Characteristics and formation of seas of clouds around Mt. Lu based on FY-4A satellite observations
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摘要: 利用FY-4A卫星等资料分析了2019—2021年的19次庐山白天云海过程(12个传统云海和7个瀑布云过程),研究了庐山云海特征及其形成机制,评估了卫星资料在云海识别中的应用。研究表明:FY-4A可见光云图可基本辨识庐山云海范围及宏观演变特征,但较难刻画出小尺度瀑布云的精细结构;FY-4A的云顶高度L2产品可用于庐山传统云海的识别,但较难识别瀑布云过程。非洋面海岛的庐山也存在云系尾流现象且频率较高(共3次过程),由绕流作用形成的尾流云系呈逗号状分布,做规律性摆动但无连续涡旋;该尾流型云海形成的主要因素是庐山为相对周边孤立的椭圆形山体、冷高压底部的强北风低空急流、山腰逆温层。庐山云海发生时大多受地面高压控制且位于850 hPa的高湿区或边缘区域,该区域的弱下沉运动形成的逆温层和低空充沛的水汽有利于庐山云海形成及维持。Abstract: The characteristics, recognition and formation of seas of clouds (SOC) around Mt. Lu are studied based on FY-4A observations and other data. From 2019 to 2021, 19 events of SOCs are recorded in the dataset, including 12 traditional SOCs (TSOC) and 7 small-scale SOCs with cloud waterfalls over Mt. Lu (SSOC). The FY-4A visible-channel cloud image can identify the area and development of TSOC but cannot capture the detailed structure of SSOC. The L2 cloud-top-height product of FY-4A can be used for the recognition of TSOC around Mt. Lu but cannot be used for the recognition except of SSOC. Normally the wake phenomena of SOC (WSOC) occur on islands over the sea, but three WSOC events are found in Mt. Lu, although it is located in the land. The WSOC in Mt. Lu exhibits a comma-type cloud pattern with regular fluctuations but without continuous vortexes. The formation of the WSOC in Mt. Lu is mainly caused by the isolated ellipse shape of Mt. Lu, the northernly low-level jet, and the temperature inversion layer below the mountain top. Besides, for most SOC events, Mt. Lu is under the control of high-pressure systems and located in high-humidity region, where the synoptic-scale subsidence inversion layer and sufficient moisture could play an important role in the forming and maintenance of SOCs over Mt. Lu.
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Key words:
- Mt. Lu /
- Sea of cloud /
- FY-4A /
- Visible channel /
- Wake phenomena
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图 1 庐山周边地形 (a) 及云海适宜观测点位置 (b) (b底图为FY-4A的0.65 μm可见光晴天灰度云图,500 m分辨率、400×400像素、约240 km×240 km范围、2021年11月11日12时30分 (北京时),下同)
Figure 1. Topography of Mt. Lu (a) and locations of observation sites (b) (the background of b is the FY-4A 500 m resolution cloud image of the 0.65 μm visible channel in a clear sky at 12:30 BT 11 November 2021,which covers an area of 240 km×240 km)
图 2 两个庐山传统云海过程的FY-4A可见光云图 (a. 2020年3月1日08时30分,b. 2020年3月1日08时53分,c. 2021年1月24日07时38分,d. 2021年1月24日08时53分;实线表示庐山山体轮廓,虚线表示因庐山山体阻挡形成的波状云带尾流轮廓,白色箭头表示云海移动方向)
Figure 2. FY-4A visible-channel cloud images of traditional seas of cloud (TSOCs) around Mt. Lu at (a. 08:30 BT 1 March 2020, b. 08:53 BT 1 March 2020,c. 07:38 BT 24 January 2021,d. 08:53 BT 24 January 2021;the solid,dashed line and arrow indicating the region of Mt. Lu,the wake zone of SOC and the movement of SOC,respectively)
图 4 庐山云海过程中FY-4A的云顶高度 (a. 2021年2月12日08时,b. 2021年4月23日08时) 和2021年4月23日08时雾区分布 (c) (紫红色圆点为庐山气象站位置)
Figure 4. FY-4A cloud top height at (a) 08:00 BT 12 February 2021,(b) 08:00 BT 23 April 2021 and fog-area detection at (c) 08:00 BT 23 April 2021 during SOCs in Mt. Lu (the purple point indicates the Mt. Lu meteorological station)
图 5 庐山传统云海 (a. 尾流型,b. 山前堆积型)、瀑布云 (c) 的ERA5海平面气压 (蓝色实线) 和850 hPa风场 (风羽) 及相对湿度 (色阶) (a. 2020年3月1日08时,b. 2021年1月24日08时,c. 2020年11月30日08时;红色圆圈为庐山气象站位置)
Figure 5. Sea level pressure (blue contours) and 850 hPa wind (barbs) and relative humidity (shaded) from ERA-5 for TSOCs (a,b) and SSOCs (c) over Mt. Lu at (a) 08:00 BT 1 March 2020,(b) 08:00 BT 24 January 2021 and (c) 08:00 BT 30 November 2020 (the red cycle marks the Mt. Lu meteorological station)
图 7 同图6,但为庐山其他传统云海和瀑布云 (分别在图例中以字母Y、P开头)
Figure 7. Same as Fig. 6 but for other TSOCs and SSOCs in Mt. Lu with a capital letter of Y and P in the legend,respectively
表 1 庐山云海特征 (2019—2021年)
Table 1. Characterists of seas of clouds (SOCs) around Mt. Lu (2019—2021)
序号 日期 时间 现象 地点 云海移向 T2m
(℃)VIS
(km)RH
(%)WS
(m/s)WD
(o)可见光云图
特征CTH 1 2019-02-28 08时 传统云海 牯岭 西—东 0.6 0.3 100 0.9 70 西部山体无云 1* 2 2019-04-10 17时 瀑布云 牯岭 西—东 5.6 8 100 1.8 360 可辨识瀑布云 0 3 2019-06-11 08时 瀑布云 仰天坪 北—南 20.0 16 36 8.0 70 可辨识瀑布云、薄云 0 4 2019-06-23 08时 瀑布云 牯岭 北—南 15.6 0.2 100 5.8 40 — 0 5 2019-07-23 09时 传统云海 五老峰 南—北 23.9 18 79 1.8 250 似冷流云,长条细胞状 1 6 2019-09-04 08时 传统云海 王家坡 东北—西南 18.9 17 81 3.1 20 西部晴空多 1 7 2019-12-18 07时 瀑布云 牯岭 西—东 0.6 0.2 96 8.0 80 — 1 8 2019-12-19 15时 传统云海 牯岭 北—南 −0.6 0.2 100 1.8 30 有尾流,北部山体无云 1 9 2019-12-31 07时 传统云海 牯岭 西—东 −5.6 4 96 9.8 90 中部山体可见 0 10 2020-03-01 07时 传统云海 三地 北—南 3.3 0.3 100 5.8 10 有尾流,北部山体无云 0 11 2020-05-22 06时 瀑布云 牯岭 东北—西南 15.6 0.2 97 4.9 30 可辨识瀑布云、多
层云1 12 2020-07-21 06时 瀑布云 牯岭 南—北 21.7 30 84 1.8 190 可辨识瀑布云、多
层云0 13 2020-11-30 07时 瀑布云 牯岭 东—西 1.7 30 70 4.9 80 可辨识瀑布云、晴
空多0 14 2020-12-10 08时 传统云海 牯岭 西南—东北 3.3 5 100 0.9 210 西部山体无云 1* 15 2021-01-24 09时 传统云海 三地 东南—西北 5.0 30 61 1.8 160 东部山前有Ω带状云 1* 16 2021-01-28 07时 传统云海 牯岭 南—北 −1.1 7 100 3.1 10 仅西边云海且移动少 1 17 2021-02-12 08时 传统云海 牯岭 西南—东北 — — — — — 西部山体无云 1* 18 2021-03-02 08时 传统云海 牯岭 东北—西南 −1.1 3 100 8.9 40 有尾流,北部山体无云 1 19 2021-04-23 08时 传统云海 牯岭 东北—西南 15.6 28 87 0.9 70 北部山体无云 1 注:此处“牯岭”包括大月山、小天池、剪刀峡、日照峰等;“三地”为牯岭、五老峰、仰天坪。T2m、VIS、RH、WS、WD分别为庐山气象站相应或相邻时刻的3 h地面气象观测要素(地面2 m气温、能见度、相对湿度、10 m风速、10 m风向)。“CTH”表示根据FY-4A云顶高度信息(4 km分辨率)能否判断有云海,“1*”表示可辨识云海,山体可见,但云顶较高约为2—4.5 km;“1”与“1*”类似,但云顶高度为0.3—2.1 km;“0”表示根据云顶高度较难辨识云海。 -
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