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Jodie Clark and Sen Chiao

Abstract

The California Baseline Ozone Transport Study (CABOTS) was a major air quality study that collected ozone measurements aloft between mid-May and mid-August of 2016. Aircraft measurements, ground-based lidar measurements, and balloon-borne ozonesondes collected precise upper-air ozone measurements across the central and Southern California valley. Utilizing daily ozonesonde data from Bodega Bay, California, and Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2), reanalysis data for 25 July to 14 August 2016, three stratospheric intrusion events are identified over Northern California influencing air masses above Bodega Bay and Sacramento simultaneously. Calculated percent daily changes in afternoon ozonesonde observations indicate increasing ozone concentrations from the point of likely stratospheric air injection with the arrival of higher potential vorticity, confirmed by ensemble back trajectories. An analysis of the onsite surface monitoring ozone data indicates ozone increases in the observations for dates of plausible low-level stratospheric air influence. Further, a comparison of Bodega Bay surface ozone observations and 14 Sacramento Valley nonattainment zone surface sites show that the surface ozone observed at the higher-elevation surface sites in the lower Sierra Nevada foothills were positively correlated with elevated ozone captured by the ozonesondes within the lowest 0.5–1 km. The strongest correlations observed (~0.61) were between elevated Bodega Bay ozonesonde data and the Placerville (~612 m) afternoon surface ozone data, an indication that these regions separated by 200 km would be influence by the same ozone source. A comparison of daily changes in afternoon ozone show that the two locales often experience similar daily ozone increases or decreases. While this study leads to a basic quantification of stratospheric influence on surface ozone in the Sacramento nonattainment zone, a future campaign that examines ozone and winds aloft at both locales is suggested to improve the quantification of stratospheric ozone.

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Sen Chiao and Gregory S. Jenkins

Abstract

Mesoscale model forecasts were carried out beginning at 0000 UTC 19 August for simulating Tropical Disturbance 4, which was named Tropical Storm Debby on 22 August 2006. The Weather Research and Forecasting model, with 25-km grid spacing and an inner nested domain of 5-km grid spacing, was used. The development of a small closed vortex at approximately 0600 UTC 20 August 2006 at 850 hPa was found off the coast of Guinea in agreement with satellite images in the 5-km simulation. Intense convection offshore and over the Guinea Highlands during the morning of 20 August 2006 led to the production of a vortex formation by 1400 UTC at 700 hPa. Sensitivity tests show that the Guinea Highlands play an important role in modulating the impinging westerly flow, in which low-level flow deflections (i.e., northward turning) enhance the cyclonic circulation of the vortex formation. Yet, the moist air can be transported by the northward deflection flow from lower latitudes to support the development of mesoscale convective systems (MCSs). Although the model forecast is not perfect, it demonstrates the predictability of the formation and development of the tropical disturbance associated with the Guinea Highlands.

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Sen Chiao and Yuh-Lang Lin

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An orographic rainfall event that occurred on 6–7 August 1999 during the passage of Tropical Storm (TS) Rachel over Taiwan is investigated by performing triply nested, nonhydrostatic numerical simulations using the Naval Research Laboratory's (NRL) Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. By examining both observational data and numerical model output, it is found that this orographic rainfall event may be separated into three distinct stages. During the first stage (0000–1200 UTC 6 August), TS Rachel was located in the South China Sea and tracked northeastward to Taiwan. Meanwhile, TS Paul was steered by the subtropical high over southwest Japan. During the second stage (1200 UTC 6 August–0000 UTC 7 August), the southwesterly monsoon current as well as the circulation of TS Rachel over southwest Taiwan strengthened and formed a low-level jet (LLJ) with high equivalent potential temperature when TS Rachel moved closer to Taiwan. In comparing the control and sensitivity (without orography) experiments, it was found that the strong LLJ triggered convective systems in the concave region of the southwest Central Mountain Range (CMR), which then produced the first episode of heavy rainfall. The second episode of heavy rainfall, which occurred during the third stage (0000–1800 7 August), was attributed to the modification of TS Rachel's own rainbands by the mountains as well as the strong southwesterly flow impinging on the mountains. The low-level convergence was propagated upstream over the sea, and the impinging flow from southwest Taiwan produced new convective cells. The orographic vertical moisture flux, which is a product of low-level horizontal velocity, mountain steepness, and moisture content, is calculated based on the fine-resolution model output. The regions of maximum moisture flux roughly coincide with the heavy-rainfall regions over the island during this event, while the regions of the general vertical moisture flux coincide with the heavy-rainfall regions over the ocean. Hence, the orographic vertical moisture flux may serve as an index for predicting this type of upslope orographic heavy rainfall. Overall, the model is able to predict the storm track, rainbands, and period of rainfall reasonably well over southern Taiwan, although the maximum rainfall may be slightly overpredicted. Nevertheless, the model results also suggest that a finer-resolution domain or vortex bogusing might be needed for the simulation of precipitation in association with a tropical storm over complex terrain.

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Mark R. Jury and Sen Chiao

Abstract

The midsummer boundary layer (BL) circulation and afternoon thunderstorm convection on the lee side of Puerto Rico is studied using observations and high-resolution models. Satellite infrared data help to identify cases on 5 and 14 June 2010 when midday surface temperatures show a 2°C gradient between land and sea and afternoon cloud-top temperatures <−60°C. Acoustic sounder profiles are analyzed for climatology, wind shear, turbulence, and diurnal cycles in the 40–300-m layer. Weather Research and Forecasting (WRF) model simulations indicate that sea-breeze flow is entrained into convective cells near Mayaguez, Puerto Rico. The simulated BL wind shear is too weak (0.5 × 10−2 s−1) in comparison with the acoustic sounder (2 × 10−2 s−1). Model 900-hPa winds are southeasterly and spread simulated convection too far north in comparison with radar. The pattern of near-surface winds in the island wake triggers afternoon thunderstorms near Mayaguez. A feature of the confluent circulation around Puerto Rico is opposing shear zones on the leeward corners of the island and a sea breeze of 5 m s−1 over the west coast during midday.

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Mark R. Jury and Sen Chiao

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The mesoscale structure of the circulation and convection over central Caribbean Antilles islands in midsummer is analyzed. Afternoon thunderstorms are frequent over islands such as Hispaniola and Jamaica as confluent trade winds circulate over heated mountainous topography. Observational data from a rain gauge network, profiles from aircraft and radiosonde, satellite estimates of rainfall, and mesoscale reanalysis fields are studied with a focus on July 2007. A statistical decomposition of 3-hourly high-resolution satellite rainfall reveals an “island mode” with afternoon convection. Trade winds pass over the mountains of Hispaniola and Jamaica with a Froude number <1, leading to a long meandering wake. The Weather Research and Forecasting model is used to simulate climatic conditions during July 2007. The model correctly locates areas of diurnal rainfall that develop because of island heat fluxes, confluent sea breezes, and mountain wakes.

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Ashley M. Merritt-Takeuchi and Sen Chiao

Abstract

This study investigates phytoplankton blooms following the passage of tropical cyclones in the Atlantic and Pacific Ocean basins. The variables of sea surface temperature (SST), chlorophyll (Chl-a), precipitation, and storm surface winds were monitored for two case studies, Typhoon Xangsane (2006) and Hurricane Earl (2010). Strong near-surface wind from tropical cyclones creates internal friction, which causes deep nutrient enriched waters to displace from the bottom of the ocean floor up toward the surface. In return, the abundance of upwelled nutrients near the surface provides an ideal environment for the growth of biological substances such as chlorophyll and phytoplankton. The inverse correlation coefficients of SST and Chl-a for this study are −0.67 and −0.26 for Xangsane and Earl, respectively. This suggests that, regardless of ocean basin, changing sea surface temperature and chlorophyll concentrations can be correlated to various characteristics of tropical cyclones including precipitation and surface wind, which in combination results in an increase of phytoplankton.

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Andre K. Pattantyus, Sen Chiao, and Stanley Czyzyk

Abstract

Numerical simulations for severe downslope winds as well as trapped lee waves in Nevada’s Las Vegas Valley were performed in this study. The goal of this study was to improve model forecasts of downslope-wind-event intensities. This was measured by assessing different planetary boundary layer (PBL) schemes in the mountain–valley region. The Weather Research and Forecasting Model was adopted for this research. The numerical experiments were constructed using two nested domains, with 4- and 1-km grid resolution. The working hypothesis was that the occurrence of low-level wind shear and surface gustiness in the Las Vegas Valley was guided by the inversion layer in the valley. The choice of boundary layer scheme and model vertical resolution will influence inversion-layer height and consequently result in significant differences in surface wind and temperature forecast error below some near-surface height. Simulations of severe downslope wind events on 15 April 2008 and on 4 October 2009 were conducted. Statistical analyses of model results from three different PBL schemes and different vertical resolutions were performed. The results from the domain with 1-km grid spacing demonstrated remarkable detail of the severe downslope winds associated with low-level wind shear and surface gustiness in the Las Vegas Valley. The simulation results demonstrated that model vertical resolution was primarily responsible for the detail of the lower-level wind and temperature structures. The inverse Froude number and Froude number are two indices that may be included as the forecasting guidance for downslope winds, lee waves, and rotors for the Las Vegas Valley.

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Sen Chiao, Yuh-Lang Lin, and Michael L. Kaplan

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This paper investigates the local circulation associated with a heavy orographic rainfall event during 19–21 September 1999 [Mesoscale Alpine Programme Intensive Observing Period 2B (MAP IOP-2B)]. This event was simulated with a 5-km horizontal grid spacing using the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). The MM5 simulation reproduced the basic features such as the timing and location of the deep trough and the associated precipitation evolution, though the total amount of precipitation is slightly higher than that measured by rain gauges (∼30% in 24 h). The near-surface flow was dominated by an easterly jet originally from the Adriatic Sea and a southerly jet from the Gulf of Genoa. A significant westward turning occurred when the southerly flow approached the south side of the Alps. This deflection was caused by boundary layer friction and rotation, as well as mountain blocking effects. Flow was generally from the south above the surface. Precipitation was mainly concentrated on the windward slopes, especially near the Lago Maggiore region. Sensitivity experiments have been conducted to investigate the effects of upstream orography, the western flank of the Alps, boundary layer friction, and horizontal resolution. The results indicate that precipitation distribution and amount over the southern upslope region of the Alps were not directly related to either the coastal Apennine Mountains or the west flank of the Alps. The boundary layer friction reduces the total amount and alters the distribution of rainfall by weakening the wind near the surface. The 1.67-km horizontal grid spacing simulation indicates that heavy rainfall tended to be concentrated in the vicinity of individual mountain peaks. The total amount of rainfall was overpredicted along the windward slopes because of the strong upward motion that occurred on the upslope of the barrier. The results indicate the importance of dynamical forcing associated with upslope-induced and near-surface horizontal velocity convergence-induced vertical motion, which increases rapidly as horizontal resolution increases.

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Yuh-Lang Lin, Darrell B. Ensley, Sen Chiao, and Ching-Yuang Huang

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In this study, a nonhydrostatic mesoscale model [Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS)] was adopted to simulate Supertyphoon Bilis (2000) and investigate the dynamics of orographic rain and track deflection accompanying the storm as it passes the Central Mountain Range (CMR) of Taiwan. Both the storm track and its associated orographic rainfall distribution are well predicted by the numerical model. The intensity of the storm is underpredicted, resulting in a discontinuous track, due to the lack of specifying a “bogus” vortex at the time of model initialization. Cyclonic curvature of the storm track over the island topography track as well as major circulation features are similar to previous studies of landfalling typhoons affecting Taiwan. The model overpredicts the total amount of accumulated rainfall. Generalization of the flux model proposed in a 2001 study by Lin and coauthors is used to help predict and understand the observed rainfall distribution by calculating both the orographic and general vertical moisture fluxes from COAMPS model-predicted wind and moisture fields. The vertical moisture flux calculated from the 15-km-resolution simulation compares reasonably well to the actual, storm-observed rainfall distribution. Results of the flux model using 5-km COAMPS model output are not necessarily better than those using the coarser 15-km-resolution results. The overall consistency between the observed rainfall distribution and that predicted by the moisture flux model of Lin and coauthors indicates that the rainfall occurring in the vicinity of the topography was strongly controlled by orographic forcing, rather than being associated with the original rainbands accompanying the typhoon as it moved onshore.

Analysis of simulation control parameters from previous studies of tropical cyclones (TCs) passing over Taiwan's CMR implies that track continuity is strongly linked to V max/Nh and V max/Rf, where V max and R are the maximum tangential wind and radius of the tropical cyclone, N the Brunt–Väisälä frequency, h the maximum mountain height, and f the Coriolis parameter. It appears that track continuity (discontinuity) is associated with higher (lower) values of these two control parameters. Numerical estimates of these two control parameters from observational data and the numerical simulation results for Supertyphoon Bilis produce results consistent with the findings shown here. Physically, V max/Nh represents the vortex-Froude number (linearity) of the outer circulation of the vortex, and V max/Rf represents the intensity (inertial stability) of the vortex. It is hypothesized that when these two control parameters are small, orographic blocking forces a greater percentage of flow around the mountain, instead of allowing the flow to pass over the topography. The vortex becomes unstable, subsequently resulting in a discontinuous surface and near-surface storm track. Analysis of control parameters from previous studies of landfalling typhoons affecting Taiwan also indicates that a westward-moving TC tends to be deflected to the north (south) when V max/Nh is large (small). The dependence of TC track deflection on the basic-flow Froude number (U/Nh) is not revealed by parameter analysis of the previous studies.

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Yuh-Lang Lin, Heather Dawn Reeves, Shu-Yun Chen, and Sen Chiao

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The dynamical impacts of an unusually strong stable layer that developed over the Po Valley and northern Ligurian Sea during Mesoscale Alpine Program (MAP) intensive observation period 8 (IOP-8) on the formation of convection over the Ligurian Sea are explored. Based on numerically simulated equivalent potential temperature, wind vectors, and by a trajectory analysis of parcels both beneath and above the stable layer, it is shown that the stable layer behaved as a material surface or “effective mountain” to the airstreams impinging on it from the south. Additional analyses show that the leading edge of the stable layer was collocated with maxima in upward motion and a strong positive moisture flux. Hence, it was further argued and demonstrated through inspection of soundings upstream of the cold dome and trajectory analyses that lifting by the stable layer enhanced convective activities over the Ligurian Sea. Finally, processes contributing to the maintenance of the stable layer during IOP-8 were explored. It was found that the differential advection of a warm, less stable air mass on top of a cooler, more stable air mass helped maintain the stable layer. The Ligurian Apennines made a secondary contribution to the stagnation of the cool air in the Po Valley by partially blocking this air mass from exiting the valley to the south.

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