Compositions of hydrogen and oxygen isotopes of precipitation in Xiamen, Southeast China Coast: Seasonal variations, synoptic processes, and typhoons impact

In this study, the δD and δ18O values of 162 precipitation samples (including 33 typhoon-related precipitation samples), collected in Xiamen, Southeast China coast, during June 2018 to August 2019, were investigated and analyzed. The results show that there are obvious seasonal variations in the δD and δ18O, which are mainly controlled by the East Asia Monsoon with significant influence of typhoon events in summer. The influence of moisture sources on δ18O values overrides the influence of precipitation fractionation process on δ18O values which leads to an inverse temperature effect in the study area. In comparison to the seasonal scale, the synoptic time-series variation of δD and δ18O is much more complicated. In general, there are three types of isotopic variations in the normal precipitation processes, which are obviously affected by re-evaporation processes and continuing equilibrium fractionation during condensation. The local meteorological parameters during normal precipitation, which mainly control the re-evaporation process, are the dominant factors for the variation patterns of δD and δ18O, whereas moisture sources control the overall isotope values of precipitation. The differences between the time-series of normal and typhoon-related precipitation are mainly controlled by the changes of physical processes and meteorologic parameters during the precipitation process. However, due to the unique atmospheric structure and dynamic processes of typhoons, the δD and δ18O of typhoon-related precipitation changes in stages gradually as the distance between the typhoon’s center and the study area changes. The uniformity of typhoon structure leads to a similar staged changes in different typhoon-related precipitation. The moisture source trajectory of typhoon-related precipitation shows a clear spiral structure (except for typhoon Yutu), and the moisture sources at different heights control the δD and δ18O values of typhoon-related precipitation. This study is important for quantifying the global changes of typhoon processes and paleotempestology studies.

Evaluation of Operational Monsoon Moisture Surveillance and Severe Weather Prediction Utilizing COSMIC-2/FORMOSAT-7 Radio Occultation Observations

Operational monsoon moisture surveillance and severe weather prediction is essential for timely water resource management and disaster risk reduction. For these purposes, this study suggests a moisture indicator using the COSMIC-2/FORMOSAT-7 radio occultation (RO) observations and evaluates numerical model experiments with RO data assimilation. The RO data quality is validated by a comparison between sampled RO profiles and nearby radiosonde profiles around Taiwan prior to the experiments. The suggested moisture indicator accurately monitors daily moisture variations in the South China Sea and the Bay of Bengal throughout the 2020 monsoon rainy season. For the numerical model experiments, the statistics of 152 moisture and rainfall forecasts for the 2020 Meiyu season in Taiwan show a neutral to slightly positive impact brought by RO data assimilation. A forecast sample with the most significant improvement reveals that both thermodynamic and dynamic fields are appropriately adjusted by model integration posterior to data assimilation. The statistics of 17 track forecasts for typhoon Hagupit (2020) also show the positive effect of RO data assimilation. A forecast sample reveals that the member with RO data assimilation simulates better typhoon structure and intensity than the member without, and the effect can be larger and faster via multi-cycle RO data assimilation.

Sensitivity of precipitation and structure of Typhoon Hato to bulk and explicit spectral bin microphysics schemes

<p>This study simulated the evolution of Typhoon Hato (2017) with the Weather Research and Forecasting model using three bulk schemes and one bin scheme. It was found that the track of the typhoon was insensitive to the microphysics scheme, whereas the degree of correspondence between the simulated precipitation and cloud structure of the typhoon was closest to the observations when using the bin scheme. The different microphysical structure of the bin and three bulk schemes was reflected mainly in the cloud water and snow content. The three bulk schemes were found to produce more cloud water because the application of saturation adjustment condensed all the water vapor at the end of each time step. The production of more snow by the bin scheme could be attributed to several causes: (1) the calculations of cloud condensation nuclei size distributions and supersaturation at every grid point that cause small droplets to form at high levels, (2) different fall velocities of different sizes of particles that mean small particles remain at a significant height, (3) sufficient water vapor at high levels, and (4) smaller amounts of cloud water that reduce the rates of riming and conversion of snow to graupel. The distribution of hydrometeors affects the thermal and dynamical structure of the typhoon. The saturation adjustment hypothesis in the bulk schemes overestimates the condensate mass. Thus, the additional latent heat makes the typhoon structure warmer, which increases vertical velocity and enhances convective precipitation in the eyewall region.</p>

An Efficient Method for Simulating Typhoon Waves Based on a Modified Holland Vortex Model

A combination of the WAVEWATCH III (WW3) model and a modified Holland vortex model is developed and studied in the present work. The Holland 2010 model is modified with two improvements: the first is a new scaling parameter, bs, that is formulated with information about the maximum wind speed (vms) and the typhoon’s forward movement velocity (vt); the second is the introduction of an asymmetric typhoon structure. In order to convert the wind speed, as reconstructed by the modified Holland model, from 1-min averaged wind inputs into 10-min averaged wind inputs to force the WW3 model, a gust factor (gf) is fitted in accordance with practical test cases. Validation against wave buoy data proves that the combination of the two models through the gust factor is robust for the estimation of typhoon waves. The proposed method can simulate typhoon waves efficiently based on easily accessible data sources.

Reconstruction of Typhoon Structure Using 3-Dimensional Doppler Radar Radial Velocity Data with the Multigrid Analysis: A Case Study in an Idealized Simulation Context

Extracting multiple-scale observational information is critical for accurately reconstructing the structure of mesoscale circulation systems such as typhoon. The Space and Time Mesoscale Analysis System (STMAS) with multigrid data assimilation developed in Earth System Research Laboratory (ESRL) in National Oceanic and Atmospheric Administration (NOAA) has addressed this issue. Previous studies have shown the capability of STMAS to retrieve multiscale information in 2-dimensional Doppler radar radial velocity observations. This study explores the application of 3-dimensional (3D) Doppler radar radial velocities with STMAS for reconstructing a 3D typhoon structure. As for the first step, here, we use an idealized simulation framework. A two-scale simulated “typhoon” field is constructed and referred to as “truth,” from which randomly distributed conventional wind data and 3D Doppler radar radial wind data are generated. These data are used to reconstruct the synthetic 3D “typhoon” structure by the STMAS and the traditional 3D variational (3D-Var) analysis. The degree by which the “truth” 3D typhoon structure is recovered is an assessment of the impact of the data type or analysis scheme being evaluated. We also examine the effects of weak constraint and strong constraint on STMAS analyses. Results show that while the STMAS is superior to the traditional 3D-Var for reconstructing the 3D typhoon structure, the strong constraint STMAS can produce better analyses on both horizontal and vertical velocities.

A Numerical Study on Impact of Taiwan Island Surface Heat Flux on Super Typhoon Haitang (2005)

Three to four tropical cyclones (TCs) by average usually impact Taiwan every year. This study, using the Developmental Tested Center (DTC) version of the Hurricane WRF (HWRF) model, examines the effects of Taiwan’s island surface heat fluxes on typhoon structure, intensity, track, and its rainfall over the island. The numerical simulation successfully reproduced the structure and intensity of super Typhoon Haitang. The model, especially, reproduced the looped path and landfall at nearly the right position. Sensitive experiments indicated that Taiwan’s surface heat fluxes have significant influence on the super Typhoon Haitang. Compared to sensible heat (SH) fluxes, latent heat (LH) is the dominant factor affecting the intensity and rainfall, but they showed opposite effects on intensity and rainfall. LH (SH) flux of Taiwan Island intensified (weakened) Typhoon Haitang’s intensity and structure by transferring more energy from (to) surface. However, only LH played a major role in the looped path before the landfall of the Typhoon Haitang.

Assimilation of T-TREC-Retrieved Winds from Single-Doppler Radar with an Ensemble Kalman Filter for the Forecast of Typhoon Jangmi (2008)

A tropical cyclone (TC) circulation Tracking Radar Echo by Correlation technique (T-TREC) developed recently is applied to derive horizontal winds from single Doppler radar reflectivity Z data (combined with radial velocity Vr data when available). The typically much longer maximum range of Z observations compared to Vr data allows for much larger spatial coverage of the T-TREC-retrieved winds (VTREC) when a TC first enters the maximum range of a coastal radar. Retrieved using data from more than one scan volume, the T-TREC winds also contain valuable cross-beam wind information. The VTREC or Vr data at 30-min intervals are assimilated into the Advanced Regional Prediction System (ARPS) model at 3-km grid spacing using an ensemble Kalman filter, over a 2-h window shortly after Typhoon Jangmi (2008) entered the Vr coverage area of an operational weather radar of Taiwan. The assimilation of VTREC data produces analyses of the typhoon structure and intensity that more closely match observations than analyses produced using Vr data or the reference Global Forecast System (GFS) analysis. Subsequent 28-h forecasts of intensity, track, structure, and precipitation are also improved by assimilating VTREC data. Further sensitivity experiments show that assimilation of VTREC data can build up a reasonably strong vortex in 1 h, while a longer assimilation period is required to spin up the vortex when assimilating Vr. Although the difference between assimilating VTREC and Vr is smaller when the assimilation window is longer, the improvement from assimilating VTREC is still evident. Assimilating Z data in addition to Vr or VTREC results in little further improvement.

Application of Buoy Observations in Determining Characteristics of Several Typhoons Passing the East China Sea in August 2012

The buoy observation network in the East China Sea is used to assist the determination of the characteristics of tropical cyclone structure in August 2012. When super typhoon “Haikui” made landfall in northern Zhejiang province, it passed over three buoys, the East China Sea Buoy, the Sea Reef Buoy, and the Channel Buoy, which were located within the radii of the 13.9 m/s winds, 24.5 m/s winds, and 24.5 m/s winds, respectively. These buoy observations verified the accuracy of typhoon intensity determined by China Meteorological Administration (CMA). The East China Sea Buoy had closely observed typhoons “Bolaven” and “Tembin,” which provided real-time guidance for forecasters to better understand the typhoon structure and were also used to quantify the air-sea interface heat exchange during the passage of the storm. The buoy-measured wind and pressure time series were also used to correct the intensity of “Damrey” initially determined by CMA.