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Jyong-En Miao
and
Ming-Jen Yang

Abstract

A severe afternoon thunderstorm (ATS) system developed within the Taipei basin on 14 June 2015, which produced intense rainfall (with a rainfall rate of 131 mm h−1) and urban-scale flooding. A control simulation (CNTL) using the Weather Research and Forecasting (WRF) Model with the horizontal grid size nested down to 500 m was performed to capture reasonably well the onset of the sea breeze, the merger of convective cells, and the evolution of the afternoon thunderstorm system. Four numerical sensitivity experiments with the increase or decrease of midlevel (700–500 hPa) relative humidity (RH) of 10% and 20% were conducted, and simulation results were compared with those from the CNTL. Although the response of convection to midlevel RH was somewhat nonlinear, sensitivity experiments showed that a dry layer at middle levels would result in stronger cold pool, more intense convection, stronger updraft, more graupel particles, stronger net latent heating above the melting level, and a much larger area of the potential flooding region [>40 mm (30 min)−1]. The estimation of bulk entrainment rate provided evidence that the entrainment rate could be reduced by stronger cold pool and the widening of moist convection area. Three terrain-removal sensitivity experiments indicated that Taipei basin modulated the response of convection intensity to midlevel RH. The basin terrain confined the outflow associated with ATS and forced it to converge with the moist sea breeze continuously, providing a favorable dynamic and thermodynamic environment for subsequent convection development. This “basin confinement effect” may be crucial for short-duration rainfall extremes over complex terrain.

Significance Statement

This study has examined the impact of midlevel moisture on the structure, evolution, and precipitation of an afternoon thunderstorm system that produced intense rainfall at Taipei using eight numerical experiments based on high-resolution model outputs. Our findings explain how a drier layer at middle levels would produce a more intense thunderstorm system, although the response of convection intensity to midlevel moisture is somewhat nonlinear. In addition, it is found that terrain could modulate the response of convection to midlevel moisture, which is rarely discussed in previous studies.

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Ming-Jen Yang
and
Robert A. Houze Jr.

Abstract

Two-dimensional and three-dimensional simulations of a midlatitude squall line with a high-resolution non-hydrostatic model suggest that the multicellular structure of the storm may be associated with gravity waves generated by convection. Time-lapse display of model output demonstrates that the commonly described “cut-off” process is actually a gravity wave phenomenon. The convective cells arise as gravity waves, which are forced by continuous strong low-level convergence at the storm's gust front. The waves propagate to both sides of the gust front. The stronger westward (front to rear) mode dominates at the mature stage of the squall line. Continuous low-level updraft is generated at the nose of the cold pool, which propagates at the speed of a density current. Updraft cells periodically break away from this persistent low-level gust-front updraft and move at phase speeds of their associated gravity waves, not at the surrounding airflow speeds as implied by the traditional multicell model.

Linear theory shows that the multicellular structure is associated with vertically trapped gravity waves in the troposphere. The waves become trapped in the mid- to upper troposphere because of the strong decrease of Scorer parameter with height as a result of strong vertical wind shear and the reduced static stability aloft. Waves are trapped in lower levels because of the rigid ground. The basic characteristics of these trapped tropospheric gravity waves are wavelengths of 16–20 km, storm-relative phase speeds of 20–25 m s−1, and periods of 11–17 min, which are consistent with the generation periods of precipitation cells at the mature stage in the leading portion of the storm. In the trailing stratiform region, these tropospheric gravity waves become more diffuse with weaker amplitudes, and their wavelengths become longer (25–35 km) with greater storm-relative phase speeds (30–40 m s−1), as described by the dispersion relationship of internal gravity waves.

The tropospheric gravity waves differ from disturbances above the tropopause, which are mechanically forced by convective cells impinging on the tropopause. These waves in the lower stratosphere have the structure of vertically propagating (rather than trapped) gravity waves.

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Ming-Jen Yang
and
Robert A. Houze Jr.

Abstract

In this paper, the authors investigate the momentum budget of a squall line with trailing stratiform precipitation by examining how the momentum balance varies with respect to the storm's internal structure. In particular, the authors determine differences between the momentum budgets of the convective and stratiform precipitation regions, which are physically distinct parts of the storm. The results from a high-resolution nonhydrostatic numerical simulation of the two-dimensional segment of the 10–11 June 1985 PRE-STORM squall line are used. The momentum equation is averaged over a 300-km-wide large-scale area for time periods of 1 h. On the 1-h timescale, the convective-scale temporal variations of horizontal and vertical velocities are nearly uncorrelated, and thus their contribution to the momentum flux is negligible. The remaining standing-eddy and mean-flow circulations account for the momentum flux on this timescale. The combination of the standing eddy and mean flow behave almost exactly like Moncrieff's idealization of two-dimensional steady-state squall line flow.

Because the standing-eddy circulation and the pressure-gradient acceleration vary from one part of the storm to another, the interplay of forces leading to the large-scale momentum tendency also differs strongly from one subregion to another. The convective precipitation region dominates the momentum budget at low levels, where the standing-eddy flux convergence produces a forward acceleration that slightly outweighs the rearward pressure-gradient acceleration. At midlevels, both the convective and stratiform precipitation regions contribute to the net large-scale momentum tendency. The pressure-gradient forces in the convective and stratiform precipitation regions are both strong but oppositely directed; however, the rearward standing-eddy flux convergence in the convective precipitation region is also strong; thus, the net large-scale momentum tendency at midlevels is rearward. At upper levels, the momentum budget is completely dominated by the stratiform precipitation region, where a strong forward-directed pressure-gradient acceleration dominates the net large-scale momentum tendency.

These differences between the momentum budgets of the convective and stratiform precipitation regions suggest that rather different large-scale momentum tendencies can arise as a function of storm structure; storms with strong convective precipitation regions and weak stratiform precipitation regions would produce momentum tendencies quite different from storms with well-developed stratiform precipitation regions.

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Ming-Jen Yang
,
Scott A. Braun
, and
Deng-Shun Chen

Abstract

Although there have been many observational and modeling studies of tropical cyclones (TCs), the understanding of TCs’ budgets of vapor and condensate and the changes of budgets after TCs’ landfall is still quite limited. In this study, high-resolution (2-km horizontal grid size and 2-min data interval) model output from a cloud-resolving simulation of Typhoon Nari (2001) is used to examine the vapor and condensate budgets and the respective changes of the budgets after Nari’s landfall on Taiwan. All budget terms are directly derived from the model except for a small residual term. For the vapor budget, while Nari is over the ocean, evaporation from the ocean surface is 11% of the inward horizontal vapor transport within 150 km of the storm center, and the net horizontal vapor convergence into the storm is 88% of the net condensation. The ocean source of water vapor in the inner core is a small portion (5.5%) of horizontal vapor import, consistent with previous studies. After landfall, Taiwan’s steep terrain enhances Nari’s secondary circulation significantly and produces stronger horizontal vapor import at low levels, resulting in a 22% increase in storm-total condensation. Precipitation efficiency, defined from either the large-scale or microphysics perspective, is increased 10%–20% over the outer-rainband region after landfall, in agreement with the enhanced surface rainfall over the complex terrain.

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Brian A. Colle
,
Bradley F. Smull
, and
Ming-Jen Yang

Abstract

This paper identifies mechanisms that led to the observed rapid evolution of a landfalling weak cold front along the steep mountainous northern California coast on 1 December 1995. This event was simulated down to 3-km horizontal grid spacing using the Pennsylvania State University–NCAR Mesoscale Model version 5 (MM5). The MM5 simulation reproduced the basic features such as the timing, location, and orientation of the cold front and associated precipitation evolution, as well as the tendency for enhanced precipitation to extend ∼50–100 km upwind of the coastal barrier, with the heaviest amounts occurring over the windward slopes (0–20 km inland); locally, however, the model underestimated the magnitude of the prefrontal terrain-enhanced flow by as much as 30% since the simulated low-level static stability was weaker than observed.

The MM5 simulations illustrate the complex thermal, wind, and precipitation structures in the coastal zone. Upstream flow blocking by the steep coastal terrain led to the development of a mesoscale pressure ridge and prefrontal terrain-enhanced winds exceeding 25 m s−1. Because of the irregular coastline and highly three-dimensional terrain, the low-level winds were not uniform along the coast. Rather, prefrontal southerly flow was significantly reduced downwind of the major capes (viz. Mendocino and Blanco), while there were localized downgradient accelerations adjacent to regions of higher topography along uninterrupted stretches of coastline. Terrain–front interactions resulted in a slowing of the front as the system made landfall, and blocking contributed to a “tipped forward” baroclinic structure below 800 mb.

The MM5 was used to investigate some of the reasons for the rapid intensification of the frontal temperature gradient and banded precipitation in the coastal zone. During this event the large-scale vertical motions increased in an environment favorable for moist convection, and a simulation without coastal topography illustrated rapid development of coastal precipitation even in the absence of local terrain influences. The coastal topography helped to further enhance and collapse the thermal gradient and associated cold-frontal rainband through enhanced deformation frontogenesis associated with the prefrontal terrain-enhanced flow. Diabatic effects from precipitation are also shown to have been important in organizing the precipitation in the coastal zone and further enhancing the frontal temperature gradient.

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Scott A. Braun
,
Robert A. Houze Jr.
, and
Ming-Jen Yang

Abstract

This comment addresses two conclusions arising from a modeling study of Chin: that the contribution of mesoscale stratiform areas to large-scale heat and moisture budgets at midlatitudes is small compared to that associated with deep convection and that longwave radiative processes are the cause of the transition zone. A review of the literature and a comparison to another simulated squall line reaffirm the long-standing result that mesoscale stratiform precipitation regions often contribute significantly to the large-scale heating and moistening and demonstrate that longwave radiation is just one of the many factors that modify the kinematic and micro-physical processes that form the transition zone.

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Ming-Jen Yang
,
Da-Lin Zhang
, and
Hsiao-Ling Huang

Abstract

Although there have been many observational and modeling studies of tropical cyclones, understanding of their intensity and structural changes after landfall is rather limited. In this study, several 84-h cloud-resolving simulations of Typhoon Nari (2001), a typhoon that produced torrential rainfall of more than 1400 mm over Taiwan, are carried out using a quadruply nested–grid mesoscale model whose finest grid size was 2 km. It is shown that the model reproduces reasonably well Nari’s kinematic and precipitation features as well as structural changes, as verified against radar and rain gauge observations. These include the storm track, the contraction and sizes of the eye and eyewall, the spiral rainbands, the rapid pressure rise (∼1.67 hPa h−1) during landfall, and the nearly constant intensity after landfall. In addition, the model captures the horizontal rainfall distribution and some local rainfall maxima associated with Taiwan’s orography.

A series of sensitivity experiments are performed in which Taiwan’s topography is reduced to examine the topographic effects on Nari’s track, intensity, rainfall distribution, and amount. Results show that the impact of island terrain on Nari’s intensity is nearly linear, with stronger storm intensity but less rainfall in lower-terrain runs. In contrast, changing the terrain heights produces nonlinear tracks with circular shapes and variable movements associated with different degrees of blocking effects. Parameter and diagnostic analyses reveal that the nonlinear track dependence on terrain heights results from the complex interactions between the environmental steering flow, Nari’s intensity, and Taiwan’s topography, whereas the terrain-induced damping effects balance the intensifying effects of latent heat release associated with the torrential rainfall in maintaining the near-constant storm intensity after landfall.

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Hsiao-Ling Huang
,
Ming-Jen Yang
, and
Chung-Hsiung Sui

Abstract

In this study, the Weather Research and Forecasting model, version 3.2, with the finest grid size of 1 km is used to explicitly simulate Typhoon Morakot (2009), which dumped rainfall of more than 2600 mm in 3 days on Taiwan. The model reasonably reproduced the track, the organization, the sizes of the eye and eyewall, and the characteristics of major convective cells in outer rainbands. The horizontal rainfall distribution and local rainfall maximum in the southwestern portion of the Central Mountain Range (CMR) are captured. The simulated rain rate and precipitation efficiency (PE) over the CMR are highly correlated. In the absence of terrain forcing, the simulated TC’s track is farther north and rainfall distribution is mainly determined by rainbands. The calculated rain rate and PE over the CMR during landfall are about 50% and 15%–20% less than those of the full-terrain control run, respectively. By following major convective cells that propagate eastward from the Taiwan Strait to the CMR, it is found that the PE and the processes of vapor condensation and raindrop evaporation are strongly influenced by orographic lifting; the PEs are 60%–75% over ocean and more than 95% over the CMR, respectively. The secondary increase of PE results from the increase of ice-phase deposition ratio when the liquid-phase condensation becomes small as the air on the lee side subsides and moves downstream. This nearly perfect PE over the CMR causes tremendous rainfall in southwestern Taiwan, triggering enormous landslides and severe flooding.

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Chung-Hsiung Sui
,
Xiaofan Li
, and
Ming-Jen Yang

Abstract

A modified definition of precipitation efficiency (PE) is proposed based on either cloud microphysics precipitation efficiency (CMPE) or water cycling processes including water vapor and hydrometeor species [large-scale precipitation efficiency (LSPE)]. These PEs are examined based on a two-dimensional cloud-resolving simulation. The model is integrated for 21 days with the imposed large-scale vertical velocity, zonal wind, and horizontal advections obtained from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). It is found that the properly defined PEs include all moisture and hydrometeor sources associated with surface rainfall processes so that they range from 0% to 100%. Furthermore, the modified LSPE and CMPE are highly correlated. Their linear correlation coefficient and root-mean-squared difference are insensitive to the spatial scales of averaged data and are moderately sensitive to the time period of averaged data.

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Yao-Chu Wu
,
Ming-Jen Yang
, and
Robert F. Rogers

Abstract

Typhoon Fanapi (2010) made landfall in Hualien in Taiwan at 0100 UTC 19 September 2010 and left Taiwan at 1200 UTC 19 September 2010, producing heavy rainfall and floods. Fanapi’s eyewall was disrupted by the Central Mountain Range (CMR) and reorganized after leaving the CMR. High-resolution simulations (nested down to 1-km horizontal grid size) using the Advanced Research Weather Research and Forecasting (WRF) Model, one simulation using the full terrain (CTL) and another set of simulations where the terrain of Taiwan was removed, were analyzed. Precipitation areas were classified into different subregions by a convective–stratiform separation algorithm to assess the impact of precipitation structure on Fanapi’s eyewall evolution. The percentage of deep convection increased from 9% to 20% when Fanapi underwent an eyewall reorganization process while departing the CMR. In the absence of terrain, moderate convection occupied most of the convective regions during the period when Fanapi moved across Taiwan Island. The low-level total vorticity stretching within the convective, stratiform, and weak-echo regions in the no-terrain experiment were of similar magnitudes, but the total vorticity stretching within the convective region at low levels was dominant in the CTL experiment. Total vorticity stretching in the region of deep convection increased after eyewall reorganization, and later became stronger than that in the moderate convection region. In the absence of the CMR, total vorticity stretching in moderate convection dominated. The total vorticity stretching within the deep convective region in the CTL experiment played an essential role in the reorganization of Fanapi’s eyewall through a bottom-up process.

Significance Statement

When a tropical cyclone makes landfall on Taiwan Island, the Central Mountain Range (CMR) usually disrupts the eyewall and changes the percentage of convective and stratiform precipitation areas. Unlike most typhoons whose eyewalls are weakened after landfall, Typhoon Fanapi’s eyewall reorganized and the percentage of deep convection increased from 9% to 20% when Fanapi moved to the west side of the CMR. Understanding how the terrain of Taiwan weakened the vortex circulation of Typhoon Fanapi during landfall and rebuilt the vorticity and eyewall after landfall is important to improve the forecast of TCs with similar track and intensity in the future.

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