Scientists study powerful wind across East Africa

Kenyan scientists are studying a very strong powerful wind that transports moisture from the Indian Ocean through Kenya as it moves to Ethiopia and northern Africa.

The wind dubbed Turkana Low Level Jet is also associated with widespread low-level divergence across East Africa, and the relatively low equatorial rainfall totals in Kenya.

Kenya Meteorological Department (KMD) studies supported by other independent research, indicate that the Turkana low level jet (LLJ), influences both flooding and droughts, and powers Africa’s largest wind farm.

Prof Gilbert Ouma, Senior Lecturer at University of Nairobi, Institute for Climate Change and Adaptation and Department of Meteorology describes the jet as an intrinsic part of the African climate system.

“It is the principle conduit for water vapour transport to the African interior from the Indian Ocean, and droughts in East Africa tend to occur when the jet is strong,” said Ouma.

Vapour transported

The jet is responsible for approximately a third of water vapour transported from the Indian Ocean across the East African Rift System to the continental interior according to Sorteberg 2013 study.

“You need moisture for rainfall and if you look at the Turkana and Eastern part of Kenya, there is hardly any source of moisture, only Lake Turkana but little bit to the north,” said Ouma.

The jet was first identified in 1980’s by Joseph Hiri Kinuthia (Kinuthia and Asnani 1982; Kinuthia 1992), a Kenya Meteorological Department officer who was based in Nairobi.

The jet was identified after pilot weather balloon observations that showed the existence of a strong southeasterly low-level jet in the Turkana channel which separated the Ethiopian Highlands and the East African highlands.

The study showed the jet existed throughout the year with speeds exceeding 30 meters per second on a number of occasions and sometimes exceeding 50 meters per second.

Observation then revealed during February and March, the mean monthly winds based on the morning observations exceeded 25 meters per second.

Stronger vertical mixing

The study also revealed the morning winds are stronger than afternoon winds, presumably due to stronger vertical mixing and dilution of the jet maximum in the afternoon.

Other parameters the Kinuthia’s observation recorded the hodograph turned in a counterclockwise direction from the lowest levels up to 0.75 km above ground level and sometimes even aloft.

Up to 0.45 km above ground level, the hodograph manifested some of the characteristics of the Southern Hemisphere Ekman layer.
Prof Ouma indicates that these pioneering observations confirm the presence of the jet, but do not reveal the full diurnal cycle, quantify the jet-related water vapour fluxes, or document the thermodynamic environment associated with jet formation.

Modern reanalysis datasets disagree with one another over the strength of jet winds and underestimate the strength of the jet by 25%–75% compared to the pilot balloon data.

To reconcile the studies, the Radiosonde Investigation for the Turkana Jet (RIFTJet)—which measured the Turkana jet for the first time in 40 years using modern technologies — was carried out from March 26 to April 23, 2021.

The investigation aimed at observing the full diurnal cycle of the Turkana jet, quantify moisture transport associated with the jet and capture the thermodynamic/dynamic environment associated with jet formation and diurnal variability.

Ouma indicated the Radiosonde data reveal a persistent low-level jet, which formed on every night of the campaign, with an average low-level maximum wind speed of 16.8 meters per second. The measurements confirm the role of the Turkana LLJ in water vapour transport: mean water vapour transport at Marsabit is 172 kg m s−1.

“This is important information for the area, we know they have floods,” he said.