Intricacies of Atmospheric Rivers

The phrase atmospheric river was first used by researchers in the early 1990s to describe narrow corridors of strong integrated water vapor transport that are frequently found in the mid and high latitudes.  Atmospheric rivers typically achieve high water vapor contents by extracting water vapor from the tropics and/or local moisture convergence.  They are often found along and ahead of cold fronts where flow convergence can concentrate moisture and the strong pre-frontal low-level jet can produce large values of integrated water vapor transport.

Atmospheric rivers are not new phenomenon.  They have played an important role in our weather and climate for ages.  What is new is the name, recognition of their role by atmospheric scientists, and the development of techniques to identify and predict atmospheric rivers.

Two variables are commonly used to identify atmospheric rivers.  The first is the integrated water vapor (IWV), which is the total mass of water vapor in the atmosphere, typically expressed as the depth the water vapor would take if it were all condensed out as rain.  One advantage of IWV is that it can be inferred using satellite and GPS receivers.  The other is the integrated water vapor transport (IVT), which is the total amount of water vapor moving over a location.  IVT is dependent on both the amount of moisture and the strength of the flow and calculating it requires profiles of both moisture and wind.  For this reason, it is most often calculated using three-dimensional gridded analyses or forecasts.

I look at IWV as a measure of moisture availability for storms and IVT as a measure of moisture delivery to storms.  I generally use IVT because it has a stronger correlation with precipitation over the mountain west than IWV, as can be seen in the plots below.  However, in some lowland areas of the western US and in the High Plains, IWV has a higher correlation.   These are areas where high moisture availability plays an important role in fueling convective storms (note, this is an instantaneous correlation – high IWV is often a result of prior moisture transport).

Source: Rutz et al. (2014)

IVT has a strong correlation with precipitation, but the correlation isn't perfect.  This is because precipitation is dependent on many factors, including a mechanism for lift.  The highest correlations between IVT and precipitation are found in the Pacific coastal ranges, Cascade Mountains, and Sierra Nevada (see left-hand panels above).  These mountains are relatively broad and are frequently experience strong cross-barrier moisture transport accompanying atmospheric rivers, with forced lifting by the mountains generating precipitation. 

The correlation is lower, however, over the western interior.  Within that region, the highest values are found in the Idaho Panhandle, central Idaho Mountains, southwest Utah, Mogollon Rim, Tetons, and San Juan Mountains.  These lower correlations reflect several factors, including the greater diversity of processes generating precipitation over the western interior and contrasts in precipitation efficiency between wide and narrow mountain barriers during some storms.

The mountains of northern and central Utah have lower correlations.  This does not mean that we can't get significant precipitation from atmospheric rivers.  Instead, it reflects the fact that we sometimes get significant precipitation from storms that don't feature large cross-barrier moisture fluxes.  An example are cold, post-cold-frontal storms that generate large amounts of snowfall when the water vapor content of the atmosphere is relatively low (in an absolute sense) and the flow weak.

Moisture transport is very important for storm dynamics, but one needs to be cautious about developing AR-myopia.  As shown above, the correlation between IVT and precipitation is not perfect, and is modest or even low in portions of the interior western U.S.  Consideration of the precipitation generation mechanisms, such as frontal or orographic lift, is also important.

As an example, below is the 0600 UTC initialized NAM IVT forecast valid 0000 UTC 7 April.  Most of the eastern Pacific off the California coast is experiencing atmospheric river conditions (IVT>250 kg/m/s), with the highest values along an axis running from about 35˚N, 130˚W to 40˚N, 125˚W.

Source: CW3E

The NAM forecast 3-h accumulated precipitation at this time is, however, nearly non-existent in that area of high IVT and is instead highest over northern California, especially in the coastal ranges, northern Sierra Nevada, and southern Cascades where there is both warm frontal and orographic forcing generating precipitation.  The AR provides the moisture.  The fronts and mountains provide the dynamics.

I hope to look at what will happen as this system moves inland in a future post.  Forecasts currently suggest something that looks like what you get when a November cyclone meets April.