Yuqing Wang - Academia.edu (original) (raw)

Papers by Yuqing Wang

Rainfall in the tropics exhibits a large, 12 h Sun-synchronous variation with coherent phase arou... more Rainfall in the tropics exhibits a large, 12 h Sun-synchronous variation with coherent phase around the globe. A long-standing, but unproved, hypothesis for this phenomenon is excitation by the prominent 12 h atmospheric tide, which itself is significantly forced remotely by solar heating of the stratospheric ozone layer. We investigated the relative roles of large-scale tidal forcing and more local effects in accounting for the 12 h variation of tropical rainfall. A model of the atmosphere run with the diurnal cycle of solar heating artificially suppressed below the stratosphere still simulated a strong coherent 12 h rainfall variation (~50% of control run), demonstrating that stratospherically forced atmospheric tide propagates downward to the troposphere and contributes to the organization of large-scale convection. The results have implications for theories of excitation of tropical atmospheric waves by moist convection, for the evaluation of climate models, and for explaining the recently discovered lunar tidal rainfall cycle.

It has been long known that cloud microphysics can have a significant impact on the simulations o... more It has been long known that cloud microphysics can have a significant impact on the simulations of precipitation; however, there have been few studies so far that have investigated the effect of cloud microphysics on tropical cyclones. In the most advanced simulation of tropical cyclones by numerical models, the use of explicit cloud microphysics becomes more and more attractive with cumulus parameterization bypassed at very high resolutions. In this study, the sensitivity of the simulated tropical cyclone structure and intensity to the choice and details of cloud microphysics parameterization is investigated using the triply nested movable mesh tropical cyclone model TCM3 described in Part I but with several refinements. Three different cloud microphysics parameterization schemes are tested, including the warm-rain-only cloud microphysics scheme (WMRN) and two mixed-icephase cloud microphysics schemes, one of which has three ice species (cloud ice-snow-graupel; CTRL) while the other has hail instead of graupel (HAIL).

A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropica... more A long-standing issue on how outer spiral rainbands affect the structure and intensity of tropical cyclones is studied through a series of numerical experiments using the cloud-resolving tropical cyclone model TCM4. Because diabatic heating due to phase changes is the main driving force of outer spiral rainbands, their effect on the tropical cyclone structure and intensity is evaluated by artificially modifying the heating and cooling rate due to cloud microphysical processes in the model. The view proposed here is that the effect of diabatic heating in outer spiral rainbands on the storm structure and intensity results mainly from hydrostatic adjustment; that is, heating (cooling) of an atmospheric column decreases (increases) the surface pressure underneath the column. The change in surface pressure due to heating in the outer spiral rainbands is significant on the inward side of the rainbands where the inertial stability is generally high. Outside the rainbands in the far field, where the inertial stability is low and internal atmospheric heating is mostly lost to gravity wave radiation and little is left to warm the atmospheric column and lower the local surface pressure, the change in surface pressure is relatively small. This strong radially dependent response reduces the horizontal pressure gradient across the radius of maximum wind and thus the storm intensity in terms of the maximum low-level tangential wind while increasing the inner-core size of the storm.

This study analyzes the diurnal cycle of precipitation simulated in a global cloud-resolving mode... more This study analyzes the diurnal cycle of precipitation simulated in a global cloud-resolving model (GCRM) named the Nonhydrostatic Icosahedral Atmospheric Model (NICAM). A 30-day integration of NICAM successfully simulates the precipitation diurnal cycle associated with the land-sea breeze and the thermally induced topographic circulations as well as the horizontal propagation of diurnal cycle signals. The first harmonic of the diurnal cycle of precipitation in the 7-km run agrees well with that from satellite observations in its geographical distributions although its amplitude is slightly overestimated. The NICAM simulation revealed that the precipitation diurnal cycle over the Maritime Continent is strongly coupled with the land-sea breeze that controls the convergence/divergence pattern in the lower troposphere around the islands. The analysis also suggests that the cold pool often forms over the open ocean where the precipitation intensity is high, and the propagation of the cold pool events is related to the precipitation diurnal cycle as well as the land-sea breeze.

Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts ... more Strong winds in a tropical cyclone over the ocean can produce high seas with substantial amounts of spray in the lower part of the atmospheric boundary layer. The effects that the evaporation of this sea spray may have on the transfer of energy between the ocean and the atmosphere, and consequent effects on the boundary layer structure, cumulus convection, and the evolution of the tropical cyclone, are largely unknown. In this study, a high-resolution tropical cyclone model with explicit cloud microphysics, developed by Y. Wang, has been used to study these potential effects. The sea spray evaporation is incorporated into the model by two bulk parameterization schemes with quite different properties.

Uncertainties in projected future changes in tropical cyclone (TC) activity are investigated usin... more Uncertainties in projected future changes in tropical cyclone (TC) activity are investigated using future (2075-2099) ensemble projections of global warming under the Intergovernmental Panel on Climate Change (IPCC) A1B scenario. Twelve ensemble experiments are performed using three different cumulus convection schemes and four different assumptions for prescribed future sea surface temperatures (SSTs). All ensemble experiments consistently project significant reductions in global and hemispheric TC genesis numbers as well as reductions in TC frequency of occurrence (TCF) and TC genesis frequency (TGF) in the western North Pacific, South Indian Ocean, and South Pacific Ocean. TCF and TGF are projected to increase over the central Pacific which is consistent with the findings of Li et al. (2010).

Two phytoplankton blooms in the South China Sea (SCS), triggered by 2 typhoons with different int... more Two phytoplankton blooms in the South China Sea (SCS), triggered by 2 typhoons with different intensities and translation speeds, were compared using remotely sensed chlorophyll a (chl a), sea surface temperature (SST), vector wind field, and best-track typhoon data. Typhoon Ling-Ling in 2001 was strong, with a maximum sustained surface wind speed of 59 m s -1 , and fast-moving with a mean translation speed of 4.52 m s -1 . Typhoon Kai-Tak in 2005 was weak with a maximum sustained surface wind speed of 46 m s -1 , and slow-moving with a mean translation speed of 2.87 m s -1 . The weak, slow-moving typhoon Kai-Tak induced phytoplankton blooms with higher chl a concentrations, while the strong, fast-moving typhoon Ling-Ling induced blooms over a larger area. On average, about 7 typhoons per year affect the SCS, among which 41% are strong (> 50 m s -1 ) and 59% are weak, while 64% are fast-moving (> 4.4 m s -1 ) and 36% are slow-moving. We conservatively estimate that typhoon periods may account for 3.5% of the annual primary production in the oligotrophic SCS.

The surface energy (entropy) flux is critical to the development and maintenance of a tropical cy... more The surface energy (entropy) flux is critical to the development and maintenance of a tropical cyclone (TC). However, it is unclear how sensitive the inner-core size and intensity of a TC could be to the radial distribution of the surface entropy flux under the TC. Such a potential sensitivity is examined in this study using the multiply nested, fully compressible, nonhydrostatic TC model TCM4. By artificially eliminating the surface entropy fluxes in different radial extent in different experiments, the effect of the surface entropy flux in the different radial ranges on the inner-core size and intensity of a simulated TC is evaluated. Consistent with recent findings from axisymmetric models, the entropy flux in the eye region of a TC is found to contribute little to the storm intensity, but it plays a role in reducing the radius of maximum wind (RMW). Although surface entropy fluxes under the eyewall contribute greatly to the storm intensity, those outside the eyewall up to a radius of about 2-2.5 times the RMW are also important. Farther outward, the surface entropy fluxes are found to be crucial to the growth of the storm inner-core size but could reduce the storm intensity. The surface entropy flux outside the inner core plays a critical role in maintaining high convective available potential energy (CAPE) outside the eyewall and thus active spiral rainbands. The latent heat release in these rainbands is responsible for the increase in the inner-core size of the simulated TC. A positive feedback is identified to explain changes in the inner-core size of the simulated storms in different experiments. Implications of the results for both observations and numerical prediction of TC structure and intensity changes are briefly discussed.

The structure and formation of an annular hurricane simulated in a fully compressible, nonhydrost... more The structure and formation of an annular hurricane simulated in a fully compressible, nonhydrostatic tropical cyclone model-TCM4-are analyzed. The model is initialized with an axisymmetric vortex on an f plane in a quiescent environment, and thus the transition from the nonannular hurricane to the annular hurricane is attributed to the internal dynamics. The simulated annular hurricane has all characteristics of those recently documented by Knaff et al. from satellite observations: quasi-axisymmetric structure, large eye and wide eyewall, high intensity, and suppressed major spiral rainbands. A striking feature of the simulated annular hurricane is its large outward tilt of the wide eyewall, which is critical to the quasi-steady high intensity and is responsible for the maintenance of the large size of the eye and eyewall of the storm. Although the annular hurricane has a quasi-axisymmetric structure, marked low-wavenumber asymmetries exist in the eyewall region.

A previous hail climatology of China was based upon observations during 1951-60.

1] The diurnal cycle of precipitation in the New Guinean region is studied on the basis of satell... more 1] The diurnal cycle of precipitation in the New Guinean region is studied on the basis of satellite observations from Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) measurements and regional atmospheric model simulations. The study focuses on the effects of both the land-sea breeze and the orographic forcing on the diurnal evolution of precipitation during the rainy season (January-March) in the region. The 7-year TRMM PR data composite reveals several distinct features of the precipitation diurnal cycle in the region. Precipitation bands develop in the inland coastal region in the late morning to early afternoon and migrate inland from both northeast and southwest sides of the New Guinean Island following the inland penetration of the sea-breeze fronts. A separate convective rainband develops over the central mountain ridge in the early afternoon as a result of the development of the upslope winds due to the elevated surface warming over the mountain in the morning hours. This mountain ridge rainband intensifies and becomes the dominant rainband as the coastal rainbands associated with the sea-breeze fronts weaken during the late afternoon and the early evening. In the midnight to the early morning the rainband over the mountaintop weakens as downslope winds develop and splits into two rainbands, propagating away from the mountain ridge, one to the north and one to the south, and weakens over the lowland some distance away from the coasts. Meanwhile a coastal rainband develops offshore on each side of the island in the late evening to midnight and remains strong through early morning before it migrates offshore. As a result, the rainfall rate peaks in the late afternoon to early evening in most land areas except for in the lowland regions between the coastlines and the mountain where the rainfall rate peaks during the midnight, while the rainfall rate peaks in the late evening to early morning in most coastal regions offshore. The distribution of the diurnal amplitude shows two maxima: one over the mountains and the other in the coastal regions offshore. Convective rainfall rate peaks in the late afternoon while stratiform rainfall rate peaks in the midnight to early morning. The latter dominates the large diurnal amplitude over the mountain areas in the early morning. The above broad features are simulated reasonably well in a control experiment with a high-resolution regional atmospheric model. A sensitivity experiment with the terrain removed is conducted to elucidate the role of orographic forcing in the diurnal evolution of both the local circulation and rainfall patterns. The results show that the orographic forcing affects the diurnal precipitation through three major processes. First, the orography increases the moisture convergence at low levels by blocking and deflecting the mean flow. Second, the upslope winds help initiate convection in the afternoon at the mountaintop. Finally, the deep convection over the mountain acts as a source of propagating gravity waves, which help initiate rainbands in the coastal regions offshore in the late evening to early morning. Implication of the results is discussed.

When Typhoon Songda (2004) was located southeast of Okinawa over the western North Pacific during... more When Typhoon Songda (2004) was located southeast of Okinawa over the western North Pacific during 2-4 September 2004, a heavy rainfall event occurred over southern central Japan and its adjacent seas, more than 1200 km from the typhoon center. The Advanced Research version of the Weather Research and Forecast (WRF-ARW) model was used to investigate the possible remote effects of Typhoon Songda on this heavy precipitation event in Japan. The National Centers for Environmental Prediction (NCEP) global final (FNL) analysis was used to provide both the initial and lateral boundary conditions for the WRF model. The model was initialized at 1800 UTC 2 September and integrated until 1800 UTC 6 September 2004, during which Songda was a supertyphoon. Two primary numerical experiments were performed. In the control experiment, a bogus vortex was inserted into the FNL analysis to enhance the initial storm intensity such that the model typhoon had an intensity that was similar to that observed at the initial time. In the no-typhoon experiment, the vortex associated with Typhoon Songda in the FNL analysis was removed by a smoothing algorithm such that the typhoon signal did not appear at the initial time. As verified against various observations, the control experiment captured reasonably well the evolution of the storm and the spatial distribution and evolution of the precipitation, whereas the remote precipitation in Japan was largely suppressed in the no-typhoon experiment, indicting the significant far-reaching effects of Typhoon Songda. Songda enhanced the remote precipitation in Japan mainly through northward moisture transport into the preconditioned precipitation region by its outer circulation. The orographic forcing of the central mountains in Japan played a small role compared with Typhoon Songda in this extreme precipitation event.

A tropical cyclone (TC) viewed as a heat engine converts heat energy extracted from the ocean int... more A tropical cyclone (TC) viewed as a heat engine converts heat energy extracted from the ocean into the kinetic energy of the TC, which is eventually dissipated due to surface friction. Since the energy production rate is a linear function while the frictional dissipation rate is a cubic power of surface wind speed, the dissipation rate is generally smaller than the production rate initially but increases faster than the production rate as the storm intensifies. When the dissipation rate eventually reaches the production rate, the TC has no excess energy to intensify. Emanuel hypothesized that a TC achieves its maximum potential intensity (E-MPI) when the surface frictional dissipation rate balances the energy production rate near the radius of maximum wind (RMW). Although the E-MPI agrees well with the maximum intensity of numerically simulated TCs in earlier axisymmetric models, the balance hypothesis near the RMW has not been evaluated. This study shows that the frictional dissipation rate in a numerically simulated mature TC is about 25% larger than the energy production rate near the RMW, while the dissipation rate is lower than the energy production rate outside the eyewall. This finding implies that the excess frictional dissipation under the eyewall should be partially balanced by the energy production outside the eyewall and thus the local balance hypothesis underestimates the TC maximum intensity. Both Lagrangian and control volume equivalent potential temperature (u e ) budget analyses demonstrate that the energy gained by boundary layer inflow air due to surface entropy fluxes outside of and prior to interaction with the eyewall contributes significantly to the energy balance in the eyewall through the lateral inward energy flux. This contribution is further verified using a sensitivity experiment in which the surface entropy fluxes are eliminated outside a radius of 30-45 km, which leads to a 13.5% reduction in the maximum sustained near-surface wind speed and a largely reduced size of the model TC.

The balanced contribution to the intensification of a tropical cyclone simulated in the three-dim... more The balanced contribution to the intensification of a tropical cyclone simulated in the three-dimensional, nonhydrostatic, full-physics tropical cyclone model version 4 (TCM4), in particular the spinup of the outercore circulation, is investigated by solving the Sawyer-Eliassen equation and by computing terms in the azimuthal-mean tangential wind tendency equation. Results demonstrate that the azimuthal-mean secondary circulation (radial and vertical circulation) and the spinup of the midtropospheric outer-core circulation in the simulated tropical cyclone are well captured by balance dynamics. The midtropospheric inflow develops in response to diabatic heating in mid-upper-tropospheric stratiform (anvil) clouds outside the eyewall in active spiral rainbands and transports absolute angular momentum inward to spin up the outer-core circulation. Although the azimuthal-mean diabatic heating rate in the eyewall is the largest, its contribution to radial winds and thus the spinup of outer-core circulation in the middle troposphere is rather weak. This is because the high inertial stability in the inner-core region resists the radial inflow in the middle troposphere, limiting the inward transport of absolute angular momentum. The result thus suggests that diabatic heating in spiral rainbands is the key to the continued growth of the storm-scale circulation.

A previous hail climatology of China was based upon observations during 1951-60.