The Impact of Rain on Ocean Surface Waves and Currents

Abstract

Precipitation is an important component of the interaction between Earth's atmosphere and oceans, modifying air-sea fluxes of momentum, heat, and gas. It has been hypothesized that rain's suppression of ocean surface gravity waves and centimeter-scale wave enhancement should alter the nature of air-sea momentum flux, resulting in increased near-surface current. Here, we use field observations to describe this impact and measure the very near-surface current response to rainfall. During heavy rain, surface-roughening ring waves were generated and longer gravity waves were suppressed; immediately following, the magnitude of the near-surface current increased in response to wind forcing but died as the rain subsided and long waves recovered. These first-of-their-kind field observations indicate that rain reduces ocean wave form drag in favor of tangential stress, resulting in the acceleration of current near the sea surface. Plain Language SummaryRain is known to be an important component of atmosphere-ocean interactions. However, the specific response of short-scale waves and very near-surface current to precipitation has not yet been described in the Earth's ocean. We observe that rainfall suppresses long waves and grows centimeter-scale ring waves. This alteration facilitates the acceleration of current, which ceases in favor of longer wave growth as the rain subsides. This is the first observation of this phenomenon in the Earth's real ocean.

Multi‐Scale Temporal Variability of Turbulent Mixing During a Monsoon Intra‐Seasonal Oscillation in the Bay of Bengal: An LES Study

“…In order to have a comprehensive understanding of the MISO variabilities, future process studies need to account for both 1D and 3D processes. Furthermore, the LT in the present study does not include the effects of misalignment between wind and wave fields (Shrestha et al, 2019;Sullivan et al, 2012;Van Roekel et al, 2012;Wang et al, 2019) as well as the changes in surface wave field during the periods of precipitation (Laxague & Zappa, 2020). Breaking surface waves can also inject a significant amount of momentum into the ML, and thus, modulate the ML properties especially when the ML is shallow during the active phase of MISO (Janssen, 2012).…”

The Response of Ocean Skin Temperature to Rain: Observations and Implications for Parameterization of Rain‐Induced Fluxes

“…3 of 16 imaging polarimeter (Laxague & Zappa, 2020;Zappa et al, 2008), while skin temperature was measured by a cooled infrared camera processed through a 5-frame median filter to remove falling raindrops from the images. Light rain (panel b) generates a few ring waves but has little effect on the thermal structure of the ocean skin.…”

“…Figure 1 displays the evolution of ocean surface slope and skin temperature over the course of a rain event observed in October 2016 in the Timor Sea (Wurl et al., 2018). Along‐look surface slope was measured by an imaging polarimeter (Laxague & Zappa, 2020; Zappa et al., 2008), while skin temperature was measured by a cooled infrared camera processed through a 5‐frame median filter to remove falling raindrops from the images. Light rain (panel b) generates a few ring waves but has little effect on the thermal structure of the ocean skin.…”

“…Secondary drops will break off from the rebounding jets, a few of which (depending on their size) are likely carried aloft by turbulence (Edson & Fairall, 1994; Mueller & Veron, 2014; Veron, 2015). At sufficiently high rain rates, the interface becomes completely disturbed by violent surface motions as impact craters and splashing droplets interact continuously with impinging raindrops, and the ring wave patterns are lost (Laxague & Zappa, 2020; Peirson et al., 2013).…”

“…This leads to additional surface renewals with increasing rain rate, and in all but the lightest of rains (<2 mm/hr) the surface renewal timescale is dominated by the precipitation effect (Craeye & Schlüssel, 1998; Schlüssel et al., 1997). Enhanced centimeter‐scale surface roughness caused by the rainfall increases tangential stress while damping wind‐wave growth, changing the nature of the air‐sea momentum flux such that a greater fraction of wind energy goes into acceleration of the near‐surface current rather than wave growth (Houk & Green, 1976; Laxague & Zappa, 2020; Poon et al., 1992; Schlüssel et al., 1997).…”