2014.07.16 GEWEX Coherent variability between diurnal cycle and intraseasonal oscilla4on in orographic precipita4on around the Meghalaya Plateau Tomonori Sato (Hokkaido University, Japan) Topography Precipita<on [m] Climatology(APHRODITE) [mm] Sato (2013, Monthly Weather Review) Precipita4on variability in Bangladesh ISO JJA Total 7-‐90 day Diurnal cycle ? 7-‐25 day Fujinami et al. (2011) • Submonthly (7-‐25 day) ISO is dominant • ISO ac<vity determines interannual varia<on Tbb < 260K June 1995 Ohsawa et al. (2001) • Night-‐morning maximum (0-‐6 LT) • Different phase with surrounding areas Ohsawa et al. (2000) Purpose of this study • To demonstrate a regional atmospheric model experiment that captures precipita4on variability around the Meghalaya Plateau including the submonthly-‐scale ISO and the diurnal cycle. • To inves4gate mesoscale circula4on that characterizes the orographic precipita4on in rela4on with ISO and diurnal cycle. 18km-‐mesh domain 3.6km-‐mesh domain Himalaya Mountains Meghalaya Plateau • • • • • • WRF (ARW core) v3.2.1 (Skamarock et al., 2008) NCEP-‐DOE reanalysis 2 (Kanamitsu et al., 2002) Weekly OISST (Reynolds et al., 2002) Period 15thJune 2004 – 25th July 2004 18km (133x133) -‐ 3.6km (151x151), 40-‐layers Physics – Microphysics: WRF single moment 6 class (Hong et al., 2004) – Cumulus: Kain-‐Fritsch scheme (Kain, 2004) Precipita4on around Meghalaya (2004 summer) 7-‐25day Filtered 2004 2004 Synop4c paderns in lower troposphere (a) Ac4ve-‐1 (5-‐9Jul) (b) Break-‐1 (11-‐15Jul) 850hPa wind This study July 2004 Moisture flux 20-‐summer composite (Ac4ve-‐Break) Fujinami et al. (2011) ・Stronger westerly over Bangladesh ・Moisture convergence Precipita4on in Meghalaya (2004 summer) 240 80 A1 B1 A2 TRMM 3B42 200 40 160 0 7-‐25day 120 -‐40 Filtered 80 -‐80 40 -‐120 0 -‐160 1-‐Jun 8-‐Jun 15-‐Jun 22-‐Jun 29-‐Jun 6-‐Jul 13-‐Jul 20-‐Jul 27-‐Jul 2004 00 21TRMM 3B42 18 15Precipita4on peak appears 12during night-‐morning corresponding to ac4ve 09phase of ISO. 06 03 00 [mm 3hr-‐1] APHRODITE A1 TRMM 3B42 A1 A1 WRF D.C. of precipita4on around Meghalaya A1 B1 24 A2 21 TRMM 18 B1 B1 B1 15 12 09 06 03 B1’ B1’ B1’ 2420 Jun 24 21 Model 1 Jul 10 Jul B1’ 20 Jul 18 15 A2 A2 A2 12 09 06 03 24 0 3 6 9 121518212427 -‐1 mm 3hr 18LT 6Jul C 21LT LLJ C’ C Color: wind speed C’ 00LT 7Jul 03LT 06LT 09LT 12LT 15LT mm hr-‐1 m sec-‐1 Diurnal cycle of low-‐level winds 900hPa-‐wind anomaly （Ac4ve period: 6-‐7 July） [hPa] Wind speed 06LT 12LT 18LT 00LT 700 800 900 Maximum in evening-‐early morning 9 6 3 210 1000 0 2 46 8101214161820 [m/sec] • Clockwise rota4on of low level wind • Consistent with radiosonde observa4ons at Dhaka (Terao et al., 2006; Murata et al., 2008). Summary Findings • A regional model experiment successfully simulated the diurnal cycle of precipita4on during the ac4ve ISO. • The magnitude of lower southwesterly winds differs significantly between ac4ve and inac4ve periods • Diurnal cycle of precipita4on is enhanced during the ac4ve period of ISO, exhibi4ng an evening to early morning maximum, as observed by the TRMM. • A prominent nocturnal LLJ appears in the lower troposphere – LLJ accelerates the preexis4ng southwesterly flow during 1800–0600 LT, regula4ng a 4ming of precipita4on maximum. Future works • Mechanism of LLJ • Many ISO-‐DC events need to be studied. Sato, T., 2013: Mechanism of Orographic Precipita4on around the Meghalaya Plateau Associated with Intraseasonal Oscilla4on and the Diurnal Cycle. Mon. Wea. Rev., 141, 2451–2466. Adainable height (a) Ac4ve (6-‐7 July) Fr2=U2/(NH)2=1 (b) Inac4ve (11-‐12 July) [m] [m sec-‐1] • Brunt-‐Vaisala frequency is very similar between ac4ve and inac4ve periods. • Wind speed in ac4ve period is sufficiently strong to reach the condensa4on level. (a) U850 A1 B1 A2 WRF(18km) NCEP2 (b) V850 Time series of (a) zonal and (b) meridional wind speed at 850 hPa over the Meghalaya Plateau (90-‐92oE, 24-‐26oN). Solid gray line and broken line indicate NCEP2 reanalysis and WRF experiment (outer domain), respec4vely. Inac4ve Ac4ve 06LT 12LT 18LT 00LT 06LT 12LT 18LT 00LT [m/sec] [m/sec] ac4ve (6-‐7July) θ inac4ve (11-‐12 July) qv θe Ver4cal profiles of poten4al temperature, equivalent poten4al temperature, specific humidity, and wind vector over 91-‐92oE, 24-‐25oN during ac4ve (6-‐7 July) and inac4ve (11-‐12 July) period. Ac4ve 900hPa wind anomaly Break [m] 06LT 09LT 12LT 15LT 18LT 21LT 00LT 03LT • Clockwise rota4on of low level wind • Consistent with radiosonde observa4ons at Dhaka (Terao et al., 2006; Murata et al., 2008).
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