1. Climatology of coastal low level jets (CLLJs) over the Bohai Sea and Yellow Sea using local and spatial-pattern based techniques
Hans von Storch1, Delei Li2 (李德磊)
1Institute of Coastal Research, Helmholtz-Zentrum Geesthacht, Germany;
2Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China.
AMS annual conference, 9 January 2018, Austin

2. A wind speed maximum in the lowest kilometers of troposphere (Stensrud 1996)
observed in many places of the word
highest occurrence during summer in both hemispheres
closely related to heavy rainfall
Significance
Strong wind: aviation safety, offshore wind energy applications
transport of moisture and modify turbulence structure: heavy rainfall, convective fog and clouds
mpact air-sea interaction processes: upwelling, aridity and dryness of coastal regions
(cg. Nicholson, 2010)
The broadest definition of a low-level jet (LLJ) is simply any lower-tropospheric maximum in the vertical profile
of the horizontal winds.
A LLJ can occur under favorable synoptic conditions anywhere in the world.
Of practical interest is their impact on the transport of moisture. Of theoretical interest is the large amount of
vertical wind shear associated with them and the observation that they are typically supergeostrophic by a large
(>50%) amount.
A large number of specific geographic locations all over the world have been identified as especially favorable for
LLJ development. Among these locations is the Great Plains region of the United States, in which one of the most
significant LLJs. Previous literature show Low level jet over the BYS, however, mostly based on case study.
Walters et al. 2008
1. Introduction

3. High occurrence frequency over the BYS
Motivation:
High occurrence frequency over the BYS
Previous studies were generally based on station observations (Sai and Miao, 2012; Wei et al. 2014) or short-term simulation (Du et al. 2014) ,
So far no detailed study of CLLJ properties and annual and decadal variability over the BYS
1. Introduction

4. Domain: Bohai and Yellow Sea
COSMO-CLM 4.14
(CCLM, Rockel et al. 2008):
Domain: Bohai and Yellow Sea
Spectral nudging technique (von Storch et al. 2000);
Forcing : ERA-Interim (0.75o, ERA-I);
Grid resolution 7 km;
Time period:
Climatological mean of observed (grey) and modeled (black) wind speed (dot) and direction (square) at four wind profile stations
2. Model, data and evaluation

5. Traditional Local definition of a CLLJ (Du et al. 2012)
height
3 km
Wind speed
Color intensity represents W_max – min(W_min, W_3km)
Wmin
Wmax
Traditional Local definition of a CLLJ (Du et al. 2012)
1) In the lowest 3000 m: Wmax ≥ 10 m/s;
2) Wmax – min(Wmin, W3km) ≥ 5 m/s;
3) Once a CLLJ was identified, the jet location, jet height, jet speed and direction were recorded
3. Identifying LLJs

6. A B Towards a spatial definition of CLLJs: example of two CLLJs (a)
Colored region: detected low level jets in 1st April 2006
Dots: Wmax ≥ 10 m/s;
Color: Wmax ≥ 10 m/s and Wmax – min(Wmin, W3km) ≥ 5 m/s
and the number N of adjacent and connected points that fit the above conditions
N=49;
N=81;
N=121;
N=169.
With the strengthening of detection criterion, the cover area of CLLJ decreases.
Two regions of LLJ can be detected by all criteria: A and B.
(c)
(d)
Color intensity represents W_max – min(W_min, W_3km)
3. Identifying LLJs

7. Towards a spatial definition of CLLJs: example of two CLLJs
A: starting from weak and small LLJ, then growing and becoming strong LLJ, then decay and disappear
B: a well developed LLJ at the beginning, then disintegrates into LLJ pieces, decaying and disappearing
Based on such maps, an algorithm for detecting centers of CLLJs, tracking their propagation and disintegration will be developed.
Cross section1: (lat: N) (lon: E)
3. Identifying LLJs

8. Stronger CLLJ over the BS and the north YS;
(a) annual mean frequency of occurrence (%) of CLLJ, (b) CLLJ mean wind speed (m/s), and (c) CLLJ mean occurrence height (m)
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 generally around 500 m.
3. Results

9. Spatial distributions of monthly occurrence frequency (%) of CLLJ
CLLJ Season
Annual cycle
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.
3. Results using local definition

10. CLLJ Season CLLJ statistics during CLLJ season jet height (%),
Spatial distributions of monthly occurrence frequency (%) of CLLJ
CLLJ Season
CLLJ statistics during CLLJ season
jet height (%),
jet wind speed distribution
jet height-wind speed distribution,
jet wind rose.
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.
3. Results using local definition

11. Diurnal variations
Diurnal variation of occurrence frequency (%) of CLLJ at a particular hour (UTC (Local Solar Time))
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
Diurnal cycle of CLLJ occurrence frequency for (a) jet height, (b) jet wind speed, and (c) jet wind direction
3. Results using local definition

12. CLLJ occurrence frequency (upper panel), mean wind speed (middle panel) and mean height (lower panel) in CLLJ-seasons of 1980s, 1990s and 2000s
Decadal differences
3. Results using local definition

13. Summary and outlooks
Summary
 Assessment of simulated CLLJ by comparing with observation
Model is robust in reproducing the climatology, diurnal variation, variability of wind profiles and specific coastal low level jet cases.
 Climatology and variability of CLLJ over the Bohai and Yellow Sea (BYS)
CLLJ over the BYS exhibits strong intra-annual and diurnal cycle variability, but weak decadal variability;
CLLJ is more frequent in April, May and June; it is generally lower than 500 m with 10 – 16 m/s intensity, and prevailing wind directions are southerly and southwesterly;
It is of nocturnal type, with highest occurrence frequency around 2300 at Local Time.
Outlook
CLLJ detection based on spatial-pattern techniques;
CLLJ influence on offshore wind farm application and local rainfall;
The contributions of remote and local forcing on CLLJ generation and variability