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Yang Yang and Yi-Leng Chen

Effects of T errain H eights and S izes on I sland-scale C irculations and R ainfall for the I sland of Hawaii during HaRP. Yang Yang and Yi-Leng Chen Department of Meteorology University of Hawaii at Manoa Honolulu, HI96822 May 8, 2007 Presented at National Central University.

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Yang Yang and Yi-Leng Chen

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  1. Effects of Terrain Heights and Sizes on Island-scale Circulations and Rainfall for the Island of Hawaii during HaRP Yang Yang and Yi-Leng Chen Department of MeteorologyUniversity of Hawaii at ManoaHonolulu, HI96822 May 8, 2007 Presented at National Central University

  2. Outlines • 1. Introduction • 2. Model description and initialization • 3. Island-scale circulations and weather without mountains • 4. Island forcing with different terrain heights • 5. Circulations and rainfall with a reduced terrain area • 6. Summary

  3. Theoretical studies of ideal airflow past idealized obstacle • For numerical studies of a stratified airflow past idealized mountains without diabatic heating, and surface friction, Fr and horizontal aspect ratio (β) are control parameters (Smolarkiewicz and Rotunno 1989, 1990; Crook et al. 1990; Schar et al. 1997; Smith 1989; Olafsson et al. 1996; Bauer et al. 2000). • The effect of bottom friction is considered by Grubišić et al. (1995) without diabatic heating. The flow past larger obstacle will be more “viscous”. • For a stratified, nonrotating, inviscid flow past obstacles with thermal forcing, the flow is controlled by Fr and a characteristic scale of thermal forcing (η) (Reisner and Smolarkiewicz 1994). The surface thermal forcing is important for flow regime transition, but crudely treated.

  4. Numerical studies on the island-scale circulations by Smolarkiewicz et al. (1988), Rasmussen et al. (1989, 1993), and Ueyoshi and Han (1991). • Land surface processes were crudely treated in their models. The diurnal cycle was not properly simulated. • Chen and Feng (2001) used MM5 to study the effects of terrain and trade-wind inversion on airflow and weather without the diurnal heating cycle. • Feng and Chen (2001) used MM5 and a simple land surface model to simulate the nighttime flow regime with the entire island covered by tropical rain forest. • Yang et al. (2005) simulated the complete diurnal cycle of airflow and cloud distribution during HaRP using MM5/LSM. In this study, the MM5/LSM will be used as a research tool.

  5. Scientific Issue: Rainfall on the eastern windward side • Carbone et al. (1998) showed that the rainfall on the windward side of the island of Hawaii increases for a higher Fr with a greater wind speed as a result of island blocking. The correlation between rainfall and Fr could be interpreted by stronger orographic lifting as the wind speed increases. A higher Fr could be achieved by lowering the mountain height without changing the wind speed. We will test if Fr is a control parameter for the rainfall production on the windward side or not.

  6. Scientific Issue: Rainfall on the Kona western Lee side From 9-year rainfall data, Schroeder et al. (1977) showed the diurnal rainfall frequency maximum in the afternoon at Kealakekua (~ 480 m elevation) and Holualoa Makai (~ 490 m elevation) along the Kona coast. At the Kona airport maximum rainfall frequencies persist through the late evening. They suggested that a convergence zone at night between the downslope mountain breezes and the lee vortices might be the reason for the prolonged evening rainfall maximum.

  7. Other possible explanations about rainfall production in Kona : • Convective clouds formed over the land or near the shore drifted inland with the sea breeze (Leopold 1949)? • The convergence between the trade-wind and the sea breeze (Giambelluca et al. 1986)? • The convergence between the two lee vortices as a result of island blocking (Patzert 1969)?

  8. Scientific Issue: Does the mountain height control whether the maximum rainfall occurs over the wind slopes or the crest of the mountain? Is the island size important? • Rainfall analyses in Hawaii (Giambelluca et al. 1986) showed that for mountains with peaks above the trade-wind inversion (~ 2 km), maximum rainfall occurs on the windward slopes. For islands with mountaintops below the trade-wind inversion, maximum rainfall occurs on the mountaintops. • The island of Kauai, the northernmost island in Hawaii with a maximum mountain elevation of 1,500 m and a diameter of 50 km, is known as one of the wettest places in the world with the rainfall maximum (12,000 mm per year) on its mountaintop (Ramage and Schroeder 1999). Does the island/terrain sizes affect the rainfall production and distribution?

  9. 2. The Model • 36 sigma levels. Four nested domains, two-way nesting with horizontal resolutions of 81, 27, 9 and 3 km, respectively. • Grell cumulus parameterization, grid-scale warm rain process (Hsie et al. 1984). • Cloud-radiation scheme (Dudhia and Moncrieff 1989). • Hong and Pan’s (1996) boundary layer scheme. • The land surface model (LSM) (Chen and Dudhia 2000) has four layers at depths of 10, 40, 100, and 200 cm. • Real terrain of the island of Hawaii (referred to as CTRL) (Yang et al. 2005). • The 1-km vegetation type, soil type, and vegetation fraction compiled by Zhang et al. (2005) were used. The initial soil moistures and soil temperatures for LSM were generated from 2-month simulations before HaRP (Hawaiian Rainband Project, 11 July-24 August,1990). • The daily simulation was initialized at 1200 UTC each day using the 24-h forecasts of the soil moistures and soil temperatures of the previous day and run for 36 h. The results from the hour 12 to hour 36 of each simulation were used to represent the simulated diurnal cycle.

  10. Terrain height shown by gray shading scale of 1000 m from light to dark. The two quadrangles in are used for statistic analysis in section 4. • c

  11. The four domains employed in this study with resolutions of 81 km, 27 km, 9 km and 3 km, respectively

  12. CTRL: with the real terrain heights (Yang et al. 2005) TER70: with 70% of the terrain heights of CTRL TER35: with 35% of the terrain heights of CTRL S_TER: with a shrunk terrain of 1/7 the size of TER35 terrain, about the size of Kauai. The shrunk terrain of the island of Hawaii from TER35 with a shading interval of 200 m.

  13. The mean surface wind vectors at 50 PAM stations during HaRP (July 11 to Aug. 24, 1990) (CTRL) 1400HST 1400HST Observation Simulation 0200HST 0200HST Simulation Observation

  14. 3. Island-scale circulations and weather without mountains • Chen and Nash (1994) suggest that the diurnal variations of rainfall and winds over the island of Hawaii are results of nonlinear interactions among land surface forcing, island blocking, and orographic lifting. In this section, to isolate the effects of land surface forcing on island-scale circulations and rainfall, we conduct simulations of the diurnal cycle without terrain for each HaRP day.

  15. The temporal mean vertical velocity at 500 m above the sea level during the HaRP period for TER00 at (a) 1700 HST with 2 cm s־¹ contour interval. Shading shows the flat terrain of the island of Hawaii.

  16. The temporal mean vertical velocity at 500 m above the sea level during the HaRP period for TER00 at (a) 2100 HST with 2 cm s־¹ contour interval.

  17. The temporal mean vertical velocity at 500 m above the sea level during the HaRP period for TER00 at 0500 HST with 1 cm s־¹ interval.

  18. 4. Island forcing with different terrain heights • In this section, a comparison is made for the simulated island-induced airflow and rainfall among CTRL, TER70, and TER35 to understand the nonlinear interactions between the land surface forcing and island blocking and rainfall production.

  19. The averaged sounding during HaRP based on the 20 aircraft upstream soundings after Chen and Feng (2001). Solid: temperature (C); dashed: dewpoint (C). The mean LFC is around 500 m.

  20. CTRL TER70 TER35 1400HST 1400HST 1400HST 1400HST 1400HST 1400HST The surface temperature deviations from the mean upstream aircraft sounding during HaRP at the same level with a contour interval of 1 K. 1400HST 1400HST 1400HST Vertical integrated cloud water content (g kg־¹ 10־³) with a contour interval of 40 at 1400 HST.

  21. 0500HST 0500HST TER70 CTRL 0500HST TER35 The surface temperature deviations from the mean upstream aircraft sounding during HaRP at the same level with a contour interval of 1 K.

  22. 1400HST 1400HST 1400HST 0200HST 0200HST 0200HST CTRL TER70 TER35 Mean zonal wind speed (m s־¹) with a contour interval of 1 m s־¹ during HaRP for the Hilo transect

  23. Some characteristics of the temporal mean upslope flow on the windward side of the island of Hawaiia 1400 HST during the HaRP period, and the spatial mean of rainfall accumulation (mm) for all the grids in the quadrangle area (Fig. 1b) on the windward side for the time period of 1100 and 1900 HST during HaRP.

  24. Control run Fr=~0.17 b b CTRL Fr=~.2 TER70 Fr=~0.25 The total rainfall (mm) with a contour interval of 100 mm during HaRP. Fr=U/Nh. U~ 7m s־¹, N=0.01 s־¹, h~4000m, 2800m, 1400m for CTRL, TER70, and TER35, respectively. TER35 Fr=~0.5 The rainband/rainfall on the windward side increases for a higher Fr with a larger U (Carbone et al. 1998). However, this study shows that rainfall on the windward side decreases for a higher Fr for a lower terrain height.

  25. 1400HST 1400HST 1400HST 1400HST 0200HST 0200HST 0200HST 0200HST CTRL TER70 TER35 Mean zonal wind speed (m s־¹) with a contour interval of 0.5 m s־¹ during HaRP for the Kona transect.

  26. The total rainfall accumulation (mm) during HaRP for S_TER and TER35 with a contour interval of 100 mm (thick solid lines). The island size for S_TER is 1/7 of TER35. Both have the same mountain heights. 5. Effect of terrain/island sizes TER35 S_TER Giambelluca et al. (1986) showed for Hawaiian islands, rainfall maxima on mountaintops with heights lower than trade-wind inversion. These mountains are smaller in size as compared with the Big Island.

  27. 6. Summary • Islands in Hawaii have different sizes and terrain heights with notable differences in climate and weather. In this study, we used MM5/LSM to conduct numerical simulations for the island of Hawaii with different model terrain heights and sizes during the diurnal cycle for the HaRP (Hawaiian Rainband Project) period. • In addition to island blocking and orographic lifting, terrain heights also affect the land surface thermal forcing throughout the diurnal cycle by the variations of orographic clouds during the day and of the longwave radiation heat loss at night. • The simulated rainfall distributions and amounts throughout the diurnal cycle are closely related to rising motions caused by nonlinear interactions among island blocking, orographic lifting, and land surface processes. • The evening rainfall maximum along the western Kona leeside coast is caused by the convergence between the westerly return flow and the offshore flow. For a lower model terrain, the westerly return flow is weaker, as a result, smaller evening rainfall amounts. • Besides the terrain/mountain height, island size is another factor that affects rainfall production and distribution in Hawaii, because island size affects orographic lifting, surface forcing, and the advection time scale for an air parcel to reach the mountaintop.The heavy rainfall maximum on the mountaintop of the island of Kauai is due to its suitable height and size.

  28. Thank you!

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