1. Climatology of coastal low level jets over the Bohai Sea and Yellow Sea and the relationship with regional atmospheric circulations
Delei Li1, Hans von Storch2, Meng Zhang2
1Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China; 2Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Germany. Contact: Paper Number: A43I-2582
1. Introduction
Coastal low level jet (CLLJ) is low-troposphere mesoscale-flow phenomenon, exhibiting wind maxima at low levels, typically below 1 km.
CLLJ affects the formation of convective fog, clouds and heavy rainfall events. It is also significant for human activities, such as aviation safety, offshore wind energy applications and fishery resources.
This study aims to present a climatology of the temporal and spatial features of CLLJ over the Bohai Sea and Yellow Sea (BYS) and its variability in different temporal scales. The relationship of CLLJ variability with regional large-scale forcing in low-frequency has also been studied as well.
2. Data and methodology
3. Evaluation of the model dataset
Link to the regional large-scale forcings Identification criteria of CLLJ
Fig. 3 (a) Observed and (b) modeled daily height–time cross-section of mean wind speeds during 2009; (c) observed and (d) modeled CLLJ cases during 0000 LST on 9 May 2009 to 0000 LST on 11 May 2009 at station SH
COSMO-CLM 4.14 (CCLM, Rockel et al. 2008):
Spectral nudging technique (von Storch et al. 2000);
Initial and boundary conditions: ERA-Interim (0.75°, ERA-I);
resolution 7 km; Time period: ;
Domain: Bohai and Yellow Sea (Fig.1)
Identification of CLLJ (Du et al. 2012)
1) Wmax of the lowest 3000 m larger than 10 m/s;,
2) Wind speed difference is larger than 5 m/s from Wmax to Wmin above or wind speed at app. 3 km.
3) Once a CLLJ was identified, the jet location, jet height, jet speed and direction were recorded.
Model data proves to be robust in reproducing wind profile climatology, diurnal cycle and variability of wind speeds as well as CLLJ cases
Fig.1 Model domain and wind profile observations
Fig.2 Climatological mean of observed and modeled wind profiles
4. Climatology and variability of CLLJ
Strong intra-annual variability of CLLJ occurrence: low occurrence frequency in winter (< 12%) and very high occurrence frequency in April, May and June (> 21%);
Larger values in the western part of the BYS and lower values in the eastern part of the BYS from March to August.
CLLJ height generally less than 500 m with wind intensities mostly ranging from 10 to 18 m/s;
Prevailing wind directions of CLLJ are southwesterly (app. 22.5%, app. 10% west-southwesterly), and then followed by southerly winds.
Annual mean frequency of CLLJ occurrence: 8% – 20%, more frequent over the Bohai Sea (BS) and western coasts of Yellow Sea (YS) ;
Stronger CLLJ over the BS and the north YS;
CLLJ mean height are are generally around 500 m
Fig. 6 CLLJ statistics during April, May and June: (a) jet height (%), (b) jet wind speed distribution (%), distribution (c) jet height-wind speed distribution, (d) jet wind rose
Fig. 5 Spatial distributions of monthly occurrence frequency (%) of CLLJ
Fig. 4 (a) annual mean frequency of occurrence (%) of CLLJ, (b) CLLJ mean wind speed (m/s), and (c) CLLJ mean occurrence height (m)
CCLM proves to be robust in reproducing the climatology, daily-cycle feature, variability of wind profiles and specific CLLJ cases;
CLLJ over the BYS are featured with strong inter-annual, intra-annual and diurnal cycle variability;
CLLJ is more frequent in April, May and June, and less frequent in winter. General features: CLLJ jet height < 500 m; Wmax : 10 – 16 m/s, wind directions are southerly and southwesterly;
Pressure gradients over the East Asia-Northwest Pacific region are significantly correlated with the variations of CLLJ occurrence frequency over the BYS in terms of the annual cycle as well as interannual variability.
5. Link to the regional large-scale forcings
6. Summary
Fig. 8 First two Canonical correlation patterns of MSLP (a and c, unit Pa) and CLLJ (c and d, Unit: %)
CCA1: pressure contrast of about 1.5 hPa between northwestern Pacific Ocean and northeast Asia is related with 0.6% to 2.4% CLLJ occurrence anomalies
CCA2: pressure contract over East Asia and one over East to Japan as well as Sea of Okhotsk will influence CLLJ occurrence
CCA1 dominates in 1998 with high frequency of CLLJ and in 1979, 1984, 1992 and 1996 with less CLLJ;
CCA2 dominates the year 1991, 1998 and 2003 with less CLLJ occurrence frequency and the year 2005 and 2006 with higher occurrence frequency.
Fig. 7 Diurnal cycle of CLLJ occurrence frequency for (a) jet height, (b) jet wind speed, and (c) jet wind direction
Strong diurnal variability, with more frequent occurrence of CLLJ in the night instead of daytime;
Prevailing CLLJ have wind speed maximum between 200 and 400 m, with strength of 10 – 16 m/s, and with dominant southerly and south-southwesterly jet flows
References
Rockel et al., 2008, Meteorol. Z.; von Storch et al., 2000, Mon. Weather Rev.; Du et al., 2012, J Meteorol Soc Jpn.