|202223W (NESAT 納沙) - Profile 檔案|
(First draft issued on November 13, 2022)
(初稿於 2022 年 11 月 13 日發出)
Brief profile of NESAT | 納沙小檔案:
|JTWC number 聯合颱風警報中心 (JTWC) 號碼||23W|
|International number 國際編號||2220|
|Period of existence 生存時期^||2022/10/15 02 HKT - 2022/10/20 14 HKT|
|Estimated maximum 1-minute wind 估計一分鐘平均最高中心風速&||
90 knots 節 (Category 2 Typhoon 二級颱風)
|Highest TC signal by HKO
|3 (Strong Wind Signal 強風信號)|
|Closest point of approach by HKO
|SSE 370 km (real-time warning 實時報告)|
|Time of closest approach by HKO
|2022/10/17 20 HKT (real-time warning 實時報告)|
|Lowest pressure recorded at HKO
^Refers to the period between the HK Observatory's first and last bulletins for the tropical cyclone (with at least tropical depression intensity). For storms formed/dissipated/transformed into an extratropical cyclone outside the area bounded by 7-36°N and 100-140°E, information from the JTWC will be used instead.
^即香港天文台為該熱帶氣旋 (熱帶低氣壓級或更高) 作出發佈的時期。至於在北緯 7 至 36 度、東經 100 至 140 度外形成／消散／轉化為溫帶氣旋的風暴，本網將參考聯合颱風警報中心的資料。
&Based on HKWW track information.
TC signals in Hong Kong* 香港曾經生效的熱帶氣旋警告*:
|Date and time
|Distance from HK
|Max. 10-min winds
|2022/10/16 (SUN) 21:20 HKT||ESE 530 km||W/WSW at 15 km/h||130 km/h (70 knots, T)|
|2022/10/17 (MON) 11:30 HKT||SE 390 km||WSW at 15 km/h||140 km/h (75 knots, T)|
|2022/10/18 (TUE) 15:40 HKT**||SSW 510 km||WSW at 15 km/h||145 km/h (80 knots, T)|
*Based on the information from real-time tropical cyclone warning bulletins.
**The Strong Monsoon Signal was issued at the same time the #3 Strong Wind Signal was cancelled.
IR imagery animation 紅外線雲圖動畫:
TC track from HKWW 本站的熱帶氣旋路徑圖:
TC track from HKO 天文台的熱帶氣旋路徑圖:
|Past HKWW Bulletins 本站發佈|
|Storm Development Overview 風暴發展概覽|
Unless otherwise stated, all times below are in Hong Kong Standard Time (UTC+8)
除非特別註明，下文時間均為香港標準時間 (協調世界時加 8 小時)
NESAT developed as tropical disturbance 97W in the northwest Pacific in the middle of October. The disturbance picked up strength on October 14, prompting the JTWC to issue a tropical cyclone formation alert in the afternoon. A gale warning was issued by the JMA later that night. The JTWC upgraded it into tropical depression 23W at 2 am October 15. Under the influence of the subtropical ridge to its north, 23W was forecast to move west to west-northwest across the Luzon Strait and into the South China Sea. There it was expected to meet a rather strong surge of northeast monsoon. As the steering mechanism switched to the continental ridge in southwestern China, 23W was expected to turn west-southwest.
The JMA upgraded 23W into a tropical storm at 2 pm October 15, naming it NESAT. While NESAT's convections were initially loose and displaced to the western side of the storm, it consolidated rather quickly in the rest of the day. By the morning of October 16, NESAT had intensified into a severe tropical storm. It further strengthened into a typhoon that afternoon as a banding eye developed.
NESAT reached the northest point along its track in the small hours of October 17. Due to the arrival of the northeast monsoon and the associated dry air mass, convections in NESAT's northern semicircle were visibly thinner than in the southern semicircle. NESAT turned west-southwest on October 17 and continued to intensify, with a maximum estimated strength of 90 knots (167 km/h) assessed by the JTWC. The continued entrainment of dry air and cooler sea temperatures in the western South China Sea eventually caused NESAT to weaken on October 18. The storm's deep convections gradually dissipated as it moved to the south of Hainan Island, leaving behind an exposed low-level circulation centre.
NESAT turned west to west-northwest on October 19 and continued to weaken. It reduced into an area of low pressure on October 20, before it was able to reach coastal Vietnam.
納沙的前身為熱帶擾動 97W，該擾動於 10 月中在西北太平洋形成。隨著 97W 於 14 日有所增強，聯合颱風警報中心於當天下午發出熱帶氣旋形成警報，而日本氣象廳當晚發出烈風警告。聯合颱風警報中心於 15 日上午 2 時將 97W 升格為熱帶低氣壓 23W。受北方的副熱帶高壓脊帶動，當時預料 23W 將向西至西北偏西移動，橫過呂宋海峽並進入南海。此後 23W 將與一股較強的東北季候風相遇，而 23W 將受位於中國西南部的大陸高壓脊影響，屆時會轉向西南偏西移動。
日本氣象廳於 15 日下午 2 時將 23W 升格為熱帶風暴，命名為納沙。雖然初時納沙的對流較為鬆散且偏離至風暴西面，但它於 15 日餘下時間快速整合。至 16 日早上，納沙已增強為強烈熱帶風暴，下午更增強為颱風，並建立一雲捲風眼。
納沙於 17 日凌晨到達其路徑的最北端位置。受東北季候風及其相關乾燥氣團的影響，納沙北半圓的對流明顯較南半圓的稀薄。納沙當天開始向西南偏西移動，並繼續增強，聯合颱風警報中心估計其最高強度為 90 節 (每小時 167 公里)。由於納沙一直捲入較為乾燥的空氣，加上南海西部海溫較低，風暴於 18 日減弱。納沙的深層對流隨風暴移至海南島以南海域而逐漸消散，剩下一個完全外露的低層環流中心。
納沙於 19 日向西至西北偏西移動，並繼續減弱。20 日，風暴於到達越南沿岸之前減弱為低壓區。
|Weather in Hong Kong During Approach 風暴影響時香港天氣狀況|
As NESAT was predicted to enter the South China Sea at about the same time as the arrival of a strong surge of northeast monsoon, strong force northerly winds were in the forecast. On October 15, the HK Observatory suggested that NESAT would maintain a distance of over 400 km from Hong Kong, thus not posing a severe direct threat to the territory. It even mentioned that the Strong Monsoon Signal would be issued depending on the situation. Northerly winds strengthened early on October 16 and the Strong Monsoon Signal was indeed issued at 8:20 am, but the signal remained in force for just over 4 hours. Soon afterwards, the Observatory mentioned that NESAT was expected to be closer to Hong Kong and stronger than previously forecast, and that the #1 Signal would be considered that night.
The #1 Standby Signal was issued at 9:20 pm October 16, when NESAT was about 530 km east-southeast of Hong Kong. Local winds generally remained stable that night. However, because elevated locations were relatively open towards the north, winds there were much stronger. Tate's Cairn registered gale force sustained winds near 11 pm and locations such as Cheung Chau, Green Island, Peng Chau, Sha Chau and Wagland Island registered strong force winds that night.
At 10 am October 17, the Observatory indicated that the #3 Strong Wind Signal would be issued at or before 11:30 am, after previously specifying 10 am to noon as the time range for the signal upgrade. The signal was indeed issued at 11:30 am, when NESAT was at a distance of about 390 km to the southeast. NESAT's outer rainbands reached Hong Kong that afternoon, leading to a drop in the temperature to around 26 degrees. Local winds strengthened somewhat in the late afternoon to the evening, with winds reaching storm force at Tate's Cairn and gale force at Green Island, Ngong Ping and Waglan Island that night.
As NESAT's distance to Hong Kong increased, the Observatory mentioned at 5 am October 18 that the chance of issuing the #8 Signal was low. The temperature at the Observatory dropped to about 22 degrees that morning. Rain became more persistent in the afternoon and the temperature dipped further to about 18 degrees. It was apparent that the northeast monsoon took over and became the dominant influence of local winds; the #3 Strong Wind Signal was replaced by the Strong Monsoon Signal at 3:40 pm.
A minimum temperature of 17.3 degrees was recorded at the Observatory in the night of October 18, the lowest temperature for mid-October since 1967. The rain gradually diminished over the night of October 18-19, and there were sunny periods on October 19. The Strong Monsoon Signal was cancelled at 8:40 am, but was re-issued that night as easterly winds strengthed again.
由於預料納沙進入南海之時剛好遇上較強的東北季候風補充，天氣報告預計本港會吹強北風。10 月 15 日，香港天文台表示納沙將與香港保持超過 400 公里，對香港的直接威脅不大，將按情況發出強烈季候風信號。隨著北風於 16 日早段增強，天文台的確於上午 8 時 20 分發出強烈季候風信號，但只維持稍多於 4 小時。不久後，天文台改稱納沙較預期接近香港，強度亦較高，將於當晚考慮發出一號信號。
一號戒備信號於 16 日下午 9 時 20 分發出，當時納沙位於香港東南偏東約 530 公里。當晚本地風勢大致平穩，但由於高地北方屏障較少，該處風勢明顯較大。大老山於下午約 11 時錄得烈風程度的持續風速，而長洲、青洲、沙洲及橫瀾島等地當晚吹強風。
繼先前表示三號強風信號會於 17 日上午 10 時至正午發出後，天文台於當早 10 時更新發出時間為 11 時 30 分或以前。該信號最終於 11 時 30 分發出，當時納沙位於香港東南約 390 公里。納沙的外圍雨帶當天下午抵港，氣溫下降至約 26 度。本地風勢下午後段至傍晚稍為增強，晚間大老山曾吹暴風，而青洲、昂坪及橫瀾島吹烈風。
隨著納沙與香港的距離有所增加，天文台於 18 日上午 5 時表示改發八號信號的機會較低。天文台早上氣溫跌至 22 度左右，而下午降雨轉趨持續，氣溫進一步下跌至約 18 度。至此，季候風對本港的影響明顯大於納沙的影響，天文台因而於下午 3 時 40 分以強烈季候風信號取代三號強風信號。
18 日晚間天文台錄得最低氣溫 17.3 度，是自 1967 年以來 10 月中旬的最低溫度。18 至 19 日晚間及凌晨雨勢轉弱，19 日日間部分時間有陽光。強烈季候風信號於當天早上 8 時 40 分取消，但當晚東風再度增強，該信號亦再次發出。
|Image Gallery 圖片庫|
Figure 1 - Satellite image of NESAT in its formative stage
圖 1 - 納沙形成初期之衛星雲圖
Figure 3 - Forecast tracks from various official agencies on October 15 (top) and 17 (bottom)
圖 3 - 10 月 15 日 (上) 及 17 日 (下) 各官方氣象台之預測路徑
Figure 4 - Steering flow chart at 8 am October 16
圖 4 - 10 月 16 日上午 8 時之駛流場分佈圖
Figure 5 - Satellite image of NESAT crossing the South China Sea; its eye is clearly discernible
圖 5 - 納沙橫過南海時之衛星雲圖，可清晰見到其風眼
Figure 6 - 24-hour sea level pressure change in southern China at 8 pm October 17; a surge of northeast monsoon was arriving
圖 6 - 10 月 17 日下午 8 時之華南 24 小時海平面氣壓變化圖，當時一股東北季候風正在抵達
Figure 7 - Wind field analysis at 9:35 am October 17 (top) and 9:17 pm October 18 (bottom)
圖 7 - 10 月 17 日上午 9 時 35 分 (上) 及 18 日下午 9 時 17 分 (下) 之風場掃描分析圖
Figure 8 - Radar image at 3 pm October 18
圖 8 - 10 月 18 日下午 3 時之雷達圖
Figure 9 - Air temperature and relative humidity time series chart for the HK Observatory on October 18
圖 9 - 10 月 18 日香港天文台之氣溫及相對濕度時間序列圖
Figure 10 - Local weather element distribution maps at 6-hour intervals from 6:30 pm October 17 to midnight October 19
圖 10 - 10 月 17 日下午 6 時 30 分至 19 日午夜每 6 小時之本港天氣元素分布圖
Figure 11 - Wind speed time series at Cheung Chau, Green Island, Ngong Ping and Tate's Cairn for October 17 and 18
圖 11 - 長洲、青洲、昂坪及大老山 (上至下) 於 10 月 17 及 18 日之風速時間序列圖
Figure 12 - Rainfall distribution map for October 18
圖 12 - 10 月 18 日之雨量分佈圖
|Winds Recorded at Reference Stations 參考站所錄得的風速|
The following table lists the maximum 10-minute average (sustained) wind speed recorded while tropical cyclone signals were in force. In some occasions, it is possible that the strongest winds were not recorded when the highest signal was in force.
下表列出各站於熱帶氣旋警告信號生效時所錄得的最高 10 分鐘平均 (持續) 風速，留意此風速不一定於最高信號生效期間錄得。
|Max. 10-min avg. wind speed
|Chek Lap Kok 赤鱲角||40 km/h||Fresh 清勁|
|Cheung Chau 長洲||60 km/h||STRONG 強風|
|Kai Tak 啟德||40 km/h||Fresh 清勁|
|Lau Fau Shan 流浮山||53 km/h||STRONG 強風|
|Sai Kung 西貢||48 km/h||STRONG 強風|
|Sha Tin 沙田||Below "fresh" threshold of 31 km/h 低於「清勁」下限的 31 km/h|
|Ta Kwu Ling 打鼓嶺||35 km/h||Fresh 清勁|
|Tsing Yi (SHL) 青衣蜆殼油庫||Below "fresh" threshold of 31 km/h 低於「清勁」下限的 31 km/h|
|Number of stations recording at least STRONG force (force 6 - 7) winds:||3|
|Number of stations recording at least GALE force (force 8 - 9) winds:||0|
|Number of stations recording at least STORM force (force 10 - 11) winds:||0|
|Number of stations recording HURRICANE force (force 12) winds:||0|
|錄得 強風 (6 - 7 級) 或更高風速的站數:||3|
|錄得 烈風 (8 - 9 級) 或更高風速的站數:||0|
|錄得 暴風 (10 - 11 級) 或更高風速的站數:||0|
|錄得 颶風 (12 級) 風速的站數:||0|
|Charts and Figures 各項數據|
Table 1: Track data from HKWW:
表一 : 香港天氣觀測站之路徑資料
YYMMDDZZ Lat Long Wind
Table 2: Maximum gust and hourly average wind speed recorded in Hong Kong from HKO:
表二 : 香港天文台公佈的各站最高每小時平均風力和陣風數據 (英文)
Table 3: Rainfall (in millimetres) contributed by NESAT from HKO (figures in brackets are based on incomplete hourly data):
表三 : 香港天文台公佈的各站雨量資料 (英文) (方括號內之數據由不完整之每小時資料擷取)
Last Accessed 最近訪問日期: Fri Dec 09 2022 00:08:30 HKT
Last Modified 最近修訂日期: Sun Nov 13 2022