Search Results

You are looking at 21 - 30 of 71 items for

  • Author or Editor: Yi Huang x
  • Refine by Access: All Content x
Clear All Modify Search
Yi Huang
,
Stephen S. Leroy
, and
James G. Anderson

Abstract

The authors investigate whether combining a data type derived from radio occultation (RO) with the infrared spectral data in an optimal detection method improves the quantification of longwave radiative forcing and feedback. Signals derived from a doubled-CO2 experiment in a theoretical study are used. When the uncertainties in both data types are conservatively estimated, jointly detecting the feedbacks of tropospheric temperature and water vapor, stratospheric temperature, and high-level cloud from the two data types should reduce the mean errors by more than 50%. This improvement is achieved because the RO measurement helps disentangle the radiance signals that are ambiguous in the infrared spectrum. The result signifies the complementary information content in infrared spectral and radio occultation data types, which can be effectively combined in optimal detection to accurately quantify the longwave radiative forcing and feedback. The results herein show that the radiative forcing of CO2 and the longwave radiative feedbacks of tropospheric temperature, tropospheric water vapor, and stratospheric temperature can be accurately quantified from the combined data types, with relative errors in their global mean values being less than 4%, 10%, 15%, and 20%, respectively.

Full access
Fahimeh Sarmadi
,
Yi Huang
,
Steven T. Siems
, and
Michael J. Manton

Abstract

The relationship between orographic precipitation, low-level thermodynamic stability, and the synoptic meteorology is explored for the Snowy Mountains of southeast Australia. A 21-yr dataset (May–October, 1995–2015) of upper-air soundings from an upwind site is used to define synoptic indicators and the low-level stability. A K-means clustering algorithm was employed to classify the daily meteorology into four synoptic classes. The initial classification, based only on six synoptic indicators, distinctly defines both the surface precipitation and the low-level stability by class. Consistent with theory, the wet classes are found to have weak low-level stability, and the dry classes have strong low-level stability. By including low-level stability as an additional input variable to the clustering method, statistically significant correlations were found between the precipitation and the low-level stability within each of the four classes. An examination of the joint PDF reveals a highly nonlinear relationship; heavy rain was associated with very weak low-level stability, and conversely, strong low-level stability was associated with very little precipitation. Building on these historical relationships, model output statistics (MOS) from a moderate resolution (12-km spatial resolution) operational forecast were used to develop stepwise regression models designed to improve the 24-h forecast of precipitation over the Snowy Mountains. A single regression model for all days was found to reduce the RMSE by 7% and the bias by 75%. A class-based regression model was found to reduce the overall RMSE by 30% and the bias by 85%.

Full access
Yi-Hsuan Huang
,
Chun-Chieh Wu
, and
Michael T. Montgomery

Abstract

This is a follow-up work to two prior studies examining secondary eyewall formation (SEF) in Typhoon Sinlaku (2008). This study shows that, in the SEF region, the majority of the elevated winds are supergradient. About two-thirds of the rapid increase in tangential wind tendencies immediately prior to SEF are attributed to agradient wind tendencies. This suggests the importance of nonlinear, unbalanced dynamical processes in SEF in addition to the classical axisymmetric balanced response to forcings of heating and momentum. In the SEF region, analyses show two distinct responsible processes for the increasing azimuthal tangential wind in two vertical intervals. Within the boundary inflow layer, the competing effect between the mean radial influx of absolute vorticity and deceleration caused by surface friction and subgrid diffusion yields a secondary maximum of positive tendency. Analyses further demonstrate the major impact of the mean radial influx of absolute vorticity on SEF. Above the boundary inflow layer, the vertical advection acts to vertically extend the tangential wind jet via the lofting of the enhanced tangential momentum farther upward. The roles of the nonlinear unbalanced dynamics in these two processes are discussed in this paper. From a Lagrangian perspective, the persistently increasing agradient force outweighs the frictional loss, effectively decelerating boundary layer inflowing air across the SEF region. This explains the sharpening of the radial gradient of boundary layer inflow, which is shown to be responsible for the buildup of a zone with concentrated boundary layer convergence. The previously proposed unbalanced dynamical pathway to SEF is elaborated upon and supported by the current results and discussion.

Full access
Guanghua Chen
,
Chun-Chieh Wu
, and
Yi-Hsuan Huang

Abstract

The effects of convective and stratiform diabatic processes in the near-core region on tropical cyclone (TC) structure and intensity change are examined by artificially modifying the convective and stratiform heating/cooling between 40- and 80-km radii. Sensitivity experiments show that the absence of convective heating in the annulus can weaken TC intensity and decrease the inner-core size. The increased convective heating generates a thick and polygonal eyewall, while the storm intensifies more gently than that in the control run. The removal of stratiform heating can slow down TC intensification with a moderate intensity, whereas the doubling of stratiform heating has little effect on the TC evolution compared to the control run. The halved stratiform cooling facilitates TC rapid intensification and a compact inner-core structure with the spiral rainbands largely suppressed. With the stratiform cooling doubled, the storm terminates intensification and eventually develops a double-eyewall-like structure accompanied by the significantly outward expansion of the inner-core size. The removal of both stratiform heating and cooling generates the strongest storm with the structure and intensity similar to those in the experiment with stratiform cooling halved. When both stratiform heating and cooling are doubled, the storm first decays rapidly, followed by the vertical connection of the updrafts at mid- to upper levels in the near-core region and at lower levels in the collapsed eyewall, which reinvigorates the eyewall convection but with a large outward slope.

Full access
Xiao-Yi Yang
,
Rui Xin Huang
, and
Dong Xiao Wang

Abstract

Using 40-yr ECMWF Re-Analysis (ERA-40) data and in situ observations, the positive trend of Southern Ocean surface wind stress during two recent decades is detected, and its close linkage with spring Antarctic ozone depletion is established. The spring Antarctic ozone depletion affects the Southern Hemisphere lower-stratospheric circulation in late spring/early summer. The positive feedback involves the strengthening and cooling of the polar vortex, the enhancement of meridional temperature gradients and the meridional and vertical potential vorticity gradients, the acceleration of the circumpolar westerlies, and the reduction of the upward wave flux. This feedback loop, together with the ozone-related photochemical interaction, leads to the upward tendency of lower-stratospheric zonal wind in austral summer. In addition, the stratosphere–troposphere coupling, facilitated by ozone-related dynamics and the Southern Annular Mode, cooperates to relay the zonal wind anomalies to the upper troposphere. The wave–mean flow interaction and the meridional circulation work together in the form of the Southern Annular Mode, which transfers anomalous wind signals downward to the surface, triggering a striking strengthening of surface wind stress over the Southern Ocean.

Full access
Clemente Lopez-Bravo
,
Claire L. Vincent
,
Yi Huang
, and
Todd P. Lane

Abstract

A West Sumatra squall line occurred on 10 January 2016, with a clear offshore propagation of convection. Satellite-derived products from Himawari-8 Advanced Himawari Imager and the Geostationary Cloud Algorithm Testbed Geocat are used to investigate the westward propagation of cloudiness from Sumatra to the Indian Ocean with a lifetime of 1.5 days. A convective mask based on deep convective cell detection and a cell-tracking algorithm are used to estimate the propagation speed of the cloud system. Two distinct mesoscale convective responses are identified: 1) a rapid development in South Sumatra is influenced by the convective environment over the Indian Ocean. The propagation speed is estimated to be ∼5 m s−1 within the first 200 km from the coast. This speed is consistent with density currents. In contrast, 2) the coupling to the inertia–gravity wave is only evident for the northwest of Sumatra with speeds of ∼12 m s−1. The analysis of brightness temperature from the 10.4-μm spectral band and cloud-top temperature showed that the lifetime of the squall line is approximately 30 h with a propagating distance of ∼1000 km. Retrieved cloud properties and tracking of the offshore propagation indicated that the cloud structure consisted of multiple types of cells, propagating as envelopes of convection, and revealed the influence of large-scale variability of the Indian Ocean. Filtered OLR anomalies, satellite-derived rainfall, moisture flux convergence, and background winds flow around Sumatra are used to explore the effects of Kelvin wave activity that likely influenced the lifetime of the squall line.

Free access
Artur Gevorgyan
,
Luis Ackermann
,
Yi Huang
,
Steven Siems
, and
Michael Manton

Abstract

The case study of a heavy precipitation event associated with the passage of cold front over the Australian Snowy Mountains (ASM) on 3 August 2018 has been examined using the observational data from an intensive field campaign and high-resolution (1 km) Weather Research and Forecasting (WRF) simulation. We divided this event into prefrontal, cold front, and postfrontal periods. The cold front and postfrontal periods were characterized by higher production of graupel, while relatively low graupel was produced in the prefrontal period. Overall, aggregation along with deposition are likely the main growth mechanisms of snow in the prefrontal clouds, while heavy rain was produced below the melting level over windward slopes of the ASM. The simulated melting level is lower compared to the observations, which is consistent with model cold bias. Stronger orographic uplift and frontal forcing were mainly responsible for the enhanced supercooled liquid water (SLW) production over the ASM in the cold front period. A drop in elevation of the freezing level and increase in low-level relative humidity further enhanced the SLW production. The production of graupel through riming processes was highly efficient in the cold front period given the high concentration of ice-phase hydrometeors in the frontal clouds and the development of clouds comprising supercooled liquid water. The orographic updrafts and embedded convection were the main dynamical processes generating postfrontal SLW clouds and graupel. Ice initiation processes were activated once SLW cloud tops reached −15°C level followed by graupel production through riming processes.

Free access
Yi Huang
,
Steven T. Siems
,
Michael J. Manton
, and
Gregory Thompson

Abstract

The representation of the marine boundary layer (BL) clouds remains a formidable challenge for state-of-the-art simulations. A recent study by Bodas-Salcedo et al. using the Met Office Unified Model highlights that the underprediction of the low/midlevel postfrontal clouds contributes to the largest bias of the surface downwelling shortwave radiation over the Southern Ocean (SO). A-Train observations and limited in situ measurements have been used to evaluate the Weather Research and Forecasting Model, version 3.3.1 (WRFV3.3.1), in simulating the postfrontal clouds over Tasmania and the SO. The simulated cloud macro/microphysical properties are compared against the observations. Experiments are also undertaken to test the sensitivity of model resolution, microphysical (MP) schemes, planetary boundary layer (PBL) schemes, and cloud condensation nuclei (CCN) concentration. The simulations demonstrate a considerable level of skill in representing the clouds during the frontal passages and, to a lesser extent, in the postfrontal environment. The simulations, however, have great difficulties in portraying the widespread marine BL clouds that are not immediately associated with fronts. This shortcoming is persistent to the changes of model configuration and physical parameterization. The representation of large-scale conditions and their connections with the BL clouds are discussed. A lack of BL moisture is the most obvious explanation for the shortcoming, which may be a consequence of either strong entrainment or weak surface fluxes. It is speculated that the BL wind shear/turbulence may be an issue over the SO. More comprehensive observations are necessary to fully investigate the deficiency of the simulations.

Full access
Chung-Chieh Wang
,
George Tai-Jen Chen
, and
Shin-Yi Huang

Abstract

In this study, the heavy-rainfall event over central Taiwan during the mei-yu season on 8 June 2007 is investigated, with an emphasis on the triggering mechanism for the deep convection that produced the rain. Observations indicate that there existed two lines of forcing with convection prior to the rain: one over the northern Taiwan Strait along the mei-yu front and the other over the southern Taiwan Strait. Yet, the convection in question developed over the central strait between these two lines, in an unstable environment with strong westerly vertical wind shear. This motivated the authors to carry out the present study.

The Cloud-Resolving Storm Simulation (CReSS) of Nagoya University was used and the event was reproduced at a horizontal grid size of 2 km, including the initiation of new convection over the central strait at the correct location and time. The model results suggest a crucial role played by the series of active, persistent, and propagating storms in the southern strait (along the aforementioned second forcing line). On their back (northern) side, these storms repeatedly produced pulses of cold outflow that traveled toward the north-northeast with positive pressure perturbation. With characteristics of gravity waves, the perturbation propagated faster than the cold air and the associated increase in forward-directed (horizontal) pressure gradient force led to northward acceleration of near-surface flow (by up to 4–5 m s−1 h−1). The stronger southerly flow in turn enhanced downstream convergence, and the deep convection was triggered in the central strait near the arrival of the gravity wave ahead of the cold air. When the convection moved eastward over Taiwan, heavy rainfall resulted. The mechanism presented here for remote triggering of convection over the ocean has not been documented near Taiwan during the mei-yu season. With a better understanding about the behavior of convection, these results can contribute to the improvement of quantitative precipitation forecasts and hazard prevention and reduction.

Full access
Yi-Hsuan Huang
,
Chun-Chieh Wu
, and
Yuqing Wang

Abstract

High-resolution simulations for Typhoon Krosa (2007) and a set of idealized experiments are conducted using a full-physics model to investigate the eminent deflection of typhoon track prior to its landfall over mountainous island topography. The terrain height of Taiwan plays the most important role in Typhoon Krosa’s looping motion at its landfall, while the surface properties, details in the topographic shape of Taiwan, and the cloud microphysics are shown to be secondary to the track deflection. A simulation with 3-km resolution and realistic model settings reproduces the observed Krosa’s track, while that with 9-km resolution fails, suggesting that high resolution to better resolve the typhoon–terrain interactions is important for the prediction and simulation of typhoon track deflection prior to landfall. Results from idealized experiments with model configurations mimicking those of Supertyphoon Krosa show that vortices approaching the northern and central topography are significantly deflected to the south before making sharp turns to the north, forming a kinked track pattern prior to and during landfall. This storm movement is consistent with the observed looping cases in Taiwan.

Both real-case and idealized simulations show strong channel winds enhanced between the storm and the terrain when deflection occurs. Backward trajectory analyses support the concept of the channeling effect, which has been previously found to be crucial to the looping motion of Typhoon Haitang (2005) as well. However, the inner-core asymmetric ventilation flow does not match the movement of a deflected typhoon perfectly, partly because the steering flow is not well defined and could not completely capture the terrain-induced deflection in the simulation and in nature.

Full access