Numerical Simulations of Airflow and Weather over the Island of Oahu

1. Numerical Simulations of Airflow and Weather over the Island of Oahu Yi-Leng Chen and Hiep Van Nguyen

2. Total area is 594 sq. miles - 44 miles long and 30 miles wide. The highest point is Kaala Peak with an elevation of 4,003 ft above sea-level.

3. Outline • Introduction • Model description • Effects of terrain and diurnal heating cycle on island weather • Effects of terrain • Effects of diurnal heating cycle • Effects of trade wind conditions on island-scale flow and weather • Strong trades • Weak trades • Sensitivity tests on terrain and diurnal heating effects • Conclusions

4. Introduction Despite its relatively small size, the island terrain of Oahu has profound influences on island-scale airflow and weather • Effects on airflow: • The mountain acts as a barrier to the approaching airflow; and • As a heat source (sink) during the day (night) (Leopold 1948, 1949; Chen and Nash 1994) resulted in mountain-valley winds and land-sea breezes (Leopold 1948 ) • Wakes off the lee-side coast

5. Introduction • Diurnal cycle of winds over Oahu: • Throughout the diurnal cycle, the surface winds are weaker than over the open ocean except over the northwestern and southeastern corners (Leopold, 1948; Ramage and Oshiro 1977) • A maximum wind speed in the afternoon, due to transfer of momentum from aloft (Leopold, 1948) • Complete reversal of wind direction occurs on the Waianae coast in the afternoon (Leopold,1948)

6. Introduction • Island effects on Oahu rainfall • The trade‑wind belt has a minimum in the global distribution of rainfall. However, trade-wind showers are frequent over the Hawaiian Islands because of terrain and local winds • Diurnal cycle of rainfall over Oahu: • Windward and mauka areas: nocturnal and morning rainfall maxima with a minimum in the afternoon (Loveridge, 1924; Leopold, 1948; Schroeder 1977; Loos 2004) • Waianae lee-side coast: afternoon maximum due to the development of sea breeze/onshore flow along the lee-side coast (Leopold, 1948; Loos 2004)

7. Previous Modeling Studies of Island effects • Theoretical studies of airflow and orographic effects in past have focused on flow past an idealized bell-shape mountain. • Land surface forcings (thermal forcing and surface friction) are either not included or crudely estimated. • NE/E Trade-wind flow is persistent during the summer over the Hawaiian islands without the presence of large-scale disturbances. The Hawaiian Island chain is an idealized situation to study island effects for islands with different heights and sizes. • In this study, we will investigate island effects on airflow and weather for Oahu using numerical models with full model physics for real cases.

8. Previous modeling studies for Oahu • Lavoie (1974) performed modeling studies over Oahu using a simple, single-layer mesoscale model: • Focus: effect of topography and the trade wind inversion • Interesting finding: Hydraulic jump-like feature to the lee of both mountain ranges • Limitation: one layer model, 3-km resolution, poor input data source. Poor treatments of land surface forcings and diurnal cycle. • Recently: • Zhang et al. (2005): use MSM/LSM to simulate temperature at three stations on Oahu. He showed that adequate descriptions of the terrain and vegetation are required to simulate diurnal cycle at these sites. • Using MM5/LSM (Land Surface Model) with adequate descriptions of the terrain and vegetation, Yang et al. (2005) were able to successfully simulate the diurnal evolution of airflow and weather over the Big Island.

9. Scientific Objectives: • MM5/LSM will used as a research tool to study island effects on rainfall and airflow for the island of Oahu. • Would it be possible to use MM5/LSM to simulate the island effects for the island of Oahu which is relatively small? • Study the relative importance of land surface forcing (daytime heating, surface friction) and orographic blocking on the production of westerly reversely flow along the lee-side coast in the afternoon hours. • Orographic effects on winds and trade-wind inversion above the lee-side slopes • What are the effects of trade-wind conditions, e.g., strong and weak trades, on the island-scale airflow and orographic rainfall over Oahu?

10. Research plan • Tool: The nonhydrostatic Mesoscale Model version 5 (MM5) (Dudhia, 1993) coupled with an advanced land surface model (F. Chen and Dudhia 2001) • With adequate depiction of the terrain and land surface properties and vegetation cover (Zhang et al., 2005; Yang et al., 2005) in the model. • Input for mm5: Global Forecast System (GFS) data provided by NCEP (National Centers for Environmental Prediction), horizontal resolution ~ 100 km • Run model for the period of July-August 2005, and compare with observations • Observations: 13 hourly stations and 69 rain gages • Simulations from 10 days of weak trades and 10 days of strong trades are averaged separately to investigate the effects of trade-wind conditions on island-scale airflow and weather • Sensitivity tests: to investigate the effects of terrain and thermal forcing on island-scale airflow and weather

11. Model descriptions • No of domains: 4 nested domains • Horizontal resolution of 1.5-km (for Oahu domain) • Vertical levels: 28 (sigma) • Cumulus parameterization scheme: Grell et al. (1994) (No cumulus parameterization for the 1.5-km Oahu domain) • Explicit precipitation: warm rain (Hsie et al., 1984) • Radiation scheme: Dudhia (1989) • Planetary boundary layer scheme: MRF (Hong and Pan, 1996) • NCEP Noah LSM • Soil moisture data are generated for continuous model run for two months prior to simulation period (Yang et al. 2005) • Each day, a 36-hour forecast is made. • 24 hours of simulation: from the 12th to the 36th hour of model output

12. Note: Horizontal resolution of 1.5-km for Oahu domain 40.5 km 13.5 km 4.5 km 1.5 km

13. Average simulated island-scale andwind observations for summer 2005. • Weak winds over central Oahu and along the lee-side coast • Strong wind speed at northwestern and southeastern corners of Oahu Observations: (red windbars) at 13 stations

14. Average simulated island-scale andwind observations for summer 2005. • Weak winds over central Oahu and along the lee-side coast • Strong wind speed at northwestern and southeastern corners of Oahu Observations: (red wind barbs) at 13 stations One flag: 5 m/s, one full barb: 1 m/s

15. Simulation of temperature diurnal cycle at stations

16. Simulation of temperature diurnal cycle at stations

17. Mean 2-m temperature anomalies (K) from an upstream point (21.67oN, 157.71oW) at the same height during July-August 2005 at (a) 14 HST

18. Mean 2-m temperature anomalies (K) from an upstream point (21.67oN, 157.71oW) at the same height during July-August 2005 at (b) 05 HST.

19. Simulation of mean mixing ratio • Mean mixing ratio simulation shows: • Moist area over windward of Ko’olau because of trade flow • Dry areas over central Oahu because of island blocking • The Driest over the Waianae mountains

20. Mean 2-m mixing ratio anomalies (g kg-1) from an upstream point (21.67oN, 157.71oW) during July-August, 2005 at (a) 14 HST

21. Mean 2-m mixing ratio anomalies (g kg-1) from an upstream point (21.67oN, 157.71oW) during July-August, 2005 at (b) 05 HST.

22. Diurnal cycle of winds • With daytime heating and nighttime cooling overland, the airflow also changes considerably during the diurnal cycleThe simulated island airflow throughout the day agrees well with observations at thirteen stations

23. Diurnal cycle of winds • With daytime heating and nighttime cooling overland, the airflow also changes considerably during the diurnal cycleThe simulated island airflow throughout the day agrees well with observations at thirteen stations

24. : 62 days • Accumulate rainfall show maximum at windward side; minimum at central Oahu and the leeside • A weak maximum on the leeside of the Waianae mountains

25. Total rainfall accumulation (mm) during July-August 2005.(a) observation, (b) simulation.

26. Three-hour rainfall accumulation during July-August, 2005: observed (left) and simulated (right) rainfall (mm).

27. Time series plots of 3-h simulated meteorological variables at 21.5oN, 157.85oW for (a) equivalent potential temperature (K), (b) deviations of vertical motions from the daily mean (cm s-1).

28. (c) zonal wind (m s-1), (d) LCL and LFC in km.

29. Diurnal cycle of winds and vertical motions cross section at 21.45 N

30. Vertical cross section of wind vector (m s-1) and vertical velocity (shaded, cm s-1) along 21.45o N for (a) mean state during July-August 2005, (b) anomalies from the mean at 08 HST.

31. Anomalies from the mean (c) 14 HST, (d) 20 HST.

32. Vertical displacements of the fluid are limited by U/N

34. Theta (K) and wind vectors

35. Theta-E (K) and wind vectors

36. Strong and weak trade wind categories + Upstream point of Oahu (21N, 152W) in reanalysis data from National Center for Environment Prediction/National Center for Atmospheric Research (NCEP/NCAR) is used to define the strong and weak trades + Strong trades: wind speed > 8m/s wind direction: 70 - 90 degree + Weak trades: wind speed < 7m/s wind direction: 60 - 90 degree

37. Mean simulation of 10 strong, 10 weak trade days show that mean wind speeds entire Oahu are weaker in weak trades mean Strong trades One flag: 5 m/s, Full barb: 1m/s Half barb:0.5m/s Weak trades Effects of trade wind conditions on mean circulation

38. Ten days of strong trade winds Ten days of weak trade winds Effect of different trade wind conditions on total rainfallMore rainfall is simulated when trades are stronger

39. At night, the cool air moves down from the windward side of the Ko’olau mountains under weak trade-wind conditions 05HST Strong trades Weak trades Effects of trade wind conditions on diurnal cycle of wind One flag: 5 m/s, Full barb: 1m/s Half barb:0.5m/s

40. In the afternoon, the westerly onshore flow along the Waianae coast and adjacent waters is more pronounced under weak trades. What is the relatively importance of orographic blocking vs. land surface forcing? 14HST Strong trades Weak trades Effects of trade wind conditions on diurnal cycle of wind One flag: 5 m/s, Full barb: 1m/s Half barb:0.5m/s

41. Sensitivity tests • Sensitivity tests are to evaluate the effect of terrain and land surface forcing on the production of lee-side wakes • Epifanio and Rotunno (2005): Blocking of the flow leads to the formation of warm anomalies over the lee slope as potentially warm air descends from aloft to replace the colder air deflected laterally around the barrier. The production of lee-side wake is caused by the temperature gradient between the warm lee-slope air and the colder fluid downstream. • Sensitivity tests: • No mountains • No mountains and no surface friction • No mountains and 75% of solar heating • No Mountains and 50% of solar heating • The sensitivity tests are run for one normal trade wind day, 09 Aug, 2005

42. No mountain case: Winds 05 HST 14 HST Control No mountain Onshore flows off the Waianae coast still exist as a result of island heating but are weaker than CTRL Wind directions are uniform but with weaker speeds over land than open ocean upstream

43. No Mountain , No frictions (NMNF)For the NMNF case, the westerly flow at 14 HST is weaker than the NM case. The Westerly flow offshore is mainly caused by solar heating.

44. No Mountains and 50% solar heating (NM50%), the afternoon westerly flow off the lee-side coast disappears.

45. Summary • Even with a relatively small size, the island of Oahu has profound influences on island airflow and weather • With a 1.5-km resolution, the island effects on airflow and weather are reasonably well simulated by the MM5/LSM model, including • windward and mauka showers at night and early morning over the Ko’olau mountains; • strong easterly downslope winds aloft on the lee-side slopes of both Ko’olau and Waianae mountains at night; • weak wind over central Oahu; • westerly onshore flow in the afternoon offshore off the Waianae coast, mainly a result of land surface heating.

46. Summary • The simulated spatial and diurnal variations of surface winds and rainfall are in good agreement with observations. • The rainfall is mainly caused by orographic lifting. More rainfall is simulated when trades are stronger. • There are two simulated rainfall maxima at the foothills and over the Ko’olau Mountains during the diurnal cycle. • The early morning rainfall maximum is caused by anomalies in rising motions on the windward side as the results of flow deceleration over the windward coastal regions and foothills when the land surface is the coolest. • The early evening rainfall maximum is caused by stronger winds aloft after sunset. • The rainfall minimum in the early afternoon is related to relatively weak orographic lifting due to relatively weak winds aloft and a relatively high LFC as a result of vertical mixing.

47. Future work • Use WRF (Weather Research and Forecast) model, developed at the National Center for Atmospheric Research, to conduct high-resolution simulations of heavy rainfall events over the Hawaiian Islands . Especially the recent record breaking prolonged wet period during February 19 – April 2, 2006. • Data assimilation to improve the initial conditions.

48. Thank you for your attention!

49. Over the Hawaiian Islands, large variations in local microclimate ranging from humid tropical on the windward slopes to hot desert over bare lava soil with different vegetation covers are typical(Juvik et al. 1978; Giambelluca et al. 1993).

50. (a) (b) (c) High-resolution (30“, ~ 1 km at 20 ºN) (a) vegetation types, (b) soil types, and (c) vegetation fraction for the island of Oahu compiled from the USGS 1:100,000-scale Land Use Land Cover Datasets for Hawaii. 13 vegetation types and 16 soil types are used. Vegetation fraction ranges from 0 to 100%.