2011 Vol. 25 No. 1
Cover Note
2011, 25(1): 1-20.
DOI:
10.1007/s13351-011-0001-3
Abstract：The possible changes in the frequency of extreme temperature events in Hong Kong in the 21st century were investigated by statistically downscaling 26 sets of the daily global climate model projections (a combination of 11 models and 3 greenhouse gas emission scenarios, namely A2, A1B, and B1) of the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. The models' performance in simulating the past climate during 1971-2000 has also been verified and discussed. The verification revealed
that the models in general have an acceptable skill in reproducing past statistics of extreme temperature events. Moreover, the models are more skillful in simulating the past climate of the hot nights and cold days than that of the very hot days. The projection results suggested that, in the 21st century, the frequency of occurrence of extremely high temperature events in Hong Kong would increase significantly while that of the extremely low temperature events is expected to drop significantly. Based on the multi-model scenario ensemble mean, the average annual numbers of very hot days and hot nights in Hong Kong are expected to increase significantly from 9 days and 16 nights in 1980-1999 to 89 days and 137 nights respectively in 2090-2099. On the other hand, the average annual number of cold days will drop from 17 days in 1980-1999 to about 1 day in 2090-2099. About 65 percent of the model-scenario combinations indicate that there will be on average less than one cold day in 2090-2099. While all the model-emission scenarios in general have projected consistent trends in the change of temperature extremes in the 21st century, there is a large
divergence in the projections between different model/emission scenarios. This reflects that there are still large uncertainties in the model simulation of the future climate of extreme temperature events.More+
Abstract：Diagnostic comparison of the East Asian subtropical jet (EASJ) and polar-front jet (EAPJ) in winter
season is carried out by using the ERA-40 dataset. The large-scale circulation characteristics and synoptic-
scale transient eddy activities (STEAs) associated with the EASJ and EAPJ are examined. The results show
that the EASJ and EAPJ in the upper-level monthly mean data have no clear geographical border, while
the distribution of the numbers of jet cores from the daily data exhibits a distinct boundary at the latitudes
of the northern Tibetan Plateau. The two areas with large numbers of jet cores correspond to the EASJ
and EAPJ regions. The analysis of STEAs over the East Asian region shows a spatial match of STEAs with
the EASJ and EAPJ in winter: the strong EASJ is located within the weak southern branch of the STEA
while the relatively weak EAPJ appears within the active northern branch of the STEA, indicating that
the EAPJ is the jet coexisting with the STEA. Further analysis shows two anomalous modes of the winter
EAPJ: the anomalous anticyclonic/cyclonic circulation and the weakened/strengthened local westerly wind.
The large-scale circulation anomalies in the Northern Hemisphere related to the first mode are concentrated
in the Eurasian mid to high latitudes, and are also influenced by the anomalous circulation in the upstream
area. When the local westerly wind over the EAPJ region is weakened/strengthened, the westerly jet in the
eastern part of the EASJ and that in the western Pacific region show opposite variations. The corresponding
anomalous atmospheric circulation demonstrates the Eurasian (EU) pattern. The EAPJ anomalies are also
closely linked with the STEA anomalies over East Asia. The anomalies in the northern branch of the STEA
propagate as a wave train along its axis into the East Asian coastal waters, and then migrate eastward to
the oceanic region. However, the ones near the southern branch are trapped over the eastern part of East
Asia and its coastal waters at 200 hPa.More+
Abstract：This study aims to examine the effect of El Ni~no and La Ni~na on the monthly and seasonal climate
of Hong Kong against the ENSO-neutral situation from a statistical perspective. Monthly and seasonal
temperature and rainfall of Hong Kong and monthly number of tropical cyclones (TCs) coming within 500
km of the city over the 59-yr (1950-2008) period are examined under three ENSO situations, namely El Ni~no,
La Ni~na, and ENSO-neutral. It is found that, compared with the ENSO-neutral situation, El Ni~no tends
to be associated with wetter winter (December-February) and spring (March-May) while La Ni~na tends
to be associated with cooler autumn (September-November) and winter. El Ni~no tends to be associated
with a later start of the tropical cyclone season of Hong Kong while La Ni~na tends to be associated with
more TCs coming within 500 km of Hong Kong in August-October. It is also found that, for April and
June-December, although the monthly number of TCs coming within 500 km of Hong Kong during El Ni~no
is generally lower than that under the ENSO-neutral situation, the difference is not statistically significant
based on the current data sample size.More+
Abstract：The variational assimilation theory is generally based on unbiased observations. In practice, however,
almost all observations suffer from biases arising from observational instruments, radiative transfer operator,
precondition of data, and so on. Therefore, a bias correction scheme is indispensable. The current scheme for
radiance bias correction in the GRAPES 3DVar system is an offline scheme. It is actually a static correction
for the radiance bias before the process of cost function minimization. In consideration of its effects on
forecast results, this kind of scheme has some shortcomings. Thus, this study provides a variational bias
correction (VarBC) scheme for the GRAPES 3DVar system following Dee's idea. In the VarBC scheme, the
observation operator is modified and a new control variable is defined by taking the predictor coefficients
as the control parameters. According to the feature of the GRAPES-3DVAR, an incremental formulation
is applied and the original bias correction scheme is maintained in the actual process of observations. The
VarBC is designed to co-exist with the original scheme, because it is a dynamic revision to the observational
operator on the basis of the old method, i.e., it adjusts the model state vector along with the control
parameters to an unbiased state in the process of minimization and the assimilation system remains consistent
with available information automatically. Preliminary experimental results show that the mean departures
of background-minus-observation and analysis-minus-observation are reduced as expected. In a case study
of the heavy rainfall that happened in South China on 11-13 June 2008, the 500-hPa geopotential height is
better simulated using the analyzed field from the VarBC as the initial condition.More+
Abstract：The sensitivity of a regional climate model (RCM) to cumulus parameterization (CUPA) schemes in modeling summer precipitation over East Asia has been investigated by using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (PSU-NCAR MM5).The feasibility of physical ensemble and the effect of interior (spectral) nudging are also assessed. The RCM simulations are evaluated against the NCEP/NCAR reanalysis data and NCEP/CPC precipitation data for three summers (JJA) in 1991, 1998, and 2003. The results show that the RCM is highly sensitive to CUPA schemes. Different CUPA schemes cause distinctive characteristics in the modeling of JJA precipitation and the intraseasonal (daily) variability of regional precipitation. The sensitivity of the RCM simulations to the CUPA schemes is reduced by adopting the spectral nudging technique, which enables the RCM to reproduce more realistic large-scale circulations at the upper levels of the atmosphere as well as near the surface, and better precipitation simulation in the selected experiments. The ensemble simulations using different CUPA schemes show higher skills than individual members for both control runs and spectral nudging runs. The physical ensemble adopting the spectral nudging technique shows the highest downscaling skill in capturing the general circulation patterns for all experiments and improved temporal distributions of precipitation in some regions.More+
Abstract：The cyclone phase space (CPS) method has been utilized to evaluate the extratropical transition (ET) of
tropical cyclones (TCs) in many recent publications. However, these studies mainly focused over the North
Atlantic basin. In this paper, the CPS characteristics of all the cyclones over the western North Pacific
are investigated and discussed, with three parameters calculated from the best-track data of the Regional
Specialized Meteorological Center in Tokyo and the Japanese 25-yr reanalysis data. It is concluded that most
TCs over the western North Pacific possess the non-frontal and warm-core structure, while a larger number
of cyclones that have undergone ET hold the frontal and cold-core structure. The spatial pattern of the CPS
parameters indicates that the areas of tropical and extratropical cyclone activities could be demarcated by
30°N. The composite and individual series of three parameters of the CPS indicate that the transformation
of -V UT from positive to negative leads to the start of ET, and could be considered as a potential predictor
in operationally forecasting an ET event.More+
Abstract：In this study, an extension of the TREC (Tracking Radar Echo by Correlations) technique, named Tropical Cyclone (TC) circulation TREC (T-TREC), is developed to retrieve the winds of landfalling typhoons in China. The T-TREC analysis is performed on a polar grid centered at the TC center, using arc-shaped correlation cells and an arc-shaped search area. The search for the best correlation match is confined along the cyclonic direction with a limited search distance in the radial direction based on the cyclonic circulation characteristics of TCs in the Northern Hemisphere. The TC center is determined objectively using reflectivity data while the Doppler radar radial velocities are incorporated to estimate the search range and create a velocity correlation matrix as auxiliary constraints.
The T-TREC was applied to the landfalling Typhoon Saomai (0608) observed by Chinese next generation Weather Surveillance Radar 1998 Doppler (CINRAD WSR-98D) on the southeast coast of China. The results show that the T-TREC has the ability to estimate the typhoon circulation with an average bias of < 4 m s-1. The incorporation of radial velocity data could distinctively improve wind retrievals for intense typhoons, especially by reducing the underestimation caused by fairly uniform reflectivity patterns in the vicinity of the eyewall and the outer rainband. A quantitative evaluation of the influence of typhoon center and cell size on the wind estimation demonstrates that the quality of the T-TREC retrieved wind circulation depends on the estimation of the typhoon center. A 4-km deviation of the TC center may result in a 10% increase in the retrieved wind error. The effect of cell size depends on the typhoon scale: better wind retrieval results can be obtained for a smaller typhoon.More+
Abstract：The 6-yr best-track data of tropical cyclones (TCs) in the western North Pacific are used to study the
statistical features of TC size and radial wind profile. A TC size is defined as the azimuthal mean radius of
34-kt surface wind.
On average, the TCs in the western North Pacific have a size of 203 km, and the size is larger for
stronger TCs. Further analyses show that larger TCs tend to move faster than smaller ones, with a 23-24
km difference in size corresponding to a difference of about 10 km h-1 in moving speed. The TCs that
recurve from westward to eastward moving have a mean size of 218 km, significantly larger than that of
those without a turning point (179 km). Regional TC distributions demonstrate that the TCs affecting
the Korean Peninsula and southwestern Japan have the largest mean size (250-280 km). There are also
some large TCs affecting southern Philippines, while TCs over the South China Sea are generally small in
size. Comparison of intensity and size of all TCs during their lifespan demonstrates that a TC tends to
reach its maximum size 6 h after it reaches its maximum intensity, and the decrease rate of size during the
weakening stage of a TC is much smaller than the increase rate of size during its developing stage. Thus,
linear regression relations between TC size and intensity are established for its developing and weakening
stages respectively, which can be used as a forecast tool for TC size.
Features of TC radial wind profile are studied by analyzing a parametric wind model based on the radius
data of 34-, 50-, and 64-kt surface winds. The results show that the shape parameter d most frequently takes
the values of 0.3, 0.4, and 0.5. It generally increases (decreases) as the TC develops (weakens), implying a
sharper (flatter) radial wind profile. Changes in d leads the tendency of intensity. The two parameters for
the asymmetric model, namely p and q, are mostly 0.85-1.05 and 0-0.2, respectively, embodying the fact
that the asymmetric component is generally much smaller than the symmetric component. The asymmetry
in 34-kt surface wind is much stronger than that in 50- and 64-kt surface winds, with the maximum radius
often in the northeast quadrant.More+
2011, 25(1): 113-127.
DOI:
10.1007/s13351-011-0009-8
Abstract：The wind data of four typhoons were obtained and analyzed. The wind speeds were measured by sonic
anemometers at four observation sites in Guangdong and Hainan provinces. Detailed analysis of the wind
data was conducted to investigate the turbulence characteristics of the typhoons. Characteristics of the
gust factor and the turbulence integral scale of the typhoons were concluded with high confidence. The
relationships among the gust factor, gust duration time, mean wind speed, roughness length, and turbulence
intensity were described. The turbulence integral scale was found to be closely related to the segment length
and turbulence intensity.More+
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