Despite consistent and heavy downpour spread almost evenly across Kerala for nearly a fortnight, the India Meteorological Department (IMD) has still not found the conditions ideal to officially declare the onset of this year's southwest monsoon.
In fact, the rainfall pattern across Kerala during May has already satisfied perhaps the most important condition for the official declaration of the onset of the monsoon: 60% of the 14 IMD rainfall stations (Minicoy, Amini, Thiruvananthapuram, Punalur, Kollam, Allapuzha, Kottayam, Kochi, Thrissur, Kozhikode, Thalassery, Kannur, Kudulu and Mangalore) should report rainfall of 2.5 mm or more for two consecutive days after May 10. If this happens, the IMD is supposed to declare the onset of the monsoon on the second day itself.
The IMD data itself shows that more than 60% of the stations have witnessed rainfall of more than 2.5 mm consecutively for more than even two days this week. In the week prior, from May 16 to 22 when the rainfall was relatively less, 13 districts in Kerala received showers that were 100% above normal ('large excess' in IMD jargon) and nine among them got rainfall that was 200% above normal. Kasaragod, with a departure of 96%, was the only district where the rainfall excess was less than 100% above normal.
Meaning, in the last 10 or more days there were periods of two or more consecutive days when a substantial chunk of Kerala was drenched in rainfall that was considerably more than 2.5 mm. To get this volume of rainfall (2.5 mm), it only has to rain modestly for a few hours. But to get a sense of the rain pouring over Kerala now, one only has to look at the amount of rainfall received by many districts south of Malappuram daily on May 27 and 28: 50 mm (many areas in Kochi and Kottayam) and even above 100 mm (certain areas in Kollam and Alappuzha).
Still, if the IMD is reluctant to officially announce the arrival of the SW monsoon, there are two reasons.
One, the westerlies (the moisture-laden monsoon winds that blow from the southwest) have still not acquired the requisite depth; in simple terms, depth is the height of the wind from ground up. The winds should grow to a height of at least four kilometres from the ground. "At the moment, it has not achieved this height," said Dr M G Manoj, a scientist with the Advanced Centre for Atmospheric Radar Research (CUSAT). The core strength of the westerly lies in the 1-2 km range, and for this core strength to intensify the westerly has to stand even taller.
Moreover, at this stage, Manoj said that the winds at the higher level was seen coming from the east in the latest satellite images put out by the IMD. "These upper-level winds should take a 180-degree turn and blow from the west. The winds have to move fully from the southwest direction for the IMD to call it monsoon," he said. If these patterns - height and direction of the winds - have not formed, it only means that the monsoon system has not yet solidified.
The second reason why the IMD is hesitant to go official is the value of the outgoing longwave radiation (OLR). OLR is about infrared radiation. Any thermodynamic system, human or cloud, with a temperature above absolute zero (zero degree kelvin or -273 degree celsius) radiates infrared/thermal energy. For instance, it is the infrared energy that radiates from humans that is caught on CCTV cameras.
IMD's OLR refers to the radiation that goes up from the clouds that have formed over the southern peninsula. These radiations are measured by the Indian National Satellite System (INSAT) placed in space by ISRO. The OLR should be below 200 watts per square metre, or in simple terms cold enough, for monsoon to be declared.
For this, deep towering skyscraper (cumulonimbus) clouds, dense with cool vapour, have to be uniformly formed over Kerala. Nonetheless, in certain areas, high-rise clouds, far taller than what normal monsoon would require, have been formed. Over Kochi, for instance. On May 28, between 9.10 a.m. and 10.10 a.m., the Kalamasserry area received 103 mm rainfall. According to the IMD, a cloudburst happens when an area receives more than 10 cm (100 mm) rain.
On the same day, the CUSAT observatory also, for the first time ever, observed 98.4 mm rainfall in one hour. Result: Flash floods in Kochi.
However, a top IMD official said that conditions were quickly coming together for the declaration of this year's monsoon.
The formation of a low-pressure area, Cyclone Remal, over the Bay of Bengal, in the northeastern area, is also accelerating the onset. The low-pressure area in the northeast is pulling moisture-bearing winds from the Arabian Sea towards it in a diagonal fashion. These winds are ramming into the Western Ghats, rising up, condensing and causing heavy rains.
"We might officially declare the onset on May 29," the official said. The IMD had said that the monsoon will touch down on Kerala on May 31, but "with a model error of + or - four days".
