Wind Speed, Surface Flux, and Intraseasonal Convection Coupling From CYGNSS Data

Abstract

This study analyzes wind speed and surface latent heat flux anomalies from the Cyclone Global Navigation Satellite System (CYGNSS), aiming to understand the physical mechanisms regulating intraseasonal convection, particularly associated with the Madden-Julian oscillation (MJO). An advantage of CYGNSS compared to other space-based data sets is that its surface wind speed retrievals have reduced attenuation by precipitation, thus providing improved information about the importance of wind-induced surface fluxes for the maintenance of convection. Consistent with previous studies from buoys, CYGNSS shows that enhanced MJO precipitation is associated with enhanced wind speeds, and that associated surface heat flux anomalies have a magnitude about 7-12% of precipitation anomalies. Thus, latent heat flux anomalies are an important maintenance mechanism for MJO convection through the column moist static energy budget. A composite analysis during boreal summer over the eastern north Pacific also supports the idea that wind-induced surface flux is important for MJO maintenance there. Plain Language Summary Enhanced surface evaporation (equivalent to a term called latent heat flux) during high wind speed periods can support precipitation through atmospheric moistening. Such evaporation anomalies form a key air-sea interaction component of the Madden-Julian oscillation (MJO). Based on the wind speed and related evaporation anomalies derived from Cyclone Global Navigation Satellite System (CYGNSS) measurements, we show that (1) enhanced MJO precipitation is associated with enhanced wind speed and surface evaporation that may help support the MJO in the Indo-Pacific warm pool and (2) enhanced local surface evaporation is important for maintenance of the MJO over the eastern north Pacific during Northern Hemisphere summer. These results verify results from previous buoy studies that suggest the importance of enhanced wind-driven surface evaporation for maintaining the MJO.

Assessment of CYGNSS Wind Speed Retrievals in Tropical Cyclones

“…The CYGNSS satellite constellation was initially designed with the objective of providing critical and frequent wind measurements in TCs, a task that, due to unforeseen calibration issues, has been proven to be extremely challenging [8][9][10]. Measurements in the low-to-moderate wind regimes have displayed better skills than anticipated and have begun to be used in a variety of applications such as assimilation experiments [54][55][56][57], deriving products such as surface fluxes [58], and air-sea interaction studies [59,60].…”

“…The CCMP_CYGNSS wind analysis is designed for air-sea interaction studies and provides insight into the wind structure in convective regions such as investigations into the Madden-Julian Oscillation [59], genesis of TCs [60], and diurnal wind variability [63,64]. CCMP_CYGNSS for the period 2017-2020 is freely available to the public at https://data.…”

The role of air–sea coupling on November–April intraseasonal rainfall variability over the South Pacific

“…To quantify the LHF-rainfall relationship, we compute a feedback parameter for latent heat flux based on the linear regression between daily anomalies of latent heat flux and rainfall (Dellaripa and Maloney 2015;Bui et al 2020). Note that both LHF and rainfall anomalies are expressed in W m −2 units.…”

“…In the absence of atmospheric feedback to local SST, the rainfall anomalies in A-DAY would grow as long as the large-scale wind anomalies and associated surface heat fluxes are favourable, as indicated by significantly higher LHF feedback parameter (Fig. 5e), based on linear regression of LHF and rainfall anomalies (Dellaripa and Maloney 2015;Bui et al 2020). The LHF anomalies at maximum positive lag are roughly 6% of the rainfall anomalies for C-CTL, which is largely exaggerated in A-DAY with a value of 9%.…”

The CYGNSS Mission: On-Going Science Team Investigations

“…A number of on-going investigations examined CYGNSS observations near tropical oceanic thunderstorms, including an examination of the characteristics of the data in storms with and without lightning [69]. The importance of wind-driven fluxes on the Madden Julian Oscillation (MJO), using CYGNSS data was high-lighted [70]. Finally, it is worth highlighting that several team members are using CYGNSS data to study the processes of air-sea interaction in weather and climate science [71].…”