1. Introduction
The ensemble Kalman filter (EnKF) is an efficient estimation tool that has been used extensively in many geophysical applications during the last two decades (Evensen 2009). It operates sequentially by combining information from a prior state of a system and its associated variance with noisy observations to produce an analyzed posterior state with an updated uncertainty (Evensen 2003). Inflation has been introduced into stochastic (e.g., EnKF) and deterministic (e.g., Anderson 2001; Hoteit et al. 2002; Whitaker and Hamill 2002; Hunt et al. 2007; Sakov and Oke 2008) ensemble filters as a way to counteract the underestimation of the true variance. Large portions of the variance are often lost during the forecast step, but the inherent bias of the EnKF, in the form of sampling errors, is in the analysis step (Bocquet 2011; van Leeuwen 1999). A separate issue is whether there are sufficient members to track all the directions of uncertainty growth (Bocquet and Carrassi 2017). Other reasons for variance underestimation can be related to the simplified model dynamics and wrong parameterizations that may introduce large biases in addition to neglected model errors. Systematic observational and representativeness errors within the analysis step are also expected to lead to underestimates of the ensemble variance (Furrer and Bengtsson 2007).
Studies within the data assimilation (DA) literature have proposed a number of techniques that deal with all kinds of system errors. The majority of these techniques, apart from localization, which removes spurious correlations from the prior covariance, can be seen as a form of inflation. These can be split into four distinct categories. The first category is background covariance inflation, which itself is divided into additive and multiplicative inflation. Additive inflation (Mitchell and Houtekamer 2000) adds a random perturbation drawn from a specific error distribution to each ensemble member during the forecast step. Additive inflation can be applied to the prior ensemble in different forms (Raanes et al. 2015) and also within the update step (Hoteit et al. 2015). Multiplicative inflation (Anderson and Anderson 1999; Zheng 2009), on the other hand, increases the prior ensemble spread by a specific factor while pushing all members away from the mean. It has been reported that multiplicative inflation can help deal with observation network-related sampling errors, while additive inflation can help mitigate the modeling errors (Whitaker and Hamill 2012). In the current study, we only look into multiplicative inflation and its spatial and temporal adaptive formulations. Observation error variance inflation falls within the second category. The idea behind inflating the observation error variance is to compensate for a wrongly specified error distribution on the observations (Liang et al. 2012) and, in other scenarios, to limit the impact of the Kalman update. Such an inflation procedure, which is often referred to as observation error moderation, has been applied in a number of ocean (e.g., Sakov et al. 2012; Karspeck 2016) and atmosphere (e.g., Li et al. 2009; Minamide and Zhang 2017) DA studies. The third inflation category includes the relaxation-to-prior perturbations (RTPP) (Zhang et al. 2004), and spread (RTPS) (Whitaker and Hamill 2012; Ying and Zhang 2015). The core of the relaxation method can be seen as a multiplicative inflation; however, it is applied only to the posterior ensemble, given the amount of ensemble spread reduction by the observations. In a recent study, Kotsuki et al. (2017) found that the adaptive versions of the RTPP and RTPS often lead to overdispersive (underdispersive) ensemble in sparse (dense) observation networks when compared to multiplicative prior inflation. The sampling error correction by Anderson (2012) can also be seen as a relaxation, in some sense. The fourth inflation category includes techniques that modify the ensemble by changing its size (Uzunoglu et al. 2007) or its physical and model-based nature (e.g., Meng and Zhang 2007; Berner et al. 2009). In addition to these four inflation categories, a few studies have focused on developing ensemble-based filters that do not require an explicit use of inflation. These methods, such as the ensemble time-local H∞ filter (EnTLHF; Luo and Hoteit 2011) and the finite-size EnKF (EnKF-N; Bocquet 2011), have some sort of a built-in inflation by design.
Anderson (2009, hereafter A09) proposed an adaptive (multiplicative) inflation algorithm that is fully based on a Bayesian approach. The algorithm deals with inflation as a spatially heterogeneous random variable following a multivariate Gaussian distribution. As we will see in the next sections, the inflation likelihood—that is not Gaussian in terms of the inflation—often peaks at values that are smaller than 1 (sometimes negative). This may cause a deflation of the ensemble, producing poor estimates, and may possibly yield filter divergence over a long assimilation run. A common strategy for countering excessive deflation (e.g., Wang et al. 2007) is to bound the inflation by rejecting values that are smaller than 1. Here, we derive a new form for the likelihood function. The new form is similar to the one proposed in A09, with an additional correction term. The correction term helps reduce the probability for deflation, particularly for small ensemble sizes. Moreover, a Gaussian inflation prior does not respect the positivity constraint, such that drawing a negative inflation value is possible. As a more suitable alternative, Raanes (2016) recently proposed the use of an inverse chi-square distribution for the inflation prior. In this study, we use an inverse gamma distribution for the prior, and we present an efficient computational procedure to switch back and forth between Gaussian and inverse gamma probability density functions (pdfs).
Section 2 briefly introduces the adaptive inflation algorithm and also provides a useful geometrical interpretation for the problem. Section 3 discusses the enhancements to the prior and likelihood distributions. Section 4 presents results from two low-order models and an idealized 3D atmospheric system. Results from A09 are compared to the ones using the proposed enhanced inflation algorithm. Section 5 provides the discussion and conclusions.
2. Adaptive inflation review
a. Definitions, background, and innovation statistics
b. Geometrical interpretation
Geometrical representation of the prior multiplicative inflation. The circle and point A describe the true background and observation error variances. Point B and its associated ellipse describe the statistics given by the ensemble. The parameters for the ellipse and the circle are displayed at the top of the graph. The horizontal axis represents the projections of the background errors, while the vertical axis represents those of the observation errors. The eccentricity e of the ellipse is computed based on Eq. (10).
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
c. Anderson’s adaptive inflation
3. Enhanced adaptive inflation algorithm
a. The likelihood
In practice, studies have shown that the result of Eq. (8) is not always positive. This often happens because of a misspecification of the observational error variance [note that Eq. (8) is derived based on the assumption that 
b. The prior
Instead of a Gaussian, we will explore representing the inflation prior by an inverse gamma (IG) distribution (Stroud and Bengtsson 2007). Such a choice is important and can be useful in two ways. First, it would prevent negative and very tiny inflation values that often deteriorate the quality of the forecasts. Second, from a coding and computational perspective, this ensures a clean algorithm without lengthy checks on the sign and magnitude of the inflation factor.
c. The posterior
d. Implementation
The enhanced adaptive inflation algorithm is implemented in the Data Assimilation Research Testbed (DART) at NCAR (www.image.ucar.edu/DAReS/DART/; Anderson et al. 2009). Listed below are some characteristics about the algorithm and its implementation procedure.
- The algorithm is coded in such a way that the user only deals with Gaussian inflation distributions. The motivation for this is to facilitate the choice of the initial inflation parameters and impose only marginal changes to old versions of DART. The algorithm operates by first transforming the Gaussian input into an inverse gamma, and then it applies the Bayesian update. The outputs are the corresponding Gaussian mean and variance.
- For computational reasons, and as outlined in A09, one may choose to keep the inflation variance unchanged during the update. One can also refrain from letting the variance increase in time and instead only accept updates that decrease the inflation variance.
- The algorithm, by construction, does not require enforcing the inflation to be nonnegative, given the IG assumption. This further simplifies the code and the list of the input parameters.
- The algorithm also features a damped inflation version to avoid overdispersion. The damping (Anderson et al. 2009), which takes place right before applying
4. Numerical experiments
The enhanced spatially and temporally varying adaptive inflation scheme is compared to the algorithm of A09. The comparison is performed using (i) the Lorenz-63 (L63; Lorenz 1963), (ii) the 40-variable model of Lorenz (L96; Lorenz 1996), and (iii) a low-resolution atmospheric GCM with more than 105 variables. All experiments use the ensemble adjustment Kalman filter (EAKF; Anderson 2003). The Kalman update is done serially using DART. All experiments use damped adaptive inflation versions of A09 and the proposed scheme (
a. Lorenz 63
For this model (see the appendix), we assume perfect modeling conditions, and we refrain from using localization. The goal is to study how well the inflation schemes can mitigate sampling errors in a strongly nonlinear regime and using a very small ensemble size. To this end, we also include results from the EnKF-N (Raanes 2016) as a benchmark. The EnKF-N accounts for errors in the prior ensemble mean and its associated covariance and is expected to outperform an EnKF with optimally tuned inflation (Bocquet 2011). Given its computational efficiency, only the dual formulation of the EnKF-N is implemented. The initial state of the model, consisting of three variables, 
Time average of forecast RMS errors and spread obtained using the L63 model for three observational network scenarios. The adaptive inflation of A09 is compared to the proposed enhanced scheme, in addition to the EnKF-N.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
b. Lorenz 96
Forty state variables are defined as equally spaced on a 
1) Inflation cap
In the following tests, perfect model conditions are assumed, and the performance is assessed based on a changing inflation lower bound (LB): either 0 or 1. The half-width c for localization is set to 0.2. In Fig. 3, the spatial average of the prior and posterior RMS and spread are plotted. Because every other variable is observed, a saw-shaped behavior is obtained. As can be seen, when LB = 1, both algorithms suggest equivalent prior and posterior accuracy. The enhanced algorithm is around 8% more accurate than A09 when LB is set to 0. The overall prior and posterior RMS errors for both schemes are smaller when the LB is 1. In terms of the spread, the enhanced algorithm yields better consistency3 in the estimates. Apparently, when the LB is 1, A09 displays a slight overdispersion in the prior estimates, with 
The temporal average of total RMS errors and AES obtained using (left) A09 and (right) enhanced inflation algorithms. The top and bottom panels use an inflation lower bound (i.e., Inf-LB) of 0 and 1, respectively. Reported in the legends are the prior and posterior overall RMS and spread average. The ratio between the overall average RMS to spread is given for both the prior and the posterior estimates in the top-right corner of the panels.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
In Fig. 4, we plot the temporal evolution of the inflation obtained using both algorithms during the last 1000 steps. The adaptive procedure in A09 (blue crosses) yields a wider inflation range, compared to the enhanced algorithm (red triangles). This explains the over- and underdispersed ensemble behavior suggested by A09 in Fig. 3. In space, A09’s inflation structure was found more heterogeneous, in the sense that the difference between inflation for observed and unobserved cells is quite big. The enhanced scheme produced a smoother inflation field. Note that large ensemble variations between neighboring cells might create instabilities in some cases.4 Related to inflation, we assessed the behavior of the eccentricity in Eq. (10) and found that the value for e suggested by the enhanced scheme is closer to 0. On average, it is around 32% smaller than that of A09.
Temporal evolution of the inflation (all 40 variables of L96) using A09 and the proposed enhanced algorithms over the last 1000 model time steps. Results are displayed for both LBs (0 and 1). The maximum value, minimum value, and the spatiotemporal average of inflation are given in the legend.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
2) Localization and model error sensitivity
The goal of the following experiments is to test the robustness of the schemes under challenging modeling and assimilation scenarios. For the localization sensitivity, the forcing is kept at 8, and the half-width parameter is varied between 0.1 and 0.8 with a step of 0.1. We tabulate the overall RMSE mean obtained using both algorithms in Table 1. As shown, the enhanced scheme suggests smaller RMSE values for all tested half-widths. For instance, when 
The average forecast RMSE resulting from A09 and the enhanced inflation algorithms for two scenarios on the lower bound for inflation, namely, LB = 0 and LB = 1. The results are shown for eight different localization length scales.
Next, 14 experiments are conducted with different forcing values in the forecast model, 
As in Table 1, but, here, the sensitivity is given in terms of 14 different forcing parameters.
Overall, the proposed enhanced algorithm performed well against A09’s scheme, suggesting high robustness to changes in model error and localization length scale. When the LB = 0, A09 performed poorly. Under perfect modeling conditions, the enhanced scheme generated small inflation values that were slightly larger than 1. In the presence of modeling errors, the scheme adequately responded and assigned necessary large inflation. We noticed that the occurrence of deflation, as compared to A09, has decreased by around 6%. These features can be attributed to the small right shift introduced to the likelihood probability density function. The IG enhancement to the prior leads to a decreased deflation and, consequently, less inflation, when compared to A09. In general, the performance with no deflation was found more accurate. This, however, cannot be generalized because it could only be a characteristic of the L96 model. As we will see in the next model results, we experience a different behavior when capping the inflation.
c. Idealized global atmospheric model
The model is a modified version of the dynamical core used in the Geophysical Fluid Dynamics Laboratory (GFDL) climate model (Anderson et al. 2004). The model (B grid) has no moisture and topography and no parameterizations for radiation, turbulence, and convection. To represent these, the model uses Held and Suarez (1994) forcing instead, which consists of Newtonian relaxation of the temperature field to a prescribed radiative equilibrium solution and Rayleigh damping of winds near the surface for dissipation. The prognostic variables in the model are surface pressure (PS), temperature T, zonal U, and meridional V wind components.
The B-grid model is configured with the minimum horizontal and vertical resolution required to generate baroclinic instabilities. The grid domain consists of 30 latitude and 60 longitude grid points and five levels spaced equally in pressure (level 1 being the highest), resulting in a total of 28 200 state variables. The time step in the model is 1 h. For more details, the reader may refer to Anderson et al. (2005). This model is available in the DART package under the name “bgrid-solo.” In the following set of DA experiments, we test the performance of the inflation algorithms for constant and varying inflation variance scenarios. We use a predefined climatological state as an initial starting point to perform the truth run over a period of 400 days; however, we only use the last 200 days for assessment of the results. Three hundred PS observations, randomly distributed in the domain, are sampled every day with a Gaussian observation error variance of 1 hPa. All results reported here are based on the prior state (and inflation) estimates. The localization half-width is set to 
1) Fixed 
In this section, the inflation is initialized with 
Evolution of spatially averaged prior RMS in time for all variables of the dry dynamical core. Except for PS, the averaging is done for all model levels. The solid lines are results from experiments with inflation LB = 0. The dash–dotted lines, on the other hand, use LB = 1. The overall RMS averages (both in space and time) are reported in the legends of each panel.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
Maps of time-averaged bias resulting from (top) A09 and (bottom) the enhanced adaptive inflation schemes. Shown on the maps with green circles are the location of the PS observations.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
Next, we plot in Fig. 7 PS inflation snapshots after 200 days (top panels), in addition to the temporal average (bottom panels) for LB = 0. At day 200, strong deflation areas in the high upper latitudes dominate A09’s map. It is surprising to see that deflation happens mostly in places where observations are available (e.g., south of Australia). The enhanced scheme, on the contrary, yields reasonable inflation patterns, such that no-observation locations are assigned 
(top) PS inflation maps after 200 days of assimilation resulting from (left) A09 and (right) the enhanced schemes. (bottom) As in (top), but displaying the overall time average (entire 400 days) instead. The green circles denote the PS observations locations.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
In addition to PS, we also analyze the behavior of the inflation along the vertical direction in the model. Figure 8 plots the average inflation maps for T at levels 1 and 5. Similar to PS, the enhanced scheme is shown to generate more reliable maps with consistent data-matching structures. Closer to the surface at L5, the inflation values are large, given the higher impact from PS observations. The maps produced by A09 consist of very large inflation regions, especially near the surface. Within these regions, few tiny no-inflation spots are observed. To better understand this behavior, we study in Fig. 9 the temporal change in the inflation’s overall minimum and maximum values, in addition to the spatial average. Both schemes yield comparable inflation average with 
(top) Time-average inflation maps for temperature from model level 1, resulting from (left) A09 and (right) the enhanced schemes. (bottom) As in (top), but for level 5. The green circles denote the PS observations locations.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
Inflation values averaged in space, in the vertical, and over all prognostic variables obtained using A09 and the enhanced inflation schemes. The minimum, mean, and maximum values for these inflation values are plotted in time. The LB here is 0. The y axis is shown in log scale.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
2) Discussion
When the inflation variance is fixed in time, the enhanced scheme with LB = 0 was shown to be the most reliable, providing accurate estimates. The results of the enhanced scheme with LB = 1 are, on average, 25% less accurate. One might argue that the main reason for using inflation is to avoid prior variance underestimation, and, thus, deflation should not be part of the scaling procedure. This is true to a certain extent; however, a more detailed and in-depth analysis is needed here.
- Preventing deflation means that Eq. (8), or Eq. (25) in our enhanced context, is no longer satisfied. As such, the innovation statistics in the following assimilation cycles, whether indicating inflation or deflation, may be incorrectly interpreted. Ignoring the likelihood information at one cycle may have consequences on future assimilations.
- The process of inflation inserts an artificial spread in the ensemble that the model is unaware of. For a biased model, this can be extremely large. This may require long cycles and successive Kalman updates before it can be removed, during which the accuracy of the forecast might be degraded. To better fit the data, deflation can be applied to get rid of the extra artificial spread. If the spread was to grow naturally in the ensemble due to the dynamics of the model alone, then deflation should not be utilized, and the Bayesian update can minimize the variance.
- A strong deflation, as we experienced with A09, can be very unhealthy, producing poor estimates. The use of deflation should be moderate—hence the introduction of the IG instead of a Gaussian. Moreover, deflation was applied using A09’s adaptive scheme 7489 times. This number was greatly reduced to 1144 times using the enhanced scheme, given the modified likelihood variance [Eq. (26)].
3) Varying 
To test the functionality of the inflation variance, we now start the assimilation by setting 
Time evolution of inflation standard deviation 
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
The effects of this behavior from both schemes are illustrated in Fig. 11, in which the RMS and AES of the estimates are displayed over time. Given the rapid decrease in the inflation variance, A09’s performance tends to degrade, as shown in the last 50 days. When the inflation variance becomes very small, the effects of the Bayesian update decrease. For a longer simulation period (i.e., >400 days), the estimates are expected to get worse as the inflation algorithm becomes technically ineffective. For all variables, the RMSE suggested by A09 is almost double that of the spread, probably indicating an ensemble collapse (divergence). On the other hand, the enhanced scheme estimates are more accurate, further suggesting a consistency of 
Evolution of spatially averaged prior RMS/AES in time for all variables. Except for PS, the averaging is done for all model levels. The overall RMS/AES averages (both in space and time) are reported in the legends of each panel. Here, LB = 1.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
Maps of the PS inflation standard deviation resulting from (top) A09 and (bottom) the enhanced adaptive inflation schemes. Shown on the maps with green circles are the location of the PS observations. Time (days) increases from left to right panels.
Citation: Monthly Weather Review 146, 2; 10.1175/MWR-D-17-0187.1
5. Summary and discussion
This paper studies the spatially and temporally varying adaptive prior covariance inflation for ensemble filters. Several enhancements are introduced to the previously documented algorithm of A09. The distance between the model forecast and the data points is assumed to be a random variable. This modifies the variance of the inflation likelihood with a factor of 
Under perfect modeling conditions, the enhanced scheme proved its efficiency in mitigating sampling errors, as compared to A09. This was verified by comparing the assimilation results to those of the EnKF-N benchmark. When the inflation is bounded below by 1, the accuracy suggested by the enhanced scheme is only marginally higher than that of A09. Moving the cap to 0, the enhanced scheme efficiently decreases both the magnitude and the frequency of deflation. This produced the most accurate and most consistent estimates in the atmospheric model. The performance of A09’s algorithm, on the other hand, degraded strongly because the Bayesian update leads to extremely tiny and large inflation values. We note here that A09’s algorithm was almost always used (at least by DART users) with an LB of 1. Allowing the inflation variance to change, the enhanced scheme offered more room for the inflation to evolve in time, and, thus, a better fit to the data was obtained. For A09, the inflation variance experienced a rapid decrease that forced the adaptive scheme to fix the prior, and this, in turn, reduced the accuracy of the estimates.
The presented results indicate that a moderate and not-so-frequent use of deflation can be quite useful. This is mainly because the mechanism by which inflation evolves in time is only based on the Bayesian update. Because the inflation distribution is kept unchanged during the forecast, an alternative way of dissipating an overinflated variance in the absence of observations is required. One example of such a scenario would be Doppler radar. Large inflation values are needed during a convective event. When the convection ends and Doppler observations vanish, the added artificial variance may be removed by other observations; however, the process can be very slow, leading to strong biases. The proposed scheme with no lower bound on the inflation can efficiently remove such variance through deflation only if there are some observations once the radar is gone. Another example is a tropical storm. To fit the data, the underestimated variance of the ensemble is heavily inflated. Once the hurricane passes, deflating the ensemble can help the model quickly forget about the hurricane and increase its prediction accuracy.
Despite the success of the presented inflation scheme, some limitations still exist.
- In terms of the inflation likelihood, the added correction term, which is based on a biased estimator for the expectation of the squared innovation sample, does not fully solve the problem. The observed inflation can still be negative when assimilating observations with large uncertainties. The correction term, however, partially mitigates the negative and deflation issues. It is important to note that the imposed right shift on the likelihood peak does not always yield larger posterior inflation values.
- An appropriate forecast model for the inflation is needed. Allowing the inflation first and second moments to evolve in time could improve the assimilation performance. Such a model should have strong correlations with the bias characteristics of the physical model, the observation network spatial structure, and how often the data become available. Ideas like deflation and damping were introduced to compensate for the absence of inflation time evolution.
- The presented inflation algorithm, unlike other schemes, such as the EnKF-N (Bocquet et al. 2015), does not account for errors in the ensemble mean and its covariance, given a finite ensemble size. Such confidence in the ensemble mean may further generate sampling errors that the adaptive scheme may not be aware of.
- When the sampling error is very large, the posterior variance may turn out to be small. One possible remediation is to inflate the posterior variance. Adapting the posterior variance inflation is not as simple as that of the prior because the observations would have been already used during the update. One could remove the impact of an observation to compute innovation and then apply the appropriate Bayesian update. This will be explored in detail in a future study. In fact, DART has supported this capability for a long time, and it has been used quite a bit, but it was not carefully documented.
The enhanced algorithm produced a robust scheme that is yet to be tested in realistic large-scale weather models. This task has been initiated, and experiments using the National Center for Atmospheric Research Community Atmosphere Model (CAM) are currently being conducted. Wind and temperature observations from radiosondes, ACARS, and aircraft, along with GPS radio occultation observations, are assimilated. Different 6-h forecasts of the atmospheric state are generated using the enhanced inflation scheme. The results of these tests will be analyzed in a follow-up publication.
The author is grateful for Patrick N. Raanes and another anonymous reviewer for their comments and suggestions. The author thanks Tim Hoar, Nancy Collins, Kevin Raeder, and Jonathan Hendricks for their immense technical support with DART. The author also thanks Jeffrey Anderson, Chris Snyder, and Alicia Karspeck for insightful discussions. The author sincerely appreciates the fruitful chats with Moustafa Gharamti. Any opinions, findings, and conclusions or recommendations expressed in this publication are those of the author and do not necessarily reflect the views of the National Science Foundation.
APPENDIX
Lorenz-63 System
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The eccentricity of an ellipse is measured as the ratio of the distance between the center of the ellipse and each focus to the length of the semimajor axis.
The left-hand side of Eq. (22) is approximated as 
The consistency in the estimates is measured here using the ratio between the overall RMS to spread averages.
We have observed similar issues when assimilating into large ocean models. Big differences in the ensemble variance can lead to CFL violations, eventually causing failure of the barotropic solver.
Note that the smallest inflation values are 0.1 and not 0 because of damping. With no damping, these become 0, and the assimilation fails.
