Essential Ingredients for Heavy Orographic Rainfall and their Potential Application for Prediction

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1 7.6 Eential Ingredient for eavy Orographic Rainfall and their Potential Application for Prediction Yuh-Lang Lin, Sen Chiao, Jame A. Thurman and Michael L. Kaplan North Carolina State Univerity, Raleigh, North Carolina, USA Ting-An Wang Civil Aeronautic and Aviation Adminitration, Taipei, Taiwan 1. Introduction By examining everal heavy orographic rainfall event in the US, everal author (e.g. Maddox et al. 1978; Pontrelli et al. 1999) found ome common ynoptic and meocale environment: (1) a low-level jet (LLJ) with high θe impinging on the mountain, (2) a deep hortwave trough propagating toward the mountain, and (3) an upper-tropopheric preure ridge to low the movement of convective ytem. Note that a high value of CAPE i not conitently oberved for thee event. In thi tudy, we examine thee condition in heavy orographic rainfall event in Alp. 2. Synoptic and meocale environment conducive to heavy orographic rainfall in Alp In analyzing ome heavy orographic precipitation event which have occurred over the outhern Alp (e.g., the Vaion-la-Romaine event, the Brig and South Ticino, and the Piedmont) during the fall eaon, Maacand et al. (1998) found that all of thee event were accompanied by a deep, narrow, and elongated uppertropopheric filament of extruded tratopheric air, which extended in a meridianally-oriented (north-outh) direction from the Britih ile to the wetern Mediterranean. In each cae, the filament of tratopheric air tranlated relatively lowly eatward, and the torm event enued a the filament forward flank approached the Alpine ridge. They propoed that thee high PV treamer act a a precuror and tend to: (i) enhance the outherly flow component toward the Alp, (ii) reduce the tatic tability beneath the upperlevel PV anomaly and (iii) trigger acent on the PV treamer forward flank to generate or enhance convection. The acent induced by the forward (eatern) flank of the upper-level trough or PV treamer may coincide with the low-level orographically induced upward motion to trigger vigorou convection. In invetigating heavy orographic rain over the outhern Alp, Buzzi and Fochini (2000) found that the following large and meocale flow feature are Correponding author addre: Yuh-Lang Lin, Department of Marine, Earth and Atmopheric Science, North Carolina State Univerity, Raleigh, NC important: (1) The flow at low level maintain a outherly direction over the wetern Mediterranean, with pronounced confluence over the wetern Alp, between the pot-frontal outhweterly flow and the pre-frontal outheaterly flow located more to the eat. (2) There exit an 850 hpa pre-frontal LLJ which erve a a warm conveyor belt that flow directly toward wetern Alp. (3) There exit a deep upper-tropopheric trough approaching the Alp. (4) There exit a quai-tationary preure ridge in the upper-tropophere, located to the eat, aociated with an anticyclone over Eatern Europe. It appear that the high PV treamer found by Maacand et al. (1998) are related to the deep hortwave trough found in Buzzi et al. tudie (e.g. 2000). In addition, if one inpect Maacand et al. figure cloely, it can be een that there alo exit a quai-tationary preure ridge to the eat of the high-pv treamer in all cae, which wa not emphaized in their paper. The low-level jet which occurred jut before the onet of the heavy orographic rainfall event were partially aociated with the approaching deep hortwave trough. In other word, the approaching deep hortwave trough not only provide the upper-level divergence for additional upward motion over the uploping topography on it forward (eatern) flank, but it alo trengthen the LLJ and help bring the deep layer of conditionally untable air from the Mediterranean Sea to interact with the Alpine mountain to produce diffluence and outflow aloft. Similar to US event, high value of CAPE are not conitently oberved in Alpine heavy orographic rainfall event. One particular flow feature aociated with the majority of Alpine heavy orographic rainfall event i that convection often tart in the concave region outh of Alp, uch a the Lago Maggiore and Ticino region (Buzzi and Fochini 2000). Thi confluent flow tend to enhance the low-level upward motion induced by the uplope, which in turn trigger the convection. Figure 1 how the accumulated 12-h rainfall ending at 00UTC and 12UTC 20 September 1999 during the MAP IOP2 heavy orographic rainfall event. The rainfall accumulated during the whole event (13UTC 19 September 1999 to 01UTC 21 September 1999) in the Lago Maggiore region range from 100 to 300 mm (Fig. 1). The rainfall region during the 12-h period ending at 00UTC 20 September (Fig. 1) extended from the Gulf of

2 Genoa to the Lago Maggiore region, with the heaviet precipitation focued in the Lago Maggiore area. The maximum 12-h rainfall amount reache 130 mm. Thi heavy rainfall area wa aociated with the impinging trough (not hown), which brought in from the Ligurian Sea air of high moiture content. Similar to the previouly dicued hitorical event, the upper-level preure ridge located to the eat of the approaching deep hortwave trough i quai-tationary (not hown). It can be een clearly from Fig. 1 that the rainfall tarted in the Lago Maggiore region (Fig. 1a), which i located in the concave topography region of the outhern lope of the European Alp (ee 500m contour in Fig. 2b). During the next 12-h period ending 12UTC 20 September, the heavy rainfall encompaed the entire outhern lope of the Alp, where the maximum rainfall amount reached a value higher than 130 mm (Fig. 1). The impinging deep hortwave trough helped induce the outherly low-level jet, which reached a maximum value of about 12.5 m -1 at Milan, Italy. CAPE value calculated from the Milan ounding were very mall (149 J kg -1 and 201 J kg -1 at 00UTC and 12UTC 20 September, repectively). A alo found in previou tudie of hitorical flah flooding event over the outhern Alp, the low-level jet i very moit, which ha a tongue of high moiture with water vapor mixing ratio ( q v ) covering northern Italy and the Lago Maggiore region. Thi high moiture tongue wa increaing during the heavy rainfall period for the next 12 h. In ummary, imilar to the US cae, the ynoptic and meocale environment conducive to heavy orographic rainfall in the European Alp are: (1) a conditionally or potentially untable airtream impinging on the mountain, (2) a very moit LLJ, (3) teep mountain to help releae intability, (4) an uppertropopheric deep hortwave trough or high-pv (potential vorticity) anomaly approaching the threat area (which help to enhance low-level upward motion), and (5) an upper-tropopheric quai-tationary high preure ridge to force the convective ytem to be quaitationary or low it forward progre over the threat area. In addition, the low-level confluent flow play an important role in helping to trigger the convection in the Alpine cae. For the MAP IOP-2 cae, a tongue of high moiture extended into the threat area. In the next ection, we will examine whether or not thee condition are eential for producing heavy orographic rainfall in Eat Aia. In examining 3 heavy orographic rain event aociated with tropical depreion approaching Taiwan and Japan, we found the following common ynoptic and meocale feature are eential for producing heavy orographic rainfall aociated with a tropical torm or depreion in Taiwan and Japan: (1) A teep mountain help to releae intability. (2) A tropical torm or depreion help enhance the low-level jet (LLJ). (3) The LLJ i highly conditionally (i.e. high CAPE) and potentially untable. (4) A quai-tationary ynoptic ytem, uch a a typhoon in Taiwanee cae or a tationary front in Japanee cae, act to impede or low the movement of the convective ytem over the mountain. 3. Some eential ingredient for producing heavy orographic rainfall Baed on Dowell et al. (1996), the total precipitation produced i determined by P = E(wq)L /, (1) c where E i the precipitation efficiency, wq the vertical moiture flux, L and c the horizontal cale and propagation peed of the convective ytem, repectively. For flow over a mountain range, the upward vertical motion may be etimated by w = woro + w env = V h + wenv, (2) where wenv i the environmentally-induced vertical motion, h(x,y) i the mountain geometry and V i the horizontal wind. The environmentally-forced upward vertical motion ( w env ) i determined by the tranient ynoptic etting, uch a the divergence aociated with an approaching deep hortwave trough. Combining Eq. (1) and (2) give [ h w env ] q L / c P E V +. (3) = Dowell et al. (1998) ha ued a form imilar to V h to diagnoe three epiode of heavy rainfall in the wetern Mediterranean. Therefore, Eq. (3) indicate that a heavy orographic rainfall require ignificant contribution from any combination of the following eential ingredient: (1) An incoming flow with high efficiency, (2) an intene low-level jet, (3) teep orography, (4) favorable (e.g. concave) mountain geometry and a confluent flow field, (5) trong environmentally-forced w, (6) a high moiture flow uptream, (7) the preence of a large, pre-exiting convective ytem, (8) impeded movement of the convective ytem, and (9) a conditionally or potentially untable low-level flow. Ingredient (9) i required for a deep convective ytem, which i often the cae for heavy orographic rainfall event. It appear that the common ynoptic and meocale condition conducive to heavy orographic rainfall over US, Alpine, Taiwan, and Japan mountain belong to a ubet of the above lited eential ingredient. Although a high CAPE i not conitently oberved for US and Alpine heavy orographic rainfall event, thi remain to be invetigated. For Alpine heavy oro-graphic rainfall event, which often tart in a concave region (uch a the Ticino and Lago Maggiore region), meocale vertical

3 motion may be produced if there exit incoming confluent flow a indicated by ingredient (4). In Table 1, we have provided rough etimate for ome of the eential ingredient apparently reponible for producing heavy orographic rain in the US, the European Alp, and Eat Aia. Thee include the lowlevel wind (U), mountain lope ( h / x ), water vapor mixing ratio (q), and CAPE. Note that w env i difficult to etimate without fine-reolution data, o that we jut denote whether or not there i an approaching ynoptic cale ytem (uch a a deep hortwave trough), which i able to enhance the low-level upward vertical motion. It i alo difficult to etimate the horizontal cale of the convective ytem and it propagation peed (c ). Thu, a a firt attempt, we only provide etimate for U, h / x, and q and compare them with the maximum oberved rainfall rate. For Alpine event, the index i > 4.7 (except for the South Ticino cae). Therefore, thi propoed index may help provide additionally valuable information for helping to predict the occurrence of uptream heavy orographic rainfall event. For the Taiwanee and Japanee cae, it ha a relatively high value (e.g. > 6) and the propoed index i roughly proportional to the oberved maximum rainfall rate. For US event, it appear that the index ha a lower value, but i till >2.7. With w env included, the index hould be able to reach a higher value (comparable to thoe etimated for the Taiwanee and Japanee cae). Fig. 2 how a ditribution of a more complete index, uch a ( V q, for MAP IOP2 by uing triply neted MM5 imulated reult. A imilar index ha alo been ued by Dowell et al. (1998). The horizontal reolution i 5 km and the predicted field are valid at 12UTC 20 September 1999, which i within the heaviet rainfall period. From the figure, it can be een that the horizontal ditribution of the propoed index (Fig. 2a) compare fairly well with oberved 12 h rainfall (Fig. 1) and model predicted rainfall ditribution (Fig. 2b). 4. Concluding Remark Similar to US event, the following common feature are alo oberved in the European and Eat Aian cae: (1) a conditionally or potentially untable airtream impinging on the mountain, (2) the preence of a very moit LLJ, (3) the preence of teep orography to help releae intability, and (4) the preence of a quai-tationary ynoptic cale ytem i required to impede or low the progre of the orographically-forced convective ytem over the threat area. A deep hortwave trough or poitive potential vorticity (PV) anomaly i found to approach the threat area in both the US and Alpine cae, but i abent in the Eat Aian cae. On the other hand, high value of CAPE are oberved in the Eat Aian cae, but are not conitently oberved in either the US or Alpine cae. The quai-tationary ynoptic ytem can be different in different region, but play imilar role a to retard or impede the convective ytem. In both the Alpine and Taiwanee cae, convection often tart in a concave region of the mountain, which induce confluent flow and enhance the upward vertical motion. Baed on an ingredient argument, we found that heavy orographic rainfall require ignificant contribution from any combination of ome eential ingredient. The common ynoptic and meocale environmental environment conducive to heavy orographic rainfall event oberved in the US, Europe, and Eat Aia appear to belong to thi et of eential ingredient. In addition, thee eential ingredient are alo ued to help explain the ynoptic and meocale environment oberved in ome heavy orographic rainfall event occurred in China, New Zealand, and India. We alo propoed an index U( h / x) q to help predict the occurrence of heavy orographic rainfall. Thi quantity appear to be promiing. By applying a more complete form, uch a ( V q, to MAP IOP2 event, we found the ditribution of the index value compare well with the rainfall data predicted by a meocale model. Acknowledgement: Dr. Joeph J. Charney and Mr. Darrell B. Enley helped in generating ome of the figure. Thi work i upported by the US NSF Grant ATM Reference Buzzi, A., and L. Fochini, 2000: Meocale meteorological feature aociated with heavy precipitation in the Southern Alpine region. Meteor. Atmo. Phy., 72, Dowell, C. A., III,. Brook, and R. Maddox, 1996: Flah flood forecating: An ingredient-baed methodology. Wea. Forecating, 11, Dowell, C. A., III, R. Romero, and S. Alono, 1998: A diagnotic tudy of three heavy precipitation epiode in the wetern Mediterranean region. Wea. Forecating, 13, Maddox, R. A., L. R. oxit, C. F. Chappell, and F. Caracena, 1978: Comparion of meteorological apect of the Big Thompon and Rapid City flah flood. Mon. Wea. Rev., 106, Maacand, A. C.,. Wernli, and. C. Davie, 1998: eavy precipitation on the Alpine outhide: An upper-level precuror. Geophy. Re. Lett., 25, Pontrelli, M.D., G. Bryan, and J. M. Fritch, 1999: The Madion County, Virginia, flah flood of 27 June Wea. and Forecating, 14,

Table 1: An etimate of the contribution from ome ingredient Event U (m/) Mtn Slope ( h / x) q (g/kg) Index [ U ( h / x) q ] w env Max.

1+ trough 260 mm/d (130 mm/6h) 383 Lago Maggiore IOP2) 12.5 0.033 11.5 4.7+ trough 300 mm/36h 90 Big Thompon 12.5 0.025 16 5.

7+ trough 519 mm/d (259 mm/6h) 628 Madion County 12.5 0.025 16 5.0+ trough 600 mm/d 286 Taiwan-1999 10.0 0.033 21 6.9 No 200 mm/d 2099 Taiwan-1959 17.5 0.033 22 12.

4 Table 1: An etimate of the contribution from ome ingredient Event U (m/) Mtn Slope ( h / x) q (g/kg) Index [ U ( h / x) q ] w env Max. Rainfall Rate CAPE (J/kg) Vaion-la Romaine 15(20) (12) 6.1+ trough 300 mm/d 2662 (383) Piedmont trough 250 mm/d 212 South Ticino trough 260 mm/d (130 mm/6h) 383 Lago Maggiore IOP2) trough 300 mm/36h 90 Big Thompon * trough 915 mm/d (305 mm/4h) 2526 Rapid City trough 1143 mm/d (381 mm/4h) (<2180) Ft. Collin trough 519 mm/d (259 mm/6h) 628 Madion County trough 600 mm/d 286 Taiwan No 200 mm/d 2099 Taiwan No 500 mm/d 2406 Japan No 150 mm/d 1149 * The ymbol + indicate that the index may be higher with the addition of w env aociated with an approaching ynoptic ytem. Fig. 1: Oberved 12-h rainfall accumulation ending at 12UTC 20 September 1999 for the MAP IOP-2 heavy rainfall event.

5 Fig. 2: (a) Orographic moiture flux index, ( V q, calculated from a 5-km reolution MM5 imulation reult, and (b) MM5 model predicted rainfall. Both are valid at 12UTC 20 September The 500 m terrain contour i alo depicted in panel

Some Common Ingredients for Heavy Orographic Rainfall

VOLUME 16 WEATHER AND FORECASTING DECEMBER 2001 Some Common Ingredients for Heavy Orographic Rainfall YUH-LANG LIN AND SEN CHIAO Department of Marine, Earth and Atmospheric Sciences, North Carolina State