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Urban effects are not currently represented in HTESSEL.  Extensive concrete and buildings are likely to have very different characteristics from HTESSEL land tiles, and possibly also provide a source of heat (the heat island effect) and even moisture (from air-conditioning units).  Forecast screen temperatures in large urban areas, particularly cities and especially coastal cities, are commonly several degrees too low when compared to observations.  The problem is accentuated on relatively clear, calm nights, and can be even worse in winter where the urban area is surrounded by snow cover.   Users should assess the potential for deficiencies in low-level parameters and adjust as necessary.

Cloud cover effects

Incorrectly analysed or incorrectly forecast cloud cover can cause 2m temperature errors.  This errors hinder or enable radiative cooling (or also heating by insolation) during the forecast process.

Commonly:

• too little cloud cover, especially over snow, increases overnight cooling and results in significantly lower forecast 2m temperatures.
• too much cloud cover reduces overnight cooling and results in anomalously high forecast minimum 2m temperatures.

Much of the cold bias of night-time 2m temperature south of 60°N is associated with an underestimation of (low) cloudiness.  The wintertime Wintertime night-time bias in Central Europe is smaller for days occasions which are (nearly) clear-sky.  

The situation by day may be different, depending also on day length.  

Underestimation However cloud cover is not solely responsible and underestimation of cloud optical depth and/or incorrect forecast of cloud type or base height could also play a part.  Errors in the prediction of the temperature structure have a strong influence on forecast cloud layer(s) and on humidity forecasts, particularly in the lowest layers (Fig9.2.1-1).Incorrectly analysed or forecast cloud cover can also cause 2m temperature errors, by hindering or enabling radiative cooling (or also heating by insolation) during the forecast process.   Commonly, too little cloud cover, especially over snow, results in significantly lower forecast 2m temperatures, whilst too much cloud reduces overnight cooling and gives anomalously high forecast minimum temperatures.  The situation by day may be different, depending also on day length.

It is also possible, though less common, to have too little cloud in the forecast, and temperatures that are too high!  These more unusual winter-time error scenarios commonly build up over a period of time.

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