Variations of the MeiYu Season Coastal Rainfall over South China Chen X F Zhang and K Zhao 2017 Influence of monsoonal wind speed and moisture content on intensity and diurnal variations of the MeiYu season coastal rainfall over South China ID: 1045452
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1. Influence of Monsoonal Wind Speed and Moisture Content on Intensity and DiurnalVariations of the Mei-Yu Season Coastal Rainfall over South ChinaChen, X., F, Zhang, and K. Zhao, 2017: Influence of monsoonal wind speed and moisture content on intensity and diurnal variations of the Mei-Yu season coastal rainfall over South China. J. Atmos. Sci., 74, 2835–2856.
2. introductioncoastal rainfall over the south China coastal region exhibits a pronounced diurnal cycle, with a maximum in the early afternoon and a secondary peak in the early morning. the mei-yu season diurnal variations of coastal rainfall are mainly controlled by the land–sea breeze. (Chen et al. 2016)The nocturnal offshore rainfall is induced by the convergence line between the onshore monsoonal wind and the land breeze, while the daytime inland-penetrating precipitation is mainly produced by the sea-breeze fronts. (Chen et al. 2016)
3. the large-scale wind speed is a crucial factor controlling the diurnal cycles of rainfall over small tropical islands, and the diurnal cycle of rainfall becomes muted when the large-scale wind is high. Wang and Sobel (2017) The intensity and evolutions of convection can be very sensitive to the small perturbations in the environmental moisture (Gilmore et al. 2004; Melhauser and Zhang 2012; Schumacher 2015).
4. Numerical modelModel: WRF, version 3.7 4 kmVertical : 50 stretched-grid vertical levelsmodel top : 50 hPaMicrophysics scheme : WRF single-moment 5-classboundary layer scheme: YSUShortwave radiation: Dudhia 1989Longwave radiation: Rapid Radiative Transfer Model (RRTM)surface types: evergreen broadleaf10-day simulations Model domain and topography: Guangzhou ground based radar (GZRD)150-km observation rangered dashed box: Pearl River Delta 珠江三角洲
5. initial and lateral boundary conditions : 6-hourly GFS NOAA GFS analysis datahigh-wind and low-wind days:daily averaged southerly wind speed at the 2-km altitude (over model domain, 2007–09 mei-yu seasons)top one-third days high-wind days (HiWind100) bottom one-third low-wind days (LoWind100)Diurnally cyclic-in-time lateral boundary condition:from 0000 to 0600 to 1200 to 1800 UTC and then back to 0000 UTC
6. 9-day average horizontal wind, 0.994-eta level (50 m altitude)onshore wind speedshadings: averaged water vapor mixing ratio below 1 kmContours: orography
7. average daily precipitationHigh-wind dayslow-wind daysContour: orographyBlack dashed box: PRD GZRDWRF
8. Hourly precipitation between 0700 and 0800 LSTThe land-breeze reaches its strongest phase during this hour in the mei-yu seasonHigh-wind dayslow-wind daysContour: orography GZRDWRF
9. Hourly precipitation between 1500 and 1600 LSTThe sea-breeze reaches its strongest phase in this hour during the mei-yu seasonHigh-wind dayslow-wind daysContour: orography GZRDWRF
10. Contours: horizontal divergenceShadings: perturbation onshore wind (0.994-eta level)Coastal region
11. (a)Streamlines: Mean wind(b)~(i)Streamlines: perturbation windHiWind100 experimentShadings: Onshore windRed contours: positive perturbation temperatureBlue contours: negative perturbation temperature
12. (a)Streamlines: Mean wind(b)~(i)Streamlines: perturbation windLoWind100 experimentShadings: Onshore windRed contours: positive perturbation temperatureBlue contours: negative perturbation temperature
13. Shadings: perturbation onshore windContours: perturbation temperature
14. Shadings: horizontal divergenceContours: positive perturbation vertical velocityShadings: perturbation onshore windContours: perturbation temperatureHigh-wind days
15. Shadings: horizontal divergenceContours: positive perturbation vertical velocityShadings: perturbation onshore windContours: perturbation temperatureLow-wind days
16. experimental design (I)lifting of the incoming oceanic air parcels induced by the differential surface friction between the land and ocean is the primary cause of the coastal rainfall maximum in the high-wind days. (Chen et al. 2014)HiWindnoDG and LoWindnoDG :The sensitivity experiments are configured the same as the control experiments HiWind100 and LoWind100, except that the inland roughness length is set equal to that over ocean in
17. Average daily precipitation
18. Contours: horizontal divergenceShadings: perturbation onshore wind (0.994-eta level)
19. experimental design (II)HiWindEXqv :high-wind days with the moisture from the low-wind daysLoWindEXqv :low-wind days with the moisture from the high-wind days
20. Average daily precipitation
21. experimental design (III)HiWindEXwind : the winds in the boundary conditions are changed to that in the low-wind daysLoWindEXwind :the winds in the boundary conditions are changed to that in the high-wind days
22. Average daily precipitation
23. experimental design (Ⅴ) To further investigate the sensitivity of accumulated coastal rainfall amount to the small perturbations in the incoming moisture.Hi/LoWind98, Hi/LoWind95, and Hi/LoWind90 :water vapor flux on the lateral boundary is reduced to 98%, 95%, and 90% on all model levels for high-wind days and low-wind days
24. Average daily precipitation
25. experimental design (Ⅵ)To further elucidate the impacts of incoming moisture on different levels on the accumulated coastal rainfall amountHi/LoWind98/0–1km, Hi/LoWind98/2–3km, and Hi/LoWind98/ 4–5km :the sensitivity of water vapor flux (reduction of 2%) only at certain altitudes (i.e., between 0 and 1, between 2 and 3, and between 4 and 5km) at the lateral boundaries
26. Average daily precipitation
27. SummaryHigh-wind days:a prominent rainfall maximum along the coastline (quasi stationary)strong horizontal convergence on the coast (differential surface friction and the small coastal hills)enhanced by the land breezeLow-wind days:coastal rainfall is much weaker (diurnal propagation)produced by the land–sea-breeze fronts (stronger land– sea temperature contrast)Environmental moisture: the accumulated amount of rainfall (especially to the moisture in the boundary layer near the surface)
28.
29. abstractthe pattern of coastal rainfall spatial distribution is mostly controlled by the ambient onshore wind speed.During the high-wind days, strong coastal rainfall is concentrated along the coastline and reaches its maximum in the early morning.The coastal lifting induced by the differential surface friction and small hills is the primary cause for the strong coastal rainfall, while land breeze enhances coastal lifting and precipitation from evening to early morning.In the low-wind days, coastal rainfall is mainly induced by the land–sea-breeze fronts, which has apparent diurnal propagation perpendicular to the coastline.With stronger land–sea temperature contrast, the land–sea breeze is stronger during the low-wind days.Both in the high-wind and low-wind days, the coastal rainfall intensity is sensitive to the incoming moisture in the upstream oceanic airflow, especially to the moisture content in the boundary layer.