CIMSS Satellite Blog

GOES-13 visible images

Blog reader Michael Lawson wrote:

Hi, I am a meteorologist working with Meridian Environmental Technology in Grand Forks, ND. A co-worker noticed a gravity wave feature in the visible over northern Indiana/northwest Ohio during the afternoon of September 15, 2008. Clouds moving into the feature would dissipate then reform once they propagated through the other side. Thought it would make an interesting topic!

Thanks for the tip, Michael. GOES-13 visible imagery (above) did indeed show the wave as it propagated northwestward (against the ambient flow) across northern Indiana/Ohio into southern Lower Michigan on 15 September 2008.

The dissipation of the stratocumulus cloud field with the passage of the wave might be explained by examining the rawinsonde data from Wilmington, Ohio (below) — note the presence of very dry air aloft, which could have been briefly mixed downward as the wave moved through (similar to what was seen with an apparent hydraulic jump over lower Michigan and southern Ontario on 08 May 2008).

Wilmington OH rawinsonde data

Mesoscale Convective Vortices (MCVs) will occasionally emerge from under the eroding cirrus canopy of a Mesoscale Convective System (MCS). Typically, an MCS will dissipate shortly after sunrise, but in atmospheres that include plentiful moisture and little vertical wind shear, the MCV that very frequently develops in an MCS can persist and serve to force subsequent convective development. (Previous MCVs have been documented in the CIMSS Satellite Blog here, here and here).

Strong convection that developed over western Texas early in the morning on Monday 11 August grew into a MCS that quickly eroded near sunrise. However, a swirl of mid-level clouds in the IR image loop above, and in the visible loop here, clearly show the persistence of an MCV. Note in the visible loop how subsequent convection develops very near the propagating MCV, first just to the northwest of the vortex (with the convection spreading south) and then to the north and east.

An MCV is driven mostly by the release of latent heat in convective clouds. Such heating will alter the stability and thus force the development of a vortex. The persistence of the vortex is a balance between the effects of ongoing convection releasing latent heat (maintaining the vortex) and the effects of strong vertical wind shear that serves to weaken the vortex. On this day, a rich moisture source is evident, as noted in the plot here of surface (yellow, in degrees Fahrenheit) and 850-mb dewpoints (white, in degrees Celsius). In addition, GFS model output of 850-500mb shear at 1200 UTC and 1800 UTC show minimal values in Texas where the MCV persisted.

Because forcing associated with the MCV can aid subsequent convective development, noting their presence — an easy task in satellite animation — is vital.

03 August 2008 marked the 22nd  consecutive day of daily high temperatures of 90º F or higher at Denver, Colorado (the old record was 18 consecutive days, set back in 1874 and 1901). AWIPS images of the MODIS Land Surface Temperature (LST) product (above) revealed daytime LST values as high as 140º F in southeastern Colorado and 134º F in southwestern Nebraska — while the surface “skin temperatures” were quite warm, the actual air temperatures (measured within shaded instrument shelters located about 5  feet above the surface) were only the in upper 90s to low 100s F.

It is interesting to note that the LST values were significantly lower across much of southcentral and southeastern Nebraska (in the upper 80s to low 90s F, green to yellow colors), even though the air temperatures were similar to those seen in eastern Colorado at that time (in the upper 90s to low 100s F).  These lower LST values were due to a much higher density of vegetation in eastern Nebraska, as shown by a comparison with the MODIS Normalized Difference Vegetation Index (NDVI) product (below) — NDVI values were greater than 0.7 in Nebraska, compared to 0.1 to 0.3 across much of eastern Colorado. In addition, very dry conditions had prevailed across eastern Colorado — Denver had only received 3.28 inches of precipitation so far in the year (the normal is 10.25 inches for the 01 January  - 31 July period) — in contrast with Hastings in eastern Nebraska, which had received 6.77 inches of precipitation so far in the year (the normal precipitation is 3.81 inches for the 01 January - 31 July period). The combination of drier soils and sparse vegetation  helped contribute to such high MODIS LST values.

MODIS true color imagery from the SSEC MODIS Today site (below) confirmed the presence of a much higher density of vegetation (denoted by darker green colors) across eastern Nebraska, compared to that found across eastern Colorado.