as clouds and aerosol) that are most unce...

Nature Communications

Understanding and prediction of tropical cyclone (TC) activity on the medium range remains challe... more Understanding and prediction of tropical cyclone (TC) activity on the medium range remains challenging. Here, we find that the pre-existing westward-moving equatorial waves can inform the risk of TC occurrence and intensification, based on a dataset obtained by synchronising objectively identified TCs and equatorial waves in a climate reanalysis. Globally, westward-moving equatorial waves can be precursors to 60–70% of pre-tropical cyclogenesis events, and to >80% of the events with the strongest vorticity, related to the favourable environmental conditions within the pouch of equatorial waves. We further find that when storms are in-phase with westward-moving equatorial waves, the intensification rate of TCs is augmented, whilst in other phases of the waves, storm intensity grows more slowly, or even decays. Coherent wave packets associated with TCs are identifiable up to two weeks ahead. Our findings show that westward-moving equatorial waves can be useful medium-range precurso...

Quarterly Journal of the Royal Meteorological Society

Equatorial waves (EWs) are synoptic- to planetary-scale propagating disturbances... more Equatorial waves (EWs) are synoptic- to planetary-scale propagating disturbances at low latitudes with periods from a few days to several weeks. Here this term includes Kelvin waves, equatorial Rossby waves, mixed-Rossby gravity waves and inertio-gravity waves, which are well described by linear wave theory, but also other tropical disturbances such as easterly waves and the intraseasonal Madden-Julian Oscillation with more complex dynamics. EWs can couple with deep convection, leading to a substantial modulation of clouds and rainfall. EWs are amongst the dynamic features of the troposphere with the longest intrinsic predictability and models are beginning to forecast them with an exploitable level of skill. Most of the methods developed to identify and objectively isolate EWs in observations and model fields rely on (or at least refer to) the adiabatic, frictionless linearized primitive equations on the sphere or the shallow water system on the equatorial &#946;-plane. Common ingredients to these methods are zonal wavenumber-frequency filtering (Fourier or wavelet) and/or projections onto predefined empirical or theoretical dynamical patterns. This paper gives an overview of six different methods to isolate EWs and their structures, discusses the underlying assumptions, evaluates the applicability to different problems and provides a systematic comparison based on a case study (20 February-20 May 2009) and a climatological analysis (2001-2018). In addition, the influence of different input fields (e.g. winds, geopotential, outgoing longwave radiation, rainfall) is investigated. Based on the results, we generally recommend employing a combination of wavenumber-frequency filtering and spatial-projection methods (and of different input fields) to check for robustness of the identified signal. In cases of disagreement, one needs to carefully investigate which assumptions made for the individual methods are most probably not fulfilled. This will help choose an approach optimally suited to a given problem at hand and avoid misinterpretation of the results.&#160;

Monthly Weather Review, 2001

A global archive of high-resolution (3-hourly, 0.5Њ latitude-longitude grid) window (11-12 m) bri... more A global archive of high-resolution (3-hourly, 0.5Њ latitude-longitude grid) window (11-12 m) brightness temperature (T b) data from multiple satellites is being developed by the European Union Cloud Archive User Service (CLAUS) project. It has been used to construct a climatology of the diurnal cycle in convection, cloudiness, and surface temperature for all regions of the Tropics. An example of the application of the climatology to the evaluation of the climate version of the U.K. Met. Office Unified Model (UM), version HadAM3, is presented. The characteristics of the diurnal cycle described by the CLAUS data agree with previous observational studies, demonstrating the universality of the characteristics of the diurnal cycle for land versus ocean, clear sky versus convective regimes. It is shown that oceanic deep convection tends to reach its maximum in the early morning. Continental convection generally peaks in the evening, although there are interesting regional variations, indicative of the effects of complex land-sea and mountain-valley breezes, as well as the life cycle of mesoscale convective systems. A striking result from the analysis of the CLAUS data has been the extent to which the strong diurnal signal over land is spread out over the adjacent oceans, probably through gravity waves of varying depths. These coherent signals can be seen for several hundred kilometers and in some instances, such as over the Bay of Bengal, can lead to substantial diurnal variations in convection and precipitation. The example of the use of the CLAUS data in the evaluation of the Met. Office UM has demonstrated that the model has considerable difficulty in capturing the observed phase of the diurnal cycle in convection, which suggests some fundamental difficulties in the model's physical parameterizations. Analysis of the diurnal cycle represents a powerful tool for identifying and correcting model deficiencies.

Annals of Geophysics, 2009

Examples of current research into systematic errors in climate models are used to demonstrate the... more Examples of current research into systematic errors in climate models are used to demonstrate the importance of scale interactions on diurnal, intraseasonal and seasonal timescales for the mean and variability of the tropical climate system. It has enabled some conclusions to be drawn about possible processes that may need to be represented, and some recommendations to be made regarding model improvements. It has been shown that the Maritime Continent heat source is a major driver of the global circulation but yet is poorly represented in GCMs. A new climatology of the diurnal cycle has been used to provide compelling evidence of important land-sea breeze and gravity wave effects, which may play a crucial role in the heat and moisture budget of this key region for the tropical and global circulation. The role of the diurnal cycle has also been emphasized for intraseasonal variability associated with the Madden Julian Oscillation (MJO). It is suggested that the diurnal cycle in Sea Surface Temperature (SST) during the suppressed phase of the MJO leads to a triggering of cumulus congestus clouds, which serve to moisten the free troposphere and hence precondition the atmosphere for the next active phase. It has been further shown that coupling between the ocean and atmosphere on intraseasonal timescales leads to a more realistic simulation of the MJO. These results stress the need for models to be able to simulate firstly, the observed tri-modal distribution of convection, and secondly, the coupling between the ocean and atmosphere on diurnal to intraseasonal timescales. It is argued, however, that the current representation of the ocean mixed layer in coupled models is not adequate to represent the complex structure of the observed mixed layer, in particular the formation of salinity barrier layers which can potentially provide much stronger local coupling between the atmosphere and ocean on diurnal to intraseasonal timescales.

Journal of Climate, 2021

This paper complements an earlier paper on the June–August Hadley cell by giving a detailed analy... more This paper complements an earlier paper on the June–August Hadley cell by giving a detailed analysis of the December–February Hadley cell as seen in a 30-yr climatology of ERA-Interim data. The focus is on the dynamics of the upper branch of the Hadley cell. There are significant differences between the Hadley cells in the two solsticial seasons. These are particularly associated with the ITCZs staying north of the equator and with mean westerlies in the equatorial regions of the east Pacific and Atlantic in December–February. The latter enables westward-moving mixed Rossby–gravity waves to be slow moving in those regions and therefore respond strongly to upstream off-equatorial active convection. However, the main result is that in both seasons it is the regions and times of active convection that predominantly lead to upper-tropospheric outflows and structures that average to give the mean flow toward the winter pole, and the steady and transient fluxes of momentum and vorticity t...

Weather and Forecasting, 2021

A novel technique is developed to identify equatorial waves in analyses and forecasts. In a real-... more A novel technique is developed to identify equatorial waves in analyses and forecasts. In a real-time operational context, it is not possible to apply a frequency filter based on a wide centered time window due to the lack of future data. Therefore, equatorial wave identification is performed based primarily on spatial projection onto wave mode horizontal structures. Spatial projection alone cannot distinguish eastward- from westward-moving waves, so a broadband frequency filter is also applied. The novelty in the real-time technique is to off-center the time window needed for frequency filtering, using forecasts to extend the window beyond the current analysis. The quality of this equatorial wave diagnosis is evaluated. First, the “edge effect” arising because the analysis is near the end of the filter time window is assessed. Second, the impact of using forecasts to extend the window beyond the current date is quantified. Both impacts are shown to be small referenced to wave diagn...

Journal of the Atmospheric Sciences, 2012

The variation of stratospheric equatorial wave characteristics with the phase of the quasi-bienni... more The variation of stratospheric equatorial wave characteristics with the phase of the quasi-biennial oscillation (QBO) is investigated using ECMWF Re-Analysis and NOAA outgoing longwave radiation (OLR) data. The impact of the QBO phases on the upward propagation of equatorial waves is found to be consistent and significant. In the easterly phase, there is larger Kelvin wave amplitude but smaller westward-moving mixed Rossby–gravity (WMRG) and n = 1 Rossby (R1) wave amplitude due to reduced propagation from the upper troposphere into the lower stratosphere, compared with the westerly phase. Differences in the wave amplitude exist in a deeper layer in summer than in winter, consistent with the seasonality of ambient zonal winds. There is a strong evidence of Kelvin wave amplitude peaking just below the descending westerly phase, suggesting that Kelvin waves act to bring the westerly phase downward. However, the corresponding evidence for WMRG and R1 waves is less clear. In the lower st...

Journal of Atmospheric and Oceanic Technology, 2000

An improved algorithm for the generation of gridded window brightness temperatures is presented. ... more An improved algorithm for the generation of gridded window brightness temperatures is presented. The primary data source is the International Satellite Cloud Climatology Project, level B3 data, covering the period from July 1983 to the present. The algorithm takes window brightness, temperatures from multiple satellites, both geostationary and polar orbiting, which have already been navigated and normalized radiometrically to the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer, and generates 3-hourly global images on a 0.5Њ by 0.5Њ latitude-longitude grid. The gridding uses a hierarchical scheme based on spherical kernel estimators. As part of the gridding procedure, the geostationary data are corrected for limb effects using a simple empirical correction to the radiances, from which the corrected temperatures are computed. This is in addition to the application of satellite zenith angle weighting to downweight limb pixels in preference to nearer-nadir pixels. The polar orbiter data are windowed on the target time with temporal weighting to account for the noncontemporaneous nature of the data. Large regions of missing data are interpolated from adjacent processed images using a form of motion compensated interpolation based on the estimation of motion vectors using an hierarchical block matching scheme. Examples are shown of the various stages in the process. Also shown are examples of the usefulness of this type of data in GCM validation.

Journal of Climate

The canonical view of the Maritime Continent (MC) diurnal cycle is deep convection occurring over... more The canonical view of the Maritime Continent (MC) diurnal cycle is deep convection occurring over land during the afternoon and evening, tending to propagate offshore overnight. However, there is considerable day-to-day variability in the convection, and the mechanism of the offshore propagation is not well understood. We test the hypothesis that large-scale drivers such as ENSO, the MJO and equatorial waves, through their modification of the local circulation, can modify the direction or strength of the propagation, or prevent the deep convection from triggering in the first place. Taking a local-to-large scale approach we use in situ observations, satellite data and reanalyses for five MC coastal regions, and show that the occurrence of the diurnal convection and its offshore propagation is closely tied to coastal wind regimes we define using the k-means cluster algorithm. Strong prevailing onshore winds are associated with a suppressed diurnal cycle of precipitation; while prevai...

International Journal of Climatology

Quarterly Journal of the Royal Meteorological Society

Equatorially trapped waves, such as Kelvin Waves, Equatorial Rossby Waves and Westward-moving Mix... more Equatorially trapped waves, such as Kelvin Waves, Equatorial Rossby Waves and Westward-moving Mixed Rossby-Gravity (WMRG) Waves, play a major role in organising tropical convection on synoptic to sub-seasonal timescales. These waves have the potential to provide an important source of predictability for high-impact weather in Southeast (SE) Asia and the Tropics more widely. To aid understanding of the role played in high-impact weather by such waves, the observed statistical relationship between identified equatorial waves and heavy rainfall in SE Asia is examined for the period 1998-2016. Increases in the amount of precipitation and the likelihood of extreme precipitation in SE Asia are linked to all three types of waves that are included in analysis; Kelvin, equatorial Rossby and WMRG waves. There is both increased mean rainfall and increased probability of occurrence of heavy rainfall on days when high-amplitude waves propagate over SE Asia. In particular, heavy precipitation can be up to three times more likely in regions of SE Asia during equatorial waves, including Malaysia, Indonesia and the Philippines. Kelvin waves have a large influence on heavy rainfall over Indonesia, WMRG and Kelvin waves impact Malaysia rainfall, and equatorial Rossby and WMRG waves are linked to increased rainfall over the Philippines. Based on this study it can be concluded that the probability of extreme precipitation in this region is dependent on equatorial wave activity. Therefore, the skill in probabilistic prediction of extreme precipitation in SE Asia would be expected to be conditional on the skill in equatorial wave prediction, and the modelled relationship between equatorial waves and convection.

Journal of the Atmospheric Sciences

A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Ros... more A connection is found between African easterly waves (AEWs), equatorial westward-moving mixed Rossby–gravity (WMRG) waves, and equivalent barotropic Rossby waves (RWs) from the Southern Hemisphere (SH). The amplitude and phase of equatorial waves is calculated by projection of broadband-filtered ERA-Interim data onto a horizontal structure basis obtained from equatorial wave theory. Mechanisms enabling interaction between the wave types are identified. AEWs are dominated by a vorticity wave that tilts eastward below the African easterly jet and westward above: the tilt necessary for baroclinic wave growth. However, a strong relationship is identified between amplifying vorticity centers within AEWs and equatorial WMRG waves. Although the waves do not phase lock, positive vorticity centers amplify whenever the cross-equatorial motion of the WMRG wave lies at the same longitude in the upper troposphere (southward flow) and east of this in the lower troposphere (northward flow). Two me...

Quarterly journal of the Royal Meteorological Society, Feb 27, 2024

Tellus B, Feb 1, 1999

Journal of Climate, Jul 21, 2023

Quarterly Journal of the Royal Meteorological Society, 2020

Journal of Climate, 2006

Nature Communications

Quarterly Journal of the Royal Meteorological Society

Monthly Weather Review, 2001

Annals of Geophysics, 2009

Journal of Climate, 2021

Weather and Forecasting, 2021

Journal of the Atmospheric Sciences, 2012

Journal of Atmospheric and Oceanic Technology, 2000

Journal of Climate

International Journal of Climatology

Quarterly Journal of the Royal Meteorological Society

Journal of the Atmospheric Sciences