Predictability of the Strong Ural blocking Event in January 2012 in the Subseasonal to Seasonal Models of Europe and Canada

Round 1

Reviewer 1 Report

Review for “Predictability of the strong Ural blocking event in January 2012 in the ECMWF and ECCC models” submitted for publication Atmosphere.

This paper analyses the predictability of a major blocking event which occurred over Eurasia in 2012 with large repercussion on weather over the continent. The authors relied on existing sub seasonal and seasonal (S2S) predictions from two state of the art models, namely the ECMWF and ECCC and compared their forecast to that of the NCEP-NCAR reanalysis. They found that the ECCC (ECMWF) model is better at predicting the growth (demise) phase of the anomalous blocking pattern. The authors relied on discerning the geostrophic vorticity equation to quantify the dominant terms responsible for the evolution of the blocking. To this end, the paper is well written and its main contribution well stated. However, I have some concerns which are outlined below and need to be addressed before acceptance for publication:

Major comments:

As the author suggested, the ECMWF model seems to be able to capture the demise face better, as evident in Figs. 4a and b. I can think of two possibilities here for the damping of the blocking event, beyond advection terms shown in equation (1). First, damping of these events are also possible from diabatic effects. Since these are anticyclonic anomalies, the main diabatic effect is from radiative cooling. (2) Frictional effects are also important in damping the anomalies.

The author should contrast better the model resolution (important for frictional effect due to resolved topography) and how these systematic effects are contrasted in the two models. The authors mentioned the analyzed resolution earlier, but these are not necessarily the nominal resolutions that these models were run at. Also, they should look into the isentropic potential vorticity equation (see Bluestein, Chap. 1.9.13). This will help you discern the differences in tendencies between the two models and observation better than the vorticity equation. This equation includes the contribution from diabatic heating, ignored by the geostrophic vorticity equation here, and probably the most important factor in the destruction of vorticity anomalies under stagnant flow fields as observed in blocked patterns.

Bluestein, H. B. (1993). Synoptic-dynamic meteorology in midlatitudes. Volume II. Observations and theory of weather systems.

The paper becomes too much descriptive on the forecast skills of these two modes and is lacking on more of the potential impacts on forewarning these events in the S2S timescales and how this paper contributes to such effort. My recommendation is to add more discussion on the main contribution and cut down on the subtle descriptions of time correlation differences in spatial patterns.

Minor comments:

For the statistical significance test, how did you identify the effective degrees of freedom?

Line 44: I suggest to modify this language. Where is "there" exactly?

Line 50: Add a comma between model and including

Line 68: Replace “form” with “from”

Line 72: When compared to what exactly?

Line 84: I suggest to label omega by the Greek letter omega as the conventional wisdom is that W is reserved for high-related vertical velocity and omega is the pressure-related counterpart.

Line 86: Is the ensemble mean computed for each model or as a grand ensemble mean combining all 15 members of the two models? I the later is the case, then, there should be some weighting applied to avoid oversampling issues with the larger ensemble from the ECMWF model. Please clarify this.

Line 103: A better term that is often used is "blocked longitude". Blocking grid is not specific as the grid itself is the global model grid. I suggest you change this throughout the paper to "blocked longitude". Please change that throughout the paper.

Line 108: How are the primes defined here? Are they deviation from the monthly mean, winter mean, from the daily climatology? More explanation is necessary here. Also, is the bandpass filter applied to V' or to its product V'V'?

Why not including the zonal component of the EKE (U'U')? Example, 1/2(U'U' + V'V')

Line 112: Term b) should be horizontal vorticity advection. And term c) is better known as the vortex stretching term. Please correct.

Line 117: Last sentence. You may want to add reference here or perform an analysis to show this.

Line 129: How you quantify anticyclonic fluxes?

Lines 130-131: This appears to late here, move it to the definition of EKE above.

Line 133: Add “the” between that and establishment.

Figure 1b: Typo in figure title. Storm Track

Line 142: Is this consistent with the definition of EKE from V’V’ before?

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

Major comments:

1. The authors mentioned in their conclusions that “The observed and predicted characteristics of the persistent UB event in January 2012 are 319 consistent with previous studies [2,43–45]”. In this regard the authors should underline better the novelty of their study along the text and also in conclusions.  

2. The authors should discuss how the different resolution of NCEP and ECMWF/ECCC data can affect the interpretation of their results. In these kind of cases it is recommended to resample all the data at the lowest resolution (NCEP in this case). Since this implies a lot of work I don't insist to do it but at least the resolution issue should be commented.  

3. Figure 2-3 and 5-8  represent a key-element of the study results. Therefore, in order to ease the reading of the results I suggest the authors to produce maps showing simply the difference between the observation (NCEP) and the predictions made by the 2 models. Also the statistical parameters could be presented separately as tables.

4. I suggest the authors to include the discussions in the Chapter 3, and not separately in a chapter 4. Also, the conclusions could be presented independently at the end of the manuscript.

5. A synthesis table showing the performance of the two models could help the reading of the results.

Specific comments:

L3: acronyms in title should be avoided.

L16: not only in East Asia.

L15-16: please avoid repetition of one word in the same phrase.

L34: high pressure center instead of anticyclone.

L35: "downstream and upstream" of what?

L35: “northern winter”;  please rephrase!

L74-77: in my opinion the paragraph is useless.

L119: the chapter deals not only with the description of the event but also with its prediction and this should be indicated from the title.

L144: "ampliefied" or "reinforced" instead of the second “deepened”; a ridge is positive structure of the geopotential field!

L145: isoline of geopotential height instead of isopleth.

Figure 4: USB on the plot A is not explained in the text of the article or in the figure caption.

L220: the authors should explain for what reason they focus on the individual terms of geostrophic vorticity tendency.

L296-298: Please rephrase the entire sentence (it does not flow fluently in English). Please pay attention to these aspects along the manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.docx