AN ANALYSIS OF OROGRAPHICALLY INDUCED RAINFALL
IN SOUTHERN NEW ENGLAND

David Vallee and Lee Czepyha
National Weather Service Forecast Office
Taunton, Massachusetts 02780

Synoptic scale storm systems producing greater than one inch of rainfall across southern New England are quite common from early fall through late spring. These systems often have favored areas of orographically induced heavier rainfall. This paper will identify the mechanisms favorable for orographically induced rainfall associated with these in southern New England. A decision tree will be formulated to allow the forecaster to identify these areas and to more accurately adjust rainfall guidance from numerical models to account for these orographic effects.

2. DATA

Eleven synoptic scale rainfall events were examined, from October 1995 through May 1996. Numerical Forecast Guidance from the 48 km Step Mountain Coordinate Model (Eta), for at least two model runs prior to the onset of precipitation, was collected for each event. This model was chosen because of its greater resolution of topography and its performance in forecasting precipitation.

As stated by Junker (1992), the development of heavy precipitation is dependent on adequate moisture and upward motion. When factoring in the orographic component, one must also closely examine the boundary layer inflow. Basic model parameters that were examined included low- and mid-level flow, vertical motion, warm air advection, and moisture availability and advection through the use of theta-e analyses.

Low- and mid-level inflow was examined through an analysis of the boundary layer wind fields from 1000 mb through 700 mb. The magnitude and track of the vertical motion fields were examined for 850 mb and 700 mb, as well as mean omega in the layer 850-500 mb. Temperature and dew point advection were examined at 850 mb and 700 mb, as well as the Laplacian of Thickness Advection at 700 mb. Layer differential positive vorticity advection and Q-vector forcing were also considered, along with moisture availability and advection, using Theta-E and precipitable water.

3. ANALYSIS

All but two of the events studied had some of the typical synoptic forcing mechanisms common to synoptic flash floods (Chappell, 1986), including a slow moving 500 mb trough or closed low, a slow moving surface frontal boundary, mid-level flow parallel to the boundary, precipitable water that was much above the climatological normal, and the presence of a strong right rear quadrant of an upper level jet. The heavy precipitation of the remaining two events were associated with cyclogenesis along the east coast and the associated left front quadrant of a strengthening jet on the east side of a 500 mb closed low. All eleven events also featured a well defined theta-e ridge which traversed the forecast area.

Three types of rainfall patterns were identified. Type A produced heavy rain over the Connecticut River Valley. Type B produced an orographically induced rainfall maximum across the east facing slope region of the Berkshires, stretching from western Massachusetts through northwest Connecticut, with a marked shadowing of the Connecticut River Valley. A second maximum was positioned over the higher elevations of northeast Connecticut, northwest Rhode Island, and central Massachusetts. This region is the dividing zone between the coastal plain and the start of the interior highlands. Type C featured heavy rainfall across the northeast portion of Massachusetts and extreme southeast New Hampshire.

3.1 Type A Pattern

This pattern produced the heaviest rainfall in the Connecticut River Valley, with amounts diminishing to the west and east. ETA Model guidance for these events was characterized by southerly flow at the surface, averaging 25-30 kts for at least 18h prior to the onset of rain. Flow at both 700 mb and 850 mb was southwesterly, averaging 45 kts at 850 mb and 65 kts at 700 mb. Significant moisture advection (dew points of at least 10EC) was present. Also, the 1000-500 mb thickness exceeded the Preferred Thickness, as reviewed by Junker (1992), by at least 100 meters. With regard to the vertical motion fields, Type A events were significantly weaker than Type B or Type C events, possessing generally 6 Mbar/sec at 850 mb and 7.5 Mbar/sec at 700 mb. Actual rainfall maxima for these events was 2 times the maximum rainfall forecasted by the ETA Model.

3.2 Type B Pattern

Type B events produced the heaviest rainfall over the east slopes of the Berkshires, with a second maximum over the interior highlands. The most notable feature was the pronounced shadowing of the Connecticut River Valley, where rainfall was generally one half to two thirds of that which occurred over higher terrain.

These events were characterized by southeasterly flow of 35-45 kts at 1000 mb. At 850 mb and 700 mb, the inflow was southerly, but with a noticeable variation in speeds, ranging from as little as 30 kts to as much as 70 kts. This range of speeds at 850-700 mb played an important role in the magnitude of the observed rainfall maxima. Even with significant theta-E and dew point advection, when the 850-700 mb inflow exceeded 50 kts, the observed rainfall maxima were much less than those associated with inflow speeds at 850-700 mb below 50 kts.

For those events where the low at 850-700 mb was below 50 kts, and theta-E and dew point advection were weak, the observed rainfall maxima was 1.5 times greater than the model forecast. For those events where the flow at 850- 700 mb was below 50 kts, but the theta-E and dew point advection were strong, the observed rainfall maxima was up to three times the maximum forecasted by the model.

3.3 Type C Pattern

Type C pattern featured the heaviest rainfall along the leading edge of the interior highlands of northeast Massachsuetts and southeast New Hampshire. These events were characterized by east-northeasterly flow at 1000 mb. These events possessed the greatest magnitude of the 2-D Laplacian of Thickness Advection and the greatest amount of theta-E advection at 850 mb. The location of the maximum precipitation corresponded with the greatest theta-E advection at 850 mb.

There was a marked variation in rainfall amounts between events of this type. This appeared to be associated with temperature advection at 700 mb. Where warm air advection was present, the observed rainfall maxima were as much as three times the model forecast. When 700 mb advection was either neutral or cold, the rainfall maxima were only 1.2 to 1.4 times greater than the model forecast.

4. THE DECISION PROCESS

The decision process for determining the favored regions for enhanced rainfall and the amount of modification to the forecast should start with an analysis of the 1000 mb flow. The inflow will identify the favored regions, as presented in Section 3. The adjustment to make to the rainfall forecast will be a factor increase above the Eta model forecast, the magnitude of which is determined by a closer examination of flow at 850 mb, theta-E advection, and 700 mb temperature and dew point advection.

4.1 Identifying Favored Areas

The first step in this process is to identify the direction of flow, based on the 1000 mb winds. The breakdown is as follows:

4.2 Modification to the Model Forecast

Type A is the most straight forward of the three event types. For this case, model rainfall forecasts should be increased by a factor of 2.

For Type B events, the factor of adjustment is related to the speed of the 850 mb and 700 mb wind fields. If the wind field is greater than 55 kts at these levels, rainfall in the favored areas should only be increased by a factor of 1.25 to 1.5. When the wind field is less than 55 kts, rainfall amounts should be increased by a factor of 1.8 to 2.0.

For Type C events, the factor of adjustment is most dependent upon the presence of warm air advection at 700 mb. If warm air advection is present, rainfall amounts should be increased by at least a factor of 2-2.5. If cold air advection is indicated, the rainfall should be increased by only a factor of 1.3.

5. CONCLUSION

Synoptic scale heavy rainfall events across southern New England possess unique characteristics in relation to the locations of the heaviest orographically induced rainfall, that are influenced by the direction of the low level flow. The magnitude of the rainfall amounts in these favored regions were influenced by the availability and advection of moisture (i.e., the magnitude of theta-E advection at 850 and 700 mb and the dew point advection at 700 mb.

The decision process developed through this research provides yet another tool for the forecaster to utilize in the preparation of Quantitative Precipitation Forecasts (QPF), and to more accurately position the maxima of QPF. This will result in more timely and accurate river flood and flash flood watches and warnings.

6. REFERENCES

Chappell, C. F., 1986: Mesoscale Meteorology and Forecasting, American Meteorological Society.

793 pp.

Junker, N. W., 1992: Heavy Rain Forecasting Manual, National Weather Service Training Center. 91 pp.