A retrospective analysis of 17 years of NCMRWF Global Forecast System (NGFS) data was conducted to understand the state and variability of cold wave episodes over northwest India. During the 2000–2016 period, a total of 21 cold wave episodes (202 cold nights) were detected, out of which 5 severe cold episodes (63 cold nights) were registered. The 10 (6) episodes occurred during La Niña (El Niño) years suggesting that both phases of El Niño-Southern Oscillation provide a favourable background for the occurrence of cold waves. The average duration of a cold wave episode was ~ 9.6 days, with the longest (shortest) episode, seen in the year 2008 (2006), lasting for 26 (6) consecutive days. The average duration of a severe cold wave episode is ≈ 4 days longer than that of a normal cold wave. In the year 2005, both the earliest (11 December) and latest (16 February) onsets of cold waves were seen. The omnipresence of intense Siberian anticyclone and the presence of western disturbance brought cold winds to the study region. Also, temperature advection and geo-potential height anomalies play vital roles in the maintenance of cold waves. The cold waves exhibit a significant intra-annual variability over northwest India. The intensity of cold waves has shown an increase of 0.11 °C per cold episode.

中文翻译：

---

使用 NGFS 回顾性分析研究印度西北部冷浪事件的变化

对 17 年的 NCMRWF 全球预报系统 (NGFS) 数据进行了回顾性分析，以了解印度西北部寒潮事件的状态和变化。2000-2016 年共检测到 21 次寒潮事件（202 个冷夜），其中记录了 5 次严重寒冷事件（63 个冷夜）。10 (6) 次事件发生在拉尼娜（厄尔尼诺）年期间，表明厄尔尼诺 - 南方涛动的两个阶段为寒潮的发生提供了有利的背景。寒潮发作的平均持续时间约为 9.6 天，最长（最短）发作出现在 2008（2006）年，连续持续 26（6）天。严重寒潮发作的平均持续时间比正常寒潮的平均持续时间长 ≈ 4 天。在 2005 年，最早（12 月 11 日）和最晚（2 月 16 日）的寒潮都出现了。强烈的西伯利亚反气旋的无处不在和西部扰动的存在给研究区域带来了冷风。此外，温度平流和地势高度异常在寒潮的维持中起着至关重要的作用。寒潮在印度西北部表现出显着的年内变化。寒潮强度显示，每次寒冷发作都会增加 0.11 °C。寒潮在印度西北部表现出显着的年内变化。寒潮强度显示，每次寒冷发作都会增加 0.11 °C。寒潮在印度西北部表现出显着的年内变化。寒潮强度显示，每次寒冷发作都会增加 0.11 °C。