Techniques and challenges in the assimilation of atmospheric water observations for numerical weather prediction towards convective scales

Abstract

While contemporary numerical weather prediction models represent the large‐scale structure of moist atmospheric processes reasonably well, they often struggle to maintain accurate forecasts of small‐scale features such as convective rainfall. Even though high‐resolution models resolve more of the flow, and are therefore arguably more accurate, moist convective flow becomes increasingly nonlinear and dynamically unstable. Importantly, the models' initial conditions are typically sub‐optimal, leaving scope to improve the accuracy of forecasts with improved data assimilation. To address issues regarding the use of atmospheric water‐related observations – especially at convective scales (also known as storm scales) – this article discusses the observation and assimilation of water‐related quantities. Special emphasis is placed on background error statistics for variational and hybrid methods which need special attention for water variables. The challenges of convective‐scale data assimilation of atmospheric water information are discussed, which are more difficult to tackle than at larger scales. Some of the most important challenges include the greater degree of inhomogeneity and lower degree of smoothness of the flow, the high volume of water‐related observations (e.g. from radar, microwave and infrared instruments), the need to analyse a range of hydrometeors, the increasing importance of position errors in forecasts, the greater sophistication of forward models to allow use of indirect observations (e.g. cloud‐ and precipitation‐affected observations), the need to account for the flow‐dependent multivariate “balance” between atmospheric water and both dynamical and mass fields, and the inherent non‐Gaussian nature of atmospheric water variables.

The ABC-DA system (v1.4): a variational data assimilation system for convective-scale assimilation research with a study of the impact of a balance constraint

“…The current version of the ABC model does not include moist processes. There is much that can be learned about convective-scale DA from a dry model, but assimilating and forecasting moisture fields is a major reason for convectivescale forecasting (Sun et al, 2014;Bannister et al, 2020). It is planned to upgrade the model to permit the advection of one or more water variables and allow condensation and evaporation processes to affect the flow.…”

“…The use of other balance relationships is possible, including statistical balance relationships (e.g. Derber and Bouttier, 1999;Chen et al, 2013;Bannister et al, 2020). An alternative balance relationship that may be applicable at convective scale is mentioned in the summary.…”

“…This makes the assumption that the eigenvectors of the horizontal covariance matrix are plane waves, and the eigenvalues in h are their variances. This is equivalent to assuming horizontal error covariances that are homogeneous (see Bartello and Mitchell, 1992;Berre, 2000;Bannister, 2008b). G is set to I in the horizontal transform.…”

Differences in tropical high clouds among reanalyses: origins and radiative impacts

“…ERA-Interim uses the scheme proposed by Tiedtke (1989), with a single pair of plumes representing updraughts and downdraughts. Deep convection is triggered when the updraught vertical velocity diagnosed at the lifting condensation level (LCL) is positive and the estimated cloud depth exceeds 200 hPa (Bechtold et al, 2006). Convection can be triggered from any level in the lowest 350 hPa of the atmosphere.…”

The “ABC-DA system” (v1.4): a variational data assimilation system for convective scale assimilation research with a study of the impact of a balance constraint

“…The current version of the ABC model does not include moist processes. There is much that can be learned about convectivescale DA from a dry model, but assimilating and forecasting moisture fields is a major reason for convective-scale forecasting (Sun et al, 2014;Bannister et al, 2019). It is planned to upgrade the model to permit the advection of one or more water https://doi.org/10.5194/gmd-2019-318 Preprint.…”