Introduction

The National Centers for Environmental Prediction (NCEP) operational job suite currently includes a 48km Eta model (the Eta-48), run at 0000 and 1200 UTC and a 29km Eta model (the Eta-29 or Meso Eta) run at 0300 and 1500 UTC. An experimental 10km Eta model (the Eta-10) is currently run at 0900 UTC over a Western Region Domain and evaluated by Western Region forecasters. This Technical Attachment (TA) describes two "bundles of changes" that are being made to the operational Eta models. (A "bundle of changes" is a collection of model changes that are made at the same time in order to reduce the operational workload at NCEP.) The first bundle of changes was implemented February 18, 1997. The most significant change, moving the Eta-29 to the early model slot, is part of the second bundle planned for spring 1997. These bundles of changes reflect a philosophy at the Environmental Modeling Center (EMC) of rapidly implementing new science and technology into the models.

Eta Model Development

The rate of model development at the EMC has increased in recent years. Prior to the 1990s, major changes to NWS operational models occurred less frequently and models were "frozen" once they were considered "mature". In order to keep pace with the rapid advancement of science and technology, the EMC no longer "freezes" models; rather model changes are implemented as soon as they can be tested and operational constraints (e.g., the front-end conversion) allow. Appendix A outlines the major developments of the Eta model. For a history of Eta model changes and their impacts on forecasting, see Staudenmaier (1996a) and Rogers et al., (1996a). Changes to model resolution (Appendix A) demonstrate the rapid development of the Eta model. Since its inception in June 1993, the Eta model has run at 80km resolution, 48km resolution (implemented September, 1993), and 29km (August, 1995). These changes in model resolution led to increased skill in low-level temperature and wind forecasts and in QPF (Staudenmaier 1997). A 15km Eta model was used for the 1996 Summer Olympics in Atlanta and currently an experimental 10km version is under evaluation by Western Region forecasters.

Forecaster evaluations have played an integral part in Eta model development by evaluating the Eta-29 and Eta-10 models and providing feedback to the EMC. The pre-implementation evaluation of the Eta-29 during the summer and fall of 1995 led to fixes in the Eta's post-processing code, in low-level winds, etc., and demonstrated the usefulness of a higher resolution model to the CAFTI committee. (All changes to NCEP models must be approved by CAFTI.) Western Region evaluations have led to several of the changes discussed below, for example, a fix for convection over high terrain and a temporary fix for surface temperatures during the melting of snow. The process of continuously improving NCEP models based on forecaster feedback has proven very successful.

The First Bundle Of Changes

At 1200 UTC 18 February 1997, several changes were implemented in the operational Eta-48 (48km/38lev) model, and at 1500 UTC 18 February 1997, the same changes were implemented in the operational Eta-29 (29km/50lev) model. A brief description of these changes follows.

List of Changes in the First Bundle

Description of Changes in the First Bundle

The largest impacts on forecasting from the first bundle are:

1. The short-wave radiation package will be modified to reduce the amount of incoming solar radiation (insolation) that reaches the earth's surface. This change is being made in response to a warm/dry bias in the Planetary Boundary Layer (PBL) reported over the plains during the summer. This warm/dry bias was detected by forecasters at NSSL and HPC. EMC examined statistics and determined that the amount of insolation in the model exceeded observed values. Several changes to the Eta radiation scheme will be made to reduce the amount of insolation. The most significant are: (1) reducing the solar constant by three percent to better approximate the absorption by trace gases; (2) including clouds from every level in the radiation scheme (rather than from three deep layers); (3) changing the earth's orbit from circular to eccentric; (4) including the effects of ozone. A concern here is that Western Region forecasters have noted a cold/wet bias this winter when cold air is trapped in valleys. Therefore, Western Region forecasters should monitor and provide feedback on temperature and moisture in the Eta's PBL.

2. A form drag scheme for the Eta terrain will be added to increase friction. Thus, terrain forced phenomena could be impacted, for example the translation of cyclones and fronts could be slowed. According to Mesinger et al. (1996), the form drag scheme could alleviate a systematic error of forecasting lows too deep and too far north in the lee of the Rocky Mountains.

3. The soil package will be changed by improving the "green fraction", allowing bare soil evaporation, and making a small change to the algorithm for the melting of snow. If any snow is melting in the model, the Eta models don't allow the skin temperature to exceed 0 degrees C. The new algorithm will ignore snow levels of less than 2.5 inches. This is an important change since Western Region forecasters noticed that in areas of thin snow cover, the Eta models often held the 2 meter temperature near 3 degrees C in locations where in reality the temperature climbed much higher. A more rigorous change to the melting snow algorithm will be made before the 1997/1998 winter. These changes to the soil package are not expected to create large impacts to QPF. However, changes in the intensity and location of precipitation are possible (Chen et al., 1996).

4. The Eta-10 model will be initiated from the operational 0300 UTC Eta-29 run rather than the experimental 0900 UTC. This change will facilitate the Western Region evaluation of the Eta-10 since the 0300 UTC run of the Eta-10 should be more reliable and timely than the 0900 UTC run.

List of Changes planned for the Second Bundle