Monthly Weather Review

Wu, Chun-Chieh; Cheung, Kevin K. W.; Lo, Ya-Yin

doi: 10.1175/2009MWR2757.1pmid: N/A

A heavy rainfall event in the Taiwan area associated with the interaction between Typhoon Babs (1998) and the East Asia winter monsoon is studied. Typhoon Babs is a case in point demonstrating the often-observed phenomenon that heavy rainfall can be induced in the eastern and/or northeastern region of Taiwan. Such heavy rainfall was caused by the joint convergent flow associated with the outer circulation of typhoons and the strengthening northeasterly monsoon in late typhoon season, even though Babs remained distant from Taiwan when it moved through the island of Luzon in the Philippines and stayed over the South China Sea. This heavy rainfall event is simulated in this study using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research (PSU–NCAR) Mesoscale Model (MM5) with three nested domains and a highest horizontal resolution of 6.67 km. The control experiments with Kain–Fritsch cumulus parameterization perform well in terms of both simulated track and intensity. The 20-km resolution simulation reproduces the correct rainfall distribution during the three days studied, and the fine domain with 6.67-km resolution further improves the maximum simulated rainfall to very close to the observations. A series of sensitivity experiments that include model physics, terrain effect, typhoon vortex structure, and monsoon strength is performed, aiming at investigating the predictability of this typhoon–monsoon–terrain system when some of its components are perturbed. The rainfall event is analyzed based on two rainfall modes of different dominant mechanisms: monsoon mode during 0000 UTC 24–25 October and topographic mode during 0000 UTC 25–26 October. Removal of the Taiwan terrain in one of the sensitivity experiments results in completely different rainfall distribution due to the lack of the convection by orographic lifting, and the terrain is also found to play a key role in changing the low-level convergence pattern between the typhoon circulation and monsoonal northeasterlies. When the radius of the bogus vortex is reduced, the cold front to the north migrates southward in a faster pace than in the control simulation, and rain rate at the front also increases such that total accumulated rainfall at northern Taiwan is comparable with that in the control simulation but with shifted maximum position. In the extreme case in which no bogus vortex is implanted at all, rainfall is mainly associated with evolution of the cold front (pure frontal mode). In addition, a technique is developed to modify the monsoon strength over China. It is found that low-level (1000–700 hPa) reduction in monsoon strength weakens interaction with the typhoon, and rain distribution remains the same as in the control simulation. However, the simulated typhoon track is considerably sensitive to the deep-layer (1000–300 hPa) monsoon strength.

Eastin, Matthew D.; Link, M. Christopher

doi: 10.1175/2009MWR2753.1pmid: N/A

Airborne Doppler radar observations are used to document the structure of three miniature supercells embedded in an outer rainband of Hurricane Ivan on 15 September 2004. The cells were located more than 100 km offshore, beyond the Doppler range of coastal radars. The combination of large CAPE, large vertical wind shear, and moderate cell-relative helicity with an apparent midlevel dry air intrusion provided an offshore environment supporting rotating storms. Each shallow cell contained a ∼5–7-km-diameter mesocyclonic updraft with midlevel updraft and vorticity maxima that exceeded 6 m s −1 and 0.008 s −1 , respectively. Such offshore structures are consistent with miniature supercells observed onshore in association with tropical cyclone tornado outbreaks. The strong updrafts resulted from a combination of kinematic convergence, thermal instability, and shear-induced vertical perturbation pressure gradients. Mesocyclone production largely resulted from the tilting and subsequent stretching of low-level horizontal streamwise vorticity into the vertical by the strong updrafts. Evidence of baroclinic contributions from inflow along cell-generated outflow boundaries was minimal. The miniature supercells persisted for at least 3 h during transit from offshore to onshore. Tornadoes were reported in association with two cells soon after moving onshore. These observations build upon a growing body of evidence suggesting that miniature supercells often develop offshore in the outer rainbands of tropical cyclones.

Zhang, Fuqing; Weng, Yonghui; Sippel, Jason A.; Meng, Zhiyong; Bishop, Craig H.

doi: 10.1175/2009MWR2645.1pmid: N/A

This study explores the assimilation of Doppler radar radial velocity observations for cloud-resolving hurricane analysis, initialization, and prediction with an ensemble Kalman filter (EnKF). The case studied is Hurricane Humberto (2007), the first landfalling hurricane in the United States since the end of the 2005 hurricane season and the most rapidly intensifying near-landfall storm in U.S. history. The storm caused extensive damage along the southeast Texas coast but was poorly predicted by operational models and forecasters. It is found that the EnKF analysis, after assimilating radial velocity observations from three Weather Surveillance Radars-1988 Doppler (WSR-88Ds) along the Gulf coast, closely represents the best-track position and intensity of Humberto. Deterministic forecasts initialized from the EnKF analysis, despite displaying considerable variability with different lead times, are also capable of predicting the rapid formation and intensification of the hurricane. These forecasts are also superior to simulations without radar data assimilation or with a three-dimensional variational scheme assimilating the same radar observations. Moreover, nearly all members from the ensemble forecasts initialized with EnKF analysis perturbations predict rapid formation and intensification of the storm. However, the large ensemble spread of peak intensity, which ranges from a tropical storm to a category 2 hurricane, echoes limited predictability in deterministic forecasts of the storm and the potential of using ensembles for probabilistic forecasts of hurricanes.

Houtekamer, P. L.; Mitchell, Herschel L.; Deng, Xingxiu

doi: 10.1175/2008MWR2737.1pmid: N/A

Since 12 January 2005, an ensemble Kalman filter (EnKF) has been used operationally at the Meteorological Service of Canada to provide the initial conditions for the medium-range forecasts of the ensemble prediction system. One issue in EnKF development is how to best account for model error. It is shown that in a perfect-model environment, without any model error or model error simulation, the EnKF spread remains representative of the ensemble mean error with respect to a truth integration. Consequently, the EnKF can be used to quantify the impact of the various error sources in a data-assimilation cycle on the quality of the ensemble mean. Using real rather than simulated observations, but still not simulating model error in any manner, the rms ensemble spread is found to be too small by approximately a factor of 2. It is then attempted to account for model error by using various combinations of the following four different approaches: (i) additive isotropic model error perturbations; (ii) different versions of the model for different ensemble members; (iii) stochastic perturbations to physical tendencies; and (iv) stochastic kinetic energy backscatter. The addition of isotropic model error perturbations is found to have the biggest impact. The identification of model error sources could lead to a more realistic, likely anisotropic, parameterization. Using different versions of the model has a small but clearly positive impact and consequently both (i) and (ii) are used in the operational EnKF. The use of approaches (iii) and (iv) did not lead to further improvements.

Anabor, Vagner; Stensrud, David J.; de Moraes, Osvaldo L. L.

doi: 10.1175/2008MWR2617.1pmid: N/A

Serial upstream-propagating mesoscale convective system (MCS) events over southeastern South America are important contributors to the local hydrologic cycle as they can provide roughly half of the total monthly summer precipitation. However, the mechanisms of upstream propagation for these events have not been explored. To remedy this situation, a numerical simulation of the composite environmental conditions from 10 observed serial MCS events is conducted. Results indicate that the 3-day simulation from the composite yields a reasonable evolution of the large-scale environment and produces a large region of organized convection in the warm sector over an extended period as seen in observations. Upstream propagation of the convective region is produced and is tied initially to the development and evolution of untrapped internal gravity waves. However, as convective downdrafts develop and begin to merge and form a surface cold pool in the simulation, the cold pool and its interaction with the environmental low-level flow also begins to play a role in convective evolution. While the internal gravity waves and cold pool interact over a several hour period to control the convective development, the cold pool eventually dominates and determines the propagation of the convective region by the end of the simulation. This upstream propagation of a South American convective region resembles the southward burst convective events described over the United States and highlights the complex interactions and feedbacks that challenge accurate forecasts of convective system evolution.

Wang, Chung-Chieh; Chen, George Tai-Jen; Yang, Shan-Chien; Tsuboki, Kazuhisa

doi: 10.1175/2008MWR2616.1pmid: N/A

Following an earlier diagnostic study, the present paper performs numerical simulations of the rare wintertime supercell storms during 19–20 December 2002 in a subtropical environment near Taiwan. Using Japan Meteorology Agency (JMA) 20-km analyses and horizontal grid spacing of 1.5 and 0.5 km, the Cloud-Resolving Storm Simulator (CReSS) of Nagoya University successfully reproduced the three major storms at the correct time and location, but the southern storm decayed too early over the Taiwan Strait. The two experiments produce similar overall results, suggesting that the 1.5-km grid spacing is sufficient even for storm dynamics. Model results are further used to examine the storm structure, kinematics, splitting process, and the variation in the mesoscale environment. Over the Taiwan Strait, the strong surface northeasterly flow enhanced low-level vertical shear and helped the storms evolve into isolated supercells. Consistent with previous studies, the vorticity budget analysis indicates that midlevel updraft rotation arose mainly from the tilting effect, and was reinforced by vertical stretching at the supercell stage. As the ultimate source of vorticity generation, the horizontal vorticity (vertical shear) was altered by the baroclinic (solenoidal) effect around the warm-core updraft, as well as the tilting of vertical vorticity onto, and rotation of vortex tubes in the x – y plane, forming a counterclockwise pattern that pointed generally northward (westward) at the right (left) flanks of the updraft. In both runs, model storms travel about 15°–20° to the left of the actual storms, and they are found to be quite sensitive to the detailed low-level thermodynamic structure of the postfrontal atmosphere and the intensity of the storms themselves, in particular whether or not the existing instability can be released by forced uplift at the gust front. In this regard, the finer 0.5-km grid did produce stronger storms that maintained longer across the strait. The disagreement in propagation direction between the model and real storms is partially attributed to the differences in environment, while the remaining part is most likely due to differences not reflected in gridded analyses. Since the conditions (in both the model and real atmosphere) over the Taiwan Strait are not uniform and depend on many detailed factors, it is anticipated that a successful simulation that agrees with the observation in all aspects over data-sparse regions like this one will remain a challenging task in the foreseeable future.

Karan, Haldun; Knupp, Kevin

doi: 10.1175/2008MWR2763.1pmid: N/A

The kinematics of a head-on collision between two gust fronts, followed by a secondary collision between a third gust front and a bore generated by the initial collision, are described using analyses of Weather Surveillance Radar-1988 Doppler (WSR-88D) and Mobile Integrated Profiling System (MIPS) data. Each gust front involved in the initial collision exhibited a nearly north–south orientation and an east–west movement. The eastward-moving boundary was 2°C colder and moved 7 m s −1 faster than the westward-moving boundary. Two-dimensional wind retrievals reveal contrasting flows within each gravity current. One exhibited a typical gravity current flow structure, while the other assumed the form of a gravity wave/current hybrid with multiple vortices atop the outflow. One of the after-collision boundaries exhibited multiple radar finelines resembling a solitary wave shortly after the collision. About 1 h after the initial collision, a vigorous gust front intersected the eastward-moving bore several minutes before both circulations were sampled by the MIPS. The MIPS measurements indicate that the gust front displaced the bore upward into a neutral residual layer. The bore apparently propagated upward even farther to the next stable layer between 2 and 3 km AGL. MIPS measurements show that the elevated turbulent bore consisted of an initial vigorous wave, with updraft/downdraft magnitudes of 3 and −6 m s −1 , respectively, followed by several (elevated) waves of decreasing amplitude.

Liu, Yimin; Thompson, Keith R.

doi: 10.1175/2008MWR2625.1pmid: N/A

A computationally efficient scheme is described for assimilating sea level measured by altimeters and vertical profiles of temperature and salinity measured by Argo floats. The scheme is based on a transformation of temperature, salinity, and sea level into a set of physically meaningful variables for which it is easier to specify spatial covariance functions. The scheme also allows for sequential correction of temperature and salinity biases and online estimation of background error covariance parameters. Two North Atlantic applications, both focused on predicting mesoscale variability, are used to assess the effectiveness of the scheme. In the first application the background is a monthly temperature and salinity climatology and skill is assessed by how well the scheme recovers Argo profiles that were not assimilated. In the second application the backgrounds are short-term forecasts made by an eddy-permitting model of the North Atlantic. Skill is assessed by the quality of forecasts with lead times of 1–60 days. Both applications show that the scheme has useful skill.

Tuovinen, Jari-Petteri; Punkka, Ari-Juhani; Rauhala, Jenni; Hohti, Harri; Schultz, David M.

doi: 10.1175/2008MWR2707.1pmid: N/A

A climatology of severe hail (2 cm in diameter or larger) in Finland was constructed by collecting newspaper, storm-spotter, and eyewitness reports. The climatology covered the warm season (1 May–14 September) during the 77-yr period of 1930–2006. Altogether, 240 severe-hail cases were found. The maximum reported severe-hail size was mainly 4 cm in diameter or less (65% of the cases), with the number of cases decreasing as hail size increased. In a few extreme cases, 7–8-cm (baseball sized) hailstones have been reported in Finland. Most of the severe-hail cases (84%) occurred from late June through early August, with July being the peak month (almost 66% of the cases). Most severe hail fell during the afternoon and early evening hours 1400–2000 local time (LT). Larger hailstones (4 cm or larger) tended to occur a little later (1600–2000 LT) than smaller (2–3.9 cm) hailstones (1400–1800 LT). Most severe-hail cases occurred in southern and western Finland, generally decreasing to the north, with the majority of the cases near population centers. The proportion of severe hail less than 4 cm in diameter is greatest over the agricultural area in southwestern Finland where crop damage caused by severe hail is more likely to be reported. The underreporting of hail is a particular problem across much of Finland because of the vast forest and lake areas, low population density, and relatively small hail swaths. Since the 1990s, a greater interest in severe weather among the general public and media, a storm-spotter network, improved communications technology, and an official Web site for reporting hail have increased the number of reported hail cases. According to the most recent 10 yr (1997–2006), Finland experiences an annual average of 10 severe-hail cases during 5 severe-hail days.

Lin, Hai; Brunet, Gilbert

doi: 10.1175/2009MWR2831.1pmid: N/A

Using the homogenized Canadian historical daily surface air temperature (SAT) for 210 relatively evenly distributed stations across Canada, the lagged composites and probability of the above- and below-normal SAT in Canada for different phases of the Madden–Julian oscillation (MJO) in the winter season are analyzed. Significant positive SAT anomalies and high probability of above-normal events in the central and eastern Canada are found 5–15 days following MJO phase 3, which corresponds to an enhanced precipitation over the Indian Ocean and Maritime Continent and a reduced convective activity near the tropical central Pacific. On the other hand, a positive SAT anomaly appears over a large part of northern and northeastern Canada about 5–15 days after the MJO is detected in phase 7. An analysis of the evolution of the 500-hPa geopotential height and sea level pressure anomalies indicates that the Canadian SAT anomaly is a result of a Rossby wave train associated with the tropical convection anomaly of the MJO. Hence, the MJO phase provides useful information for the extended-range forecast of Canadian winter surface air temperature. This result also provides an important reference for numerical model verifications.