THE INFLUENCE OF MODEL RESOLUTION ON AN EXPRESSION OF THE ATMOSPHERIC BOUNDARY LAYER IN A SINGLE-COLUMN MODEL

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1 THE INFLUENCE OF MODEL RESOLUTION ON AN EXPRESSION OF THE ATMOSPHERIC BOUNDARY LAYER IN A SINGLE-COLUMN MODEL P3.1 Kot Iwmur*, Hiroto Kitgw Jpn Meteorologil Ageny 1. INTRODUCTION Jpn Meteorologil Ageny (JMA) hs joined the seond interomprison projet of the GEWEX Atmospheri Boundry Lyer Study (GABLS). This interomprison tht hd purpose of understnding representtion of the diurnl yle in tmospheri oundry lyer (ABL) over lnd reveled the differene in expressions of the ABL etween vrious numeril models. It ws pointed out in the projet tht the different results etween models my prtly ome from differene in their model resolutions (Svensson, 5). A Numeril model expresses nturl phenomen like flow of tmosphere using finite grid points, tht is, its governing equtions re trnsformed into finite differene expressions. Therefore model resolution (sptil resolution nd time step length) my influene on n expression of the ABL. The resolutions of JMA opertionl glol NWP model (GSM) is 55 km (horizontl), 4 lyers (vertil) nd se (time step length). Due to the limited omputer power, it is not esy to upgrde our model to higher resolution. In this study, we revel the influene of vertil resolution nd time step length on representtion of ABL using the single-olumn model (SCM). 2. EXPERIMENTS We reserh the influene of numer of vertil lyers nd time steps on expression of oundry lyer nd surfe wether * Corresponding uthor ddress: Kot Iwmur, Jpn Meteorologil Ageny, Ote-mhi, Chiyod-ku Tokyo, Jpn, -8122; e-mil: iwmur@met.kishou.go.jp (temperture nd wind speed) using the GABLS seond interomprison experimentl se. Mjor fetures of the se re: -Single-olumn version of JMA GSM is used. -Surfe skin temperture hnges diurnlly nd is given s oundry ondition. -Geostrophi wind lows onstntly with time nd height. Turulene sheme of SCM is Mellor nd Ymd level 2., whih is lmost the sme s the one used in JMA opertionl NWP model. In this study, vertil lyers of SCM is deided y the following eqution, Z Z + r( Z Z ). 1 (1) n+ 1 = n n n Z n mens n ltitude of n-th lyer. Ftor r in eq.(1) is seleted using the three prmeters (KMAX, Z 1, nd Z KMAX ). The first prmeter KMAX is the numer of vertil lyers, Z 1 is thikness of lowest lyer, nd Z KMAX is the height of the top lyer. In present study, we set 2 m nd 1 km to Z 1 nd Z KMAX, respetively, nd investigte the impt y vrious KMAXs. 3. RESULTS 3.1 Flux Rihrdson Numer Figure 1 ( to ) shows vertil profiles of flux Rihrdson numer (Rf) y vertil resolution. The height where Rf rehes minimum vlue inreses with the numer of vertil lyers in the dytime (Fig.1 ). For exmple, in the se where the numer of vertil lyers (KMAX) is 2, this height is out 4 m (6th model lyer, 32-51m), nd in the se where KMAX is, this height is out 7 m (27th model lyer, 68-7m) t lol time (LT) 1 (15UTC, 2 hours fter strting time integrtion). The height where

2 Rf rehes lso inreses with the numer of vertil lyers. In other words, unstle lyers expnd upwrd with inrese of the resolution. This influene is signifint prtiulrly when the tmospheri oundry lyer grows up. The height of Rf= is out m nd 17 m in the se where KMAX re 2 nd t LT 1, respetively (Fig.1 ). The minimum vlues of flux Rihrdson numer dereses with inrese of KMAX t LT 1 (Fig.1 ). These fetures re not lerly in stle strtifition t LT 7 (Fig.1 ). Flux Rihrdson numer lso depends on time step length (Fig.2 ). Profiles of flux Rihrdson numer with longer time steps shows similr hrteristis to low vertil resolution. 3.2 Het Flux Profiles of het fluxes re strongly influened y the numer of vertil lyers. In figure 1(d), the negtive het flux tends to grow with inresed KMAX in the erly morning (LT 7). After 3 hours, the positive het flux lso tends to grow with the numer of vertil lyers t LT 1 (Fig.1 e). The height where het flux rehes mximum (LT 1, 13) or minimum (LT 7) rises with the numer of vertil lyers (Fig.1 d to f). This feture is onsistent with the resolution dependeny seen in the flux Rihrdson numer profiles. 3.3 Momentum Flux The influene on momentum flux y the vertil resolution is similr to the one on het flux in the dytime (Fig.1 h nd i). The momentum flux ner the surfe with high resolution is out twie s lrge s the one with low resolution. Although the signs of momentum flux nd het flux re in the opposite, shpes of the profiles re similr to eh other t LT 7 (Fig.1 d nd g). Momentum nd het flux expressions depend on lso time step length (Fig.2 nd ). The profile with long time steps is loser to the one with low vertil resolution. 3.4 Potentil Temperture As numer of vertil lyers dereses, the potentil temperture elow m level inreses t LT 7 (Fig.1 j). The differene in potentil tempertures t m level etween low resolution (2 lyers) nd high resolution ( lyers) is more thn 1.5 K t LT 7 (Fig.1 j). However, the differene etween KMAXs is osure in the viinity of surfe t LT 1 (Fig.1 k). In the dytime, s numer of vertil lyers dereses, the potentil temperture elow m level lso dereses t LT 13 (Fig.1 l). The differene in the profile of potentil temperture is regrded s result of different expressions of the vertil het flux. 3.5 Wind Speed Although the differene of wind speed etween vertil resolutions is osure t LT 7 (Fig.1 m), it is ler in the dytime (Fig.1 n nd o). The differene of wind speed t m level etween low resolution (2 lyers) nd high resolution ( lyers) is more thn 1.5 m/s t LT 13 (Fig.1 o). Wind speed with low resolution is lwys higher thn the one with high resolution ner the surfe (Fig.1 m to o), nd is loser to the geostrophi wind (out 9.5m/s in this study). The feture well orresponds with the hrteristis of resolution dependeny in momentum flux. The dependeny of time steps on wind speed nd potentil temperture is smll ompred with the Rihrdson numer nd vertil fluxes (Fig.2 d nd e). 3.6 Boundry Lyer Height The oundry lyer height (BLH) is defined s the height, where the vlue of momentum flux flls to 95% of its surfe vlue, divided y.95. This definition is the sme s the GABLS experiment. The BLH tends to inrese with KMAX (Fig.3 ), espeilly in the night-time. 3.7 Surfe Wind Speed Wind speed t 1 m ove ground (U1m) is

3 estimted y Monin-Oukhov similrity theory with surfe roughness length of.3m. U1m is strongly influened y the vertil resolution (Fig.3 ). U1m with 2 lyers is out 3% higher thn the one with lyers. The influene of vertil resolution on U1m is onsistent with the result presented in setion 3.5, tht wind speed in the low level of the ABL tends to derese with inresed KMAX (Fig.1 n nd o) 3.8 Surfe Air Temperture Temperture t 2 m ove ground (T2m) is lso estimted y the sme mnner s wind. The influene of vertil resolution on T2m is onsistent with the result of the potentil temperture (3.4). In the night, T2m with low resolution is higher thn one with high resolution, nd, on the other hnds, the reltionship goes into reverse in the dytime (Fig.3 ). In other words, the mplitude of the diurnl yle of T2m with low resolution is smller thn the one with high resolution. 4. DISCCUSION It is not esy to ompre the ABL etween oservtions nd numeril models. Sine oundry lyer grows ove m level in the dytime over lnd, it is diffiult to diretly oserve it up to suh height. The height of tower oservtions does not reh more thn hundreds of meters. Therefore, in this study, we ssume the profile with high resolution to e more orret. We found the following fetures of influene y model resolution. -The differene of flux Rihrdson numer due to resolutions is emphsized when strtifition is unstle (Rf < ) (Fig.1 nd ). -The solute vlues of het flux nd momentum flux tend to inrese with the resolution (Fig.1 d to i). -The differenes of oundry lyer height in resolutions is emphsized in the night (Fig.3 ). -The mplitude of the diurnl yle of surfe temperture with low resolution is smller thn the one with high resolution (Fig.3 ). -The influene of time steps on flux Rihrdson numer nd flux is lrger thn the one on potentil temperture nd wind speed (Fig. 2). In the se where the vertil resolution is low, Rf is lrge (Fig.1 ) or unstle lyer is shllow (Fig.1 ). As onsequene, ABL strtifition represented with low resolution is not unstle ompred to the high resolution. Therefore het nd momentum flux re wek (Fig.1 d to i), nd the diurnl yle of surfe temperture (Fig.3 ) nd wind speed (Fig.1 m to o) lso is smll. The vertil resolution of JMA opertionl GSM is 4 lyers from surfe to.4 hp. JMA opertionl model is perhps influened y the vertil resolution, sine its resolution is muh lower thn the resolution used in this study. The next opertionl model is plned to enhne to 6 lyers from surfe to.1 hp in CONCLUSION We reserhed the influene y the vertil resolution (the numer of the vertil lyers) nd time step length on potentil temperture, wind speed, flux Rihrdson numer, momentum flux, het flux, surfe wind, surfe ir temperture, nd oundry lyer height. All of the elements depended on vertil resolution. This influene ws espeilly ler in lower resolution nd longer time steps. The mgnitude of the influene depends on the element nd lol time. JMA opertionl model is perhps influened y the vertil resolution. If it is not esy to improve the model resolution, we should tune up the representtion of ABL to orrespond with high resolution model. 6. REFERENCE Svensson, G., 5: Introdution seond GABLS 1D Interomprison se nd set up, presenttion in "GLASS nd GABLS workshop on lol lnd-tmosphere oupling".

4 CASES99 Rihrdson Num. (dt=1se LT7) n=2 n=3 n=5 n= n= n= CASES99 Rihrdson Num. (dt=1se LT1) n=2 n=3 n=5 n= n= n= CASES99 Rihrdson Num. (dt=1se LT13) n=2 n=3 n=5 n= n= n= CASES99 Het Flux (dt=1se LT7) CASES99 Het Flux (dt=1se LT1) CASES99 Het Flux (dt=1se LT13) d n=2 n=3 n=5 n= n= n= e n=2 n=3 n=5 n= n= n= f n=2 n=3 n=5 n= n= n= CASES99 Momentum Flux (dt=1se LT7) CASES99 Momentum Flux (dt=1se LT1) CASES99 Momentum Flux (dt=1se LT13) g n=2 n=3 n=5 n= n= n= h n=2 n=3 n=5 n= n= n= i n=2 n=3 n=5 n= n= n= CASES99 Potentil Temp. (dt=1se LT7) CASES99 Potentil Temp. (dt=1se LT1) CASES99 Potentil Temp. (dt=1se LT13) j n=2 n=3 n=5 n= n= n= k n=2 n=3 n=5 n= n= n= l n=2 n=3 n=5 n= n= n= CASES99 Wind Speed (dt=1se LT7) CASES99 Wind Speed (dt=1se LT1) CASES99 Wind Speed (dt=1se LT13) m n=2 n=3 n=5 n= n= n= n n=2 n=3 n=5 n= n= n= o n=2 n=3 n=5 n= n= n= Figure 1. Expression of ABL y vrious vertil resolutions. The red, green, lue, purple, sky lue, nd gry lines indite the expression y the model whose numers of vertil lyers re 2, 3, 5,,, nd, respetively. Flux Rihrdson numer, het flux, momentum flux, potentil temperture, nd wind speed re given from top to ottom pnels, respetively. Left, enter, nd right olumns show snpshots t lol time 7, 1, nd 13, respetively.

5 CASES99 Rihrdson Num. (n= LT13) CASES99 Momentum Flux (n= LT13) CASES99 Het Flux (n= LT13) dt=1 dt=2 dt=6 dt=18 dt=1 dt=2 dt=6 dt=18 dt=1 dt=2 dt=6 dt= d CASES99 Wind Speed (n= LT13) dt=1 dt=2 dt=6 dt=18 e CASES99 Potentil Temp. (n= LT13) dt=1 dt=2 dt=6 dt= Figure 2. Expression of ABL y vrious time step length. These pnels show vertil profiles of () flux Rihrdson numer, () momentum flux, () het flux, (d) wind speed, nd (e) potentil temperture t lol time 13. Red, green, lue, nd purple lines indite the time steps of 1se, 2se, 6se, nd 18se, respetively. CASES99 Boundry Lyer Height (delt=1se) CASES99 1m Wind Speed (delt=1se) CASES99 2m Temprure (delt=1se) BLH [m] n=2 n=3 n=5 n= n= n= U1m [m/s] n=2 n=3 n=5 n= n= n= T2m [K] n=2 n=3 n=5 n= n= n= Figure 3. Time series of ABL y vrious vertil resolutions. These pnels show time series of () Boundry lyer height, () wind speed t 1 m level, nd () ir temperture t 2 m level. The eh olor of line shows the sme s figure 1.