Optimization of Gravity Wave Source Parameters for Improved Seasonal Prediction of the Quasi-Biennial Oscillation

Abstract

A methodology is presented for objectively optimizing nonorographic gravity wave source parameters to minimize forecast error for target regions and forecast lead times. In this study, we employ a high-altitude version of the Navy Global Environmental Model (NAVGEM-HA) to ascertain the forcing needed to minimize hindcast errors in the equatorial lower stratospheric zonal-mean zonal winds in order to improve forecasts of the quasi-biennial oscillation (QBO) over seasonal time scales. Because subgrid-scale wave effects play a large role in driving the QBO, this method leverages the nonorographic gravity wave drag (GWD) parameterization scheme to provide the necessary forcing. To better constrain the GWD source parameters, we utilize ensembles of NAVGEM-HA hindcasts over the 2014–16 period with perturbed source parameters and develop a cost function to minimize errors in the equatorial lower stratosphere compared to analysis. Thus, we may determine the set of GWD source parameters that yields a forecast state that most closely agrees with observed QBO winds over each optimization time interval. Results show that the source momentum flux and phase speed spectrum width are the most important parameters. The seasonal evolution of optimal parameter value, specifically a robust semiannual periodicity in the source strength, is also revealed. Changes in optimal source parameters with increasing forecast lead time are seen, as the GWD parameterization takes on a more active role as QBO driver at longer forecast lengths. Implementation of a semiannually varying source function at the equator provides RMS error improvement in QBO winds over the default constant value.

Intercomparison of tropospheric and stratospheric mesoscale kinetic energy resolved by the high‐resolution global reanalysis datasets

“…According to Añel et al (2007), the number of soundings reaching 20 km in the Integrated Global Radiosonde Archive is lower in the Tropics than in the extratropics, and the number of detections decreases from high latitudes to the Equator. From the perspective of system configuration, one main reason may be the non-orographic GW drag parametrization used in each reanalysis dataset, which will affect the generation and features of the QBO in models (e.g., Rind et al, 2014;Yu et al, 2017) and improve its prediction (e.g., Barton et al, 2019). Nonetheless, this parametrization is highly uncertain in atmospheric models (Polichtchouk et al, 2018).…”

The Ensemble Tangent Linear Model Applied to Nonorographic Gravity Wave Drag

“…The NAVGEM NGWD scheme is described in Eckermann (2011), Barton et al (2019), andAllen et al (2022, hereafter A22). It models the upward propagation of GWs launched in the troposphere.…”

Calibration and Uncertainty Quantification of a Gravity Wave Parameterization: A Case Study of the Quasi‐Biennial Oscillation in an Intermediate Complexity Climate Model

“…Importantly, the main goal of parameterizations is to obtain climate model output consistent with the macrophysical climate state (i.e., large‐scale circulation and variability), rather than the microphysical (i.e., gravity wave drag). Therefore, the typical approach is to tune the parameterization to obtain a consistent climate state (e.g., Barton et al., 2019; Couvreux et al., 2021; Donner et al., 2011; Dunbar et al., 2021; Scaife et al., 2002).…”

“…Based on more recent models that obtain a spontaneous QBO, at least half of the forcing required is contributed from non‐orographic gravity wave parameterizations (Holt et al., 2020). This makes the QBO a useful phenomenon to consider when calibrating the gravity wave parameterization (Anstey et al., 2016; Barton et al., 2019; Scaife et al., 2002).…”